7 Expressions [expr]

The > token following the type-id in a dynamic_cast, static_cast, reinterpret_cast, or const_cast can be the product of replacing a >> token by two consecutive > tokens ([temp.names]).

— end note]

7.6.1.2 Subscripting [expr.sub]

A subscript expression is a postfix expression followed by square brackets containing a possibly empty, comma-separated list of initializer-clauses that constitute the arguments to the subscript operator.

The postfix-expression and the initialization of the object parameter of any applicable subscript operator function is sequenced before each expression in the expression-list and also before any default argument.

The initialization of a non-object parameter of a subscript operator function S ([over.sub]), including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other non-object parameter of S.

With the built-in subscript operator, an expression-list shall be present, consisting of a single assignment-expression.

One of the expressions shall be a glvalue of type “array of T” or a prvalue of type “pointer to T” and the other shall be a prvalue of unscoped enumeration or integral type.

The result is of type “T”.

The type “T” shall be a completely-defined object type.50

The expression E1[E2] is identical (by definition) to *((E1)+(E2)), except that in the case of an array operand, the result is an lvalue if that operand is an lvalue and an xvalue otherwise.

[Note 1:

Despite its asymmetric appearance, subscripting is a commutative operation except for sequencing.

See [expr.unary] and [expr.add] for details of * and + and [dcl.array] for details of array types.

— end note]

50)50)

This is true even if the subscript operator is used in the following common idiom: &x[0].

7.6.1.3 Function call [expr.call]

A function call is a postfix expression followed by parentheses containing a possibly empty, comma-separated list of initializer-clauses which constitute the arguments to the function.

[Note 1:

If the postfix expression is a function or member function name, the appropriate function and the validity of the call are determined according to the rules in [over.match].

— end note]

The postfix expression shall have function type or function pointer type.

For a call to a non-member function or to a static member function, the postfix expression shall either be an lvalue that refers to a function (in which case the function-to-pointer standard conversion ([conv.func]) is suppressed on the postfix expression), or have function pointer type.

If the selected function is non-virtual, or if the id-expression in the class member access expression is a qualified-id, that function is called.

Otherwise, its final overrider in the dynamic type of the object expression is called; such a call is referred to as a virtual function call.

[Note 2:

The dynamic type is the type of the object referred to by the current value of the object expression.

[class.cdtor] describes the behavior of virtual function calls when the object expression refers to an object under construction or destruction.

— end note]

[Note 3:

If a function or member function name is used, and name lookup does not find a declaration of that name, the program is ill-formed.

No function is implicitly declared by such a call.

— end note]

If the postfix-expression names a destructor or pseudo-destructor ([expr.prim.id.dtor]), the type of the function call expression is void; otherwise, the type of the function call expression is the return type of the statically chosen function (i.e., ignoring the virtual keyword), even if the type of the function actually called is different.

If the postfix-expression names a pseudo-destructor (in which case the postfix-expression is a possibly-parenthesized class member access), the function call destroys the object of scalar type denoted by the object expression of the class member access ([expr.ref], [basic.life]).

Calling a function through an expression whose function type E is different from the function type F of the called function's definition results in undefined behavior unless the type “pointer to F” can be converted to the type “pointer to E” via a function pointer conversion ([conv.fctptr]).

[Note 4:

The exception applies when the expression has the type of a potentially-throwing function, but the called function has a non-throwing exception specification, and the function types are otherwise the same.

— end note]

When a function is called, each parameter ([dcl.fct]) is initialized ([dcl.init], [class.copy.ctor]) with its corresponding argument.

If the function is an explicit object member function and there is an implied object argument ([over.call.func]), the list of provided arguments is preceded by the implied object argument for the purposes of this correspondence.

If there is no corresponding argument, the default argument for the parameter is used.

[Example 1: template<typename ...T> int f(int n = 0, T ...t); int x = f<int>(); // error: no argument for second function parameter — end example]

If the function is an implicit object member function, the this parameter of the function ([expr.prim.this]) is initialized with a pointer to the object of the call, converted as if by an explicit type conversion .

[Note 5:

There is no access or ambiguity checking on this conversion; the access checking and disambiguation are done as part of the (possibly implicit) class member access operator.

See [class.member.lookup], [class.access.base], and [expr.ref].

— end note]

When a function is called, the type of any parameter shall not be a class type that is either incomplete or abstract.

[Note 6:

This still allows a parameter to be a pointer or reference to such a type.

However, it prevents a passed-by-value parameter to have an incomplete or abstract class type.

— end note]

It is implementation-defined whether the lifetime of a parameter ends when the function in which it is defined returns or at the end of the enclosing full-expression.

The initialization and destruction of each parameter occurs within the context of the calling function.

[Example 2:

The access of the constructor, conversion functions or destructor is checked at the point of call in the calling function.

If a constructor or destructor for a function parameter throws an exception, the search for a handler starts in the calling function; in particular, if the function called has a function-try-block ([except.pre]) with a handler that can handle the exception, this handler is not considered.

— end example]

The postfix-expression is sequenced before each expression in the expression-list and any default argument.

The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter.

[Note 7:

All side effects of argument evaluations are sequenced before the function is entered (see [intro.execution]).

— end note]

[Example 3: void f() { std::string s = "but I have heard it works even if you don't believe in it"; s.replace(0, 4, "").replace(s.find("even"), 4, "only").replace(s.find(" don't"), 6, ""); assert(s == "I have heard it works only if you believe in it"); // OK } — end example]

[Note 8:

If an operator function is invoked using operator notation, argument evaluation is sequenced as specified for the built-in operator; see [over.match.oper].

— end note]

[Example 4: struct S { S(int); }; int operator<<(S, int); int i, j; int x = S(i=1) << (i=2); int y = operator<<(S(j=1), j=2);

After performing the initializations, the value of i is 2 (see [expr.shift]), but it is unspecified whether the value of j is 1 or 2.

— end example]

The result of a function call is the result of the possibly-converted operand of the return statement ([stmt.return]) that transferred control out of the called function (if any), except in a virtual function call if the return type of the final overrider is different from the return type of the statically chosen function, the value returned from the final overrider is converted to the return type of the statically chosen function.

[Note 9:

A function can change the values of its non-const parameters, but these changes cannot affect the values of the arguments except where a parameter is of a reference type ([dcl.ref]); if the reference is to a const-qualified type, const_cast is required to be used to cast away the constness in order to modify the argument's value.

Where a parameter is of const reference type a temporary object is introduced if needed ([dcl.type], [lex.literal], [lex.string], [dcl.array], [class.temporary]).

In addition, it is possible to modify the values of non-constant objects through pointer parameters.

— end note]

A function can be declared to accept fewer arguments (by declaring default arguments) or more arguments (by using the ellipsis, ..., or a function parameter pack ([dcl.fct])) than the number of parameters in the function definition .

[Note 10:

This implies that, except where the ellipsis (...) or a function parameter pack is used, a parameter is available for each argument.

— end note]

When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_arg .

[Note 11:

This paragraph does not apply to arguments passed to a function parameter pack.

Function parameter packs are expanded during template instantiation ([temp.variadic]), thus each such argument has a corresponding parameter when a function template specialization is actually called.

— end note]

The lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the argument expression.

An argument that has type cv std::nullptr_t is converted to type void* ([conv.ptr]).

After these conversions, if the argument does not have arithmetic, enumeration, pointer, pointer-to-member, or class type, the program is ill-formed.

Passing a potentially-evaluated argument of a scoped enumeration type or of a class type ([class]) having an eligible non-trivial copy constructor, an eligible non-trivial move constructor, or a non-trivial destructor ([special]), with no corresponding parameter, is conditionally-supported with implementation-defined semantics.

If the argument has integral or enumeration type that is subject to the integral promotions, or a floating-point type that is subject to the floating-point promotion, the value of the argument is converted to the promoted type before the call.

These promotions are referred to as the default argument promotions.

Recursive calls are permitted, except to the main function .

A function call is an lvalue if the result type is an lvalue reference type or an rvalue reference to function type, an xvalue if the result type is an rvalue reference to object type, and a prvalue otherwise.

7.6.1.4 Explicit type conversion (functional notation) [expr.type.conv]

A simple-type-specifier or typename-specifier followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer.

If the type is a placeholder for a deduced class type, it is replaced by the return type of the function selected by overload resolution for class template deduction for the remainder of this subclause.

Otherwise, if the type contains a placeholder type, it is replaced by the type determined by placeholder type deduction ([dcl.type.auto.deduct]).

[Example 1: struct A {}; void f(A&); // #1 void f(A&&); // #2 A& g(); void h() { f(g()); // calls #1 f(A(g())); // calls #2 with a temporary object f(auto(g())); // calls #2 with a temporary object } — end example]

If the initializer is a parenthesized single expression, the type conversion expression is equivalent to the corresponding cast expression .

Otherwise, if the type is cv void and the initializer is () or {} (after pack expansion, if any), the expression is a prvalue of type void that performs no initialization.

Otherwise, the expression is a prvalue of the specified type whose result object is direct-initialized with the initializer.

If the initializer is a parenthesized optional expression-list, the specified type shall not be an array type.

7.6.1.5 Class member access [expr.ref]

A postfix expression followed by a dot . or an arrow ->, optionally followed by the keyword template, and then followed by an id-expression, is a postfix expression.

The postfix expression before the dot or arrow is evaluated;51 the result of that evaluation, together with the id-expression, determines the result of the entire postfix expression.

[Note 1:

If the keyword template is used, the following unqualified name is considered to refer to a template ([temp.names]).

If a simple-template-id results and is followed by a ::, the id-expression is a qualified-id.

— end note]

For the first option (dot) the first expression shall be a glvalue.

For the second option (arrow) the first expression shall be a prvalue having pointer type.

The expression E1->E2 is converted to the equivalent form (*(E1)).E2; the remainder of [expr.ref] will address only the first option (dot).52

Abbreviating postfix-expression.id-expression as E1.E2, E1 is called the object expression.

If the object expression is of scalar type, E2 shall name the pseudo-destructor of that same type (ignoring cv-qualifications) and E1.E2 is a prvalue of type “function of () returning void”.

[Note 2:

This value can only be used for a notional function call ([expr.prim.id.dtor]).

— end note]

Otherwise, the object expression shall be of class type.

The class type shall be complete unless the class member access appears in the definition of that class.

[Note 3:

The program is ill-formed if the result differs from that when the class is complete ([class.member.lookup]).

— end note]

[Note 4:

[basic.lookup.qual] describes how names are looked up after the . and -> operators.

— end note]

If E2 is a bit-field, E1.E2 is a bit-field.

The type and value category of E1.E2 are determined as follows.

In the remainder of [expr.ref], cq represents either const or the absence of const and vq represents either volatile or the absence of volatile.

cv represents an arbitrary set of cv-qualifiers, as defined in [basic.type.qualifier].

If E2 is declared to have type “reference to T”, then E1.E2 is an lvalue of type T.

If E2 is a static data member, E1.E2 designates the object or function to which the reference is bound, otherwise E1.E2 designates the object or function to which the corresponding reference member of E1 is bound.

Otherwise, one of the following rules applies.

(6.1)
If E2 is a static data member and the type of E2 is T, then E1.E2 is an lvalue; the expression designates the named member of the class.

The type of E1.E2 is T.
(6.2)
If E2 is a non-static data member and the type of E1 is “cq1 vq1 X”, and the type of E2 is “cq2 vq2 T”, the expression designates the corresponding member subobject of the object designated by the first expression.

If E1 is an lvalue, then E1.E2 is an lvalue; otherwise E1.E2 is an xvalue.

Let the notation vq12 stand for the “union” of vq1 and vq2; that is, if vq1 or vq2 is volatile, then vq12 is volatile.

Similarly, let the notation cq12 stand for the “union” of cq1 and cq2; that is, if cq1 or cq2 is const, then cq12 is const.

If E2 is declared to be a mutable member, then the type of E1.E2 is “vq12 T”.

If E2 is not declared to be a mutable member, then the type of E1.E2 is “cq12 vq12 T”.
(6.3)
If E2 is an overload set, function overload resolution ([over.match]) is used to select the function to which E2 refers.

The type of E1.E2 is the type of E2 and E1.E2 refers to the function referred to by E2.
- (6.3.1)
  If E2 refers to a static member function, E1.E2 is an lvalue.
- (6.3.2)
  Otherwise (when E2 refers to a non-static member function), E1.E2 is a prvalue.
  
  The expression can be used only as the left-hand operand of a member function call ([class.mfct]).
  
  [Note 5:
  Any redundant set of parentheses surrounding the expression is ignored ([expr.prim.paren]).
  — end note]
(6.4)
If E2 is a nested type, the expression E1.E2 is ill-formed.
(6.5)
If E2 is a member enumerator and the type of E2 is T, the expression E1.E2 is a prvalue of type T whose value is the value of the enumerator.

If E2 is a non-static member, the program is ill-formed if the class of which E2 is directly a member is an ambiguous base ([class.member.lookup]) of the naming class ([class.access.base]) of E2.

[Note 6:

The program is also ill-formed if the naming class is an ambiguous base of the class type of the object expression; see [class.access.base].

— end note]

If E2 is a non-static member and the result of E1 is an object whose type is not similar ([conv.qual]) to the type of E1, the behavior is undefined.

[Example 1: struct A { int i; }; struct B { int j; }; struct D : A, B {}; void f() { D d; static_cast<B&>(d).j; // OK, object expression designates the B subobject of d reinterpret_cast<B&>(d).j; // undefined behavior } — end example]

51)51)

If the class member access expression is evaluated, the subexpression evaluation happens even if the result is unnecessary to determine the value of the entire postfix expression, for example if the id-expression denotes a static member.

52)52)

Note that (*(E1)) is an lvalue.

7.6.1.6 Increment and decrement [expr.post.incr]

The value of a postfix ++ expression is the value of its operand.

[Note 1:

The value obtained is a copy of the original value.

— end note]

The operand shall be a modifiable lvalue.

The type of the operand shall be an arithmetic type other than cv bool, or a pointer to a complete object type.

An operand with volatile-qualified type is deprecated; see [depr.volatile.type].

The value of the operand object is modified ([defns.access]) by adding 1 to it.

The value computation of the ++ expression is sequenced before the modification of the operand object.

With respect to an indeterminately-sequenced function call, the operation of postfix ++ is a single evaluation.

[Note 2:

Therefore, a function call cannot intervene between the lvalue-to-rvalue conversion and the side effect associated with any single postfix ++ operator.

— end note]

The result is a prvalue.

The type of the result is the cv-unqualified version of the type of the operand.

If the operand is a bit-field that cannot represent the incremented value, the resulting value of the bit-field is implementation-defined.

7.6.1.7 Dynamic cast [expr.dynamic.cast]

The result of the expression dynamic_cast<T>(v) is the result of converting the expression v to type T.

T shall be a pointer or reference to a complete class type, or “pointer to cv void”.

The dynamic_cast operator shall not cast away constness ([expr.const.cast]).

If T is a pointer type, v shall be a prvalue of a pointer to complete class type, and the result is a prvalue of type T.

If T is an lvalue reference type, v shall be an lvalue of a complete class type, and the result is an lvalue of the type referred to by T.

If T is an rvalue reference type, v shall be a glvalue having a complete class type, and the result is an xvalue of the type referred to by T.

If the type of v is the same as T (ignoring cv-qualifications), the result is v (converted if necessary).

If T is “pointer to cv1 B” and v has type “pointer to cv2 D” such that B is a base class of D, the result is a pointer to the unique B subobject of the D object pointed to by v, or a null pointer value if v is a null pointer value.

Similarly, if T is “reference to cv1 B” and v has type cv2 D such that B is a base class of D, the result is the unique B subobject of the D object referred to by v.53

In both the pointer and reference cases, the program is ill-formed if B is an inaccessible or ambiguous base class of D.

[Example 1: struct B { }; struct D : B { }; void foo(D* dp) { B* bp = dynamic_cast<B*>(dp); // equivalent to B* bp = dp; } — end example]

Otherwise, v shall be a pointer to or a glvalue of a polymorphic type .

If v is a null pointer value, the result is a null pointer value.

If T is “pointer to cv void”, then the result is a pointer to the most derived object pointed to by v.

Otherwise, a runtime check is applied to see if the object pointed or referred to by v can be converted to the type pointed or referred to by T.

Let C be the class type to which T points or refers.

The runtime check logically executes as follows:

(8.1)
If, in the most derived object pointed (referred) to by v, v points (refers) to a public base class subobject of a C object, and if only one object of type C is derived from the subobject pointed (referred) to by v the result points (refers) to that C object.
(8.2)
Otherwise, if v points (refers) to a public base class subobject of the most derived object, and the type of the most derived object has a base class, of type C, that is unambiguous and public, the result points (refers) to the C subobject of the most derived object.
(8.3)
Otherwise, the runtime check fails.

The value of a failed cast to pointer type is the null pointer value of the required result type.

A failed cast to reference type throws an exception of a type that would match a handler of type std::bad_cast .

[Example 2: class A { virtual void f(); }; class B { virtual void g(); }; class D : public virtual A, private B { }; void g() { D d; B* bp = (B*)&d; // cast needed to break protection A* ap = &d; // public derivation, no cast needed D& dr = dynamic_cast<D&>(*bp); // fails ap = dynamic_cast<A*>(bp); // fails bp = dynamic_cast<B*>(ap); // fails ap = dynamic_cast<A*>(&d); // succeeds bp = dynamic_cast<B*>(&d); // ill-formed (not a runtime check) } class E : public D, public B { }; class F : public E, public D { }; void h() { F f; A* ap = &f; // succeeds: finds unique A D* dp = dynamic_cast<D*>(ap); // fails: yields null; f has two D subobjects E* ep = (E*)ap; // error: cast from virtual base E* ep1 = dynamic_cast<E*>(ap); // succeeds } — end example]

[Note 1:

Subclause [class.cdtor] describes the behavior of a dynamic_cast applied to an object under construction or destruction.

— end note]

53)53)

The most derived object ([intro.object]) pointed or referred to by v can contain other B objects as base classes, but these are ignored.

7.6.1.8 Type identification [expr.typeid]

The result of a typeid expression is an lvalue of static type const std::type_info ([type.info]) and dynamic type const std::type_info or const name where name is an implementation-defined class publicly derived from std::type_info which preserves the behavior described in [type.info].54

The lifetime of the object referred to by the lvalue extends to the end of the program.

Whether or not the destructor is called for the std::type_info object at the end of the program is unspecified.

If the type of the expression or type-id operand is a (possibly cv-qualified) class type or a reference to (possibly cv-qualified) class type, that class shall be completely defined.

When typeid is applied to a glvalue whose type is a polymorphic class type ([class.virtual]), the result refers to a std::type_info object representing the type of the most derived object ([intro.object]) (that is, the dynamic type) to which the glvalue refers.

If the glvalue is obtained by applying the unary * operator to a pointer55 and the pointer is a null pointer value ([basic.compound]), the typeid expression throws an exception ([except.throw]) of a type that would match a handler of type std::bad_typeid exception ([bad.typeid]).

When typeid is applied to an expression other than a glvalue of a polymorphic class type, the result refers to a std::type_info object representing the static type of the expression.

Lvalue-to-rvalue, array-to-pointer, and function-to-pointer conversions are not applied to the expression.

If the expression is a prvalue, the temporary materialization conversion is applied.

The expression is an unevaluated operand .

When typeid is applied to a type-id, the result refers to a std::type_info object representing the type of the type-id.

If the type of the type-id is a reference to a possibly cv-qualified type, the result of the typeid expression refers to a std::type_info object representing the cv-unqualified referenced type.

[Note 1:

The type-id cannot denote a function type with a cv-qualifier-seq or a ref-qualifier ([dcl.fct]).

— end note]

If the type of the expression or type-id is a cv-qualified type, the result of the typeid expression refers to a std::type_info object representing the cv-unqualified type.

[Example 1: class D { /* ... */ }; D d1; const D d2; typeid(d1) == typeid(d2); // yields true typeid(D) == typeid(const D); // yields true typeid(D) == typeid(d2); // yields true typeid(D) == typeid(const D&); // yields true — end example]

The type std::type_info ([type.info]) is not predefined; if a standard library declaration ([typeinfo.syn], [std.modules]) of std::type_info does not precede ([basic.lookup.general]) a typeid expression, the program is ill-formed.

[Note 2:

Subclause [class.cdtor] describes the behavior of typeid applied to an object under construction or destruction.

— end note]

54)54)

The recommended name for such a class is extended_type_info.

55)55)

If p is an expression of pointer type, then *p, (*p), *(p), ((*p)), *((p)), and so on all meet this requirement.

7.6.1.9 Static cast [expr.static.cast]

The result of the expression static_cast<T>(v) is the result of converting the expression v to type T.

The static_cast operator shall not cast away constness ([expr.const.cast]).

An lvalue of type “cv1 B”, where B is a class type, can be cast to type “reference to cv2 D”, where D is a class derived from B, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.

If B is a virtual base class of D or a base class of a virtual base class of D, or if no valid standard conversion from “pointer to D” to “pointer to B” exists ([conv.ptr]), the program is ill-formed.

An xvalue of type “cv1 B” can be cast to type “rvalue reference to cv2 D” with the same constraints as for an lvalue of type “cv1 B”.

If the object of type “cv1 B” is actually a base class subobject of an object of type D, the result refers to the enclosing object of type D.

Otherwise, the behavior is undefined.

[Example 1: struct B { }; struct D : public B { }; D d; B &br = d; static_cast<D&>(br); // produces lvalue denoting the original d object — end example]

An lvalue of type T1 can be cast to type “rvalue reference to T2” if T2 is reference-compatible with T1 ([dcl.init.ref]).

If the value is not a bit-field, the result refers to the object or the specified base class subobject thereof; otherwise, the lvalue-to-rvalue conversion is applied to the bit-field and the resulting prvalue is used as the operand of the static_cast for the remainder of this subclause.

If T2 is an inaccessible ([class.access]) or ambiguous ([class.member.lookup]) base class of T1, a program that necessitates such a cast is ill-formed.

An expression E can be explicitly converted to a type T if there is an implicit conversion sequence ([over.best.ics]) from E to T, if overload resolution for a direct-initialization ([dcl.init]) of an object or reference of type T from E would find at least one viable function ([over.match.viable]), or if T is an aggregate type ([dcl.init.aggr]) having a first element x and there is an implicit conversion sequence from E to the type of x.

If T is a reference type, the effect is the same as performing the declaration and initialization T t(E); for some invented temporary variable t ([dcl.init]) and then using the temporary variable as the result of the conversion.

Otherwise, the result object is direct-initialized from E.

[Note 1:

The conversion is ill-formed when attempting to convert an expression of class type to an inaccessible or ambiguous base class.

— end note]

[Note 2:

If T is “array of unknown bound of U”, this direct-initialization defines the type of the expression as U[1].

— end note]

Otherwise, the static_cast shall perform one of the conversions listed below.

No other conversion shall be performed explicitly using a static_cast.

Any expression can be explicitly converted to type cv void, in which case the operand is a discarded-value expression ([expr.prop]).

[Note 3:

Such a static_cast has no result as it is a prvalue of type void; see [basic.lval].

— end note]

[Note 4:

However, if the value is in a temporary object, the destructor for that object is not executed until the usual time, and the value of the object is preserved for the purpose of executing the destructor.

— end note]

The inverse of any standard conversion sequence ([conv]) not containing an lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), function-to-pointer ([conv.func]), null pointer ([conv.ptr]), null member pointer ([conv.mem]), boolean ([conv.bool]), or function pointer ([conv.fctptr]) conversion, can be performed explicitly using static_cast.

A program is ill-formed if it uses static_cast to perform the inverse of an ill-formed standard conversion sequence.

[Example 2: struct B { }; struct D : private B { }; void f() { static_cast<D*>((B*)0); // error: B is a private base of D static_cast<int B::*>((int D::*)0); // error: B is a private base of D } — end example]

The lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) conversions are applied to the operand.

Such a static_cast is subject to the restriction that the explicit conversion does not cast away constness ([expr.const.cast]), and the following additional rules for specific cases:

A value of a scoped enumeration type ([dcl.enum]) can be explicitly converted to an integral type; the result is the same as that of converting to the enumeration's underlying type and then to the destination type.

A value of a scoped enumeration type can also be explicitly converted to a floating-point type; the result is the same as that of converting from the original value to the floating-point type.

A value of integral or enumeration type can be explicitly converted to a complete enumeration type.

If the enumeration type has a fixed underlying type, the value is first converted to that type by integral promotion ([conv.prom]) or integral conversion ([conv.integral]), if necessary, and then to the enumeration type.

If the enumeration type does not have a fixed underlying type, the value is unchanged if the original value is within the range of the enumeration values ([dcl.enum]), and otherwise, the behavior is undefined.

A value of floating-point type can also be explicitly converted to an enumeration type.

The resulting value is the same as converting the original value to the underlying type of the enumeration ([conv.fpint]), and subsequently to the enumeration type.

A prvalue of floating-point type can be explicitly converted to any other floating-point type.

If the source value can be exactly represented in the destination type, the result of the conversion has that exact representation.

If the source value is between two adjacent destination values, the result of the conversion is an implementation-defined choice of either of those values.

Otherwise, the behavior is undefined.

A prvalue of type “pointer to cv1 B”, where B is a class type, can be converted to a prvalue of type “pointer to cv2 D”, where D is a complete class derived from B, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.

The null pointer value ([basic.compound]) is converted to the null pointer value of the destination type.

If the prvalue of type “pointer to cv1 B” points to a B that is actually a base class subobject of an object of type D, the resulting pointer points to the enclosing object of type D.

Otherwise, the behavior is undefined.

A prvalue of type “pointer to member of D of type cv1 T” can be converted to a prvalue of type “pointer to member of B of type cv2 T”, where D is a complete class type and B is a base class of D, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.

[Note 5:

Function types (including those used in pointer-to-member-function types) are never cv-qualified ([dcl.fct]).

— end note]

If no valid standard conversion from “pointer to member of B of type T” to “pointer to member of D of type T” exists ([conv.mem]), the program is ill-formed.

The null member pointer value is converted to the null member pointer value of the destination type.

If class B contains the original member, or is a base or derived class of the class containing the original member, the resulting pointer to member points to the original member.

Otherwise, the behavior is undefined.

[Note 6:

Although class B need not contain the original member, the dynamic type of the object with which indirection through the pointer to member is performed must contain the original member; see [expr.mptr.oper].

— end note]

A prvalue of type “pointer to cv1 void” can be converted to a prvalue of type “pointer to cv2 T”, where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.

If the original pointer value represents the address A of a byte in memory and A does not satisfy the alignment requirement of T, then the resulting pointer value is unspecified.

Otherwise, if the original pointer value points to an object a, and there is an object b of type similar to T that is pointer-interconvertible with a, the result is a pointer to b.

Otherwise, the pointer value is unchanged by the conversion.

[Example 3: T* p1 = new T; const T* p2 = static_cast<const T*>(static_cast<void*>(p1)); bool b = p1 == p2; // b will have the value true. — end example]

7.6.1.10 Reinterpret cast [expr.reinterpret.cast]

The result of the expression reinterpret_cast<T>(v) is the result of converting the expression v to type T.

If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the expression v.

Conversions that can be performed explicitly using reinterpret_cast are listed below.

No other conversion can be performed explicitly using reinterpret_cast.

The reinterpret_cast operator shall not cast away constness ([expr.const.cast]).

An expression of integral, enumeration, pointer, or pointer-to-member type can be explicitly converted to its own type; such a cast yields the value of its operand.

[Note 1:

The mapping performed by reinterpret_cast might, or might not, produce a representation different from the original value.

— end note]

A pointer can be explicitly converted to any integral type large enough to hold all values of its type.

The mapping function is implementation-defined.

[Note 2:

It is intended to be unsurprising to those who know the addressing structure of the underlying machine.

— end note]

A value of type std::nullptr_t can be converted to an integral type; the conversion has the same meaning and validity as a conversion of (void*)0 to the integral type.

[Note 3:

A reinterpret_cast cannot be used to convert a value of any type to the type std::nullptr_t.

— end note]

A value of integral type or enumeration type can be explicitly converted to a pointer.

A pointer converted to an integer of sufficient size (if any such exists on the implementation) and back to the same pointer type will have its original value; mappings between pointers and integers are otherwise implementation-defined.

A function pointer can be explicitly converted to a function pointer of a different type.

[Note 4:

The effect of calling a function through a pointer to a function type ([dcl.fct]) that is not the same as the type used in the definition of the function is undefined ([expr.call]).

— end note]

Except that converting a prvalue of type “pointer to T1” to the type “pointer to T2” (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified.

[Note 5:

See also [conv.ptr] for more details of pointer conversions.

— end note]

An object pointer can be explicitly converted to an object pointer of a different type.56

When a prvalue v of object pointer type is converted to the object pointer type “pointer to cv T”, the result is static_cast<cv T*>(static_cast<cv void*>(v)).

[Note 6:

Converting a pointer of type “pointer to T1” that points to an object of type T1 to the type “pointer to T2” (where T2 is an object type and the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer value.

— end note]

Converting a function pointer to an object pointer type or vice versa is conditionally-supported.

The meaning of such a conversion is implementation-defined, except that if an implementation supports conversions in both directions, converting a prvalue of one type to the other type and back, possibly with different cv-qualification, shall yield the original pointer value.

The null pointer value ([basic.compound]) is converted to the null pointer value of the destination type.

[Note 7:

A null pointer constant of type std::nullptr_t cannot be converted to a pointer type, and a null pointer constant of integral type is not necessarily converted to a null pointer value.

— end note]

A prvalue of type “pointer to member of X of type T1” can be explicitly converted to a prvalue of a different type “pointer to member of Y of type T2” if T1 and T2 are both function types or both object types.57

The null member pointer value ([conv.mem]) is converted to the null member pointer value of the destination type.

The result of this conversion is unspecified, except in the following cases:

(10.1)
Converting a prvalue of type “pointer to member function” to a different pointer-to-member-function type and back to its original type yields the original pointer-to-member value.
(10.2)
Converting a prvalue of type “pointer to data member of X of type T1” to the type “pointer to data member of Y of type T2” (where the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer-to-member value.

A glvalue of type T1, designating an object x, can be cast to the type “reference to T2” if an expression of type “pointer to T1” can be explicitly converted to the type “pointer to T2” using a reinterpret_cast.

The result is that of *reinterpret_cast<T2 *>(p) where p is a pointer to x of type “pointer to T1”.

No temporary is created, no copy is made, and no constructors ([class.ctor]) or conversion functions ([class.conv]) are called.58

56)56)

The types can have different cv-qualifiers, subject to the overall restriction that a reinterpret_cast cannot cast away constness.

57)57)

T1 and T2 can have different cv-qualifiers, subject to the overall restriction that a reinterpret_cast cannot cast away constness.

58)58)

This is sometimes referred to as a type pun when the result refers to the same object as the source glvalue.

7.6.1.11 Const cast [expr.const.cast]

The result of the expression const_cast<T>(v) is of type T.

If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the expression v.

Conversions that can be performed explicitly using const_cast are listed below.

No other conversion shall be performed explicitly using const_cast.

[Note 1:

Subject to the restrictions in this subclause, an expression can be cast to its own type using a const_cast operator.

— end note]

For two similar types T1 and T2, a prvalue of type T1 may be explicitly converted to the type T2 using a const_cast if, considering the qualification-decompositions of both types, each

P_{i}^{1}

is the same as

P_{i}^{2}

for all i.

The result of a const_cast refers to the original entity.

[Example 1: typedef int *A[3]; // array of 3 pointer to int typedef const int *const CA[3]; // array of 3 const pointer to const int CA &&r = A{}; // OK, reference binds to temporary array object // after qualification conversion to type CA A &&r1 = const_cast<A>(CA{}); // error: temporary array decayed to pointer A &&r2 = const_cast<A&&>(CA{}); // OK — end example]

For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:

(4.1)
an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;
(4.2)
a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and
(4.3)
if T1 is a class type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>.

The result of a reference const_cast refers to the original object if the operand is a glvalue and to the result of applying the temporary materialization conversion otherwise.

A null pointer value ([basic.compound]) is converted to the null pointer value of the destination type.

The null member pointer value ([conv.mem]) is converted to the null member pointer value of the destination type.

[Note 2:

Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member resulting from a const_cast that casts away a const-qualifier59 can produce undefined behavior ([dcl.type.cv]).

— end note]

A conversion from a type T1 to a type T2 casts away constness if T1 and T2 are different, there is a qualification-decomposition ([conv.qual]) of T1 yielding n such that T2 has a qualification-decomposition of the form

c v_{0}^{2}

P_{0}^{2}

c v_{1}^{2}

P_{1}^{2}

⋯

c v_{n - 1}^{2}

P_{n - 1}^{2}

c v_{n}^{2}

U_{2}

, and there is no qualification conversion that converts T1 to

c v_{0}^{2}

P_{0}^{1}

c v_{1}^{2}

P_{1}^{1}

⋯

c v_{n - 1}^{2}

P_{n - 1}^{1}

c v_{n}^{2}

U_{1}

Casting from an lvalue of type T1 to an lvalue of type T2 using an lvalue reference cast or casting from an expression of type T1 to an xvalue of type T2 using an rvalue reference cast casts away constness if a cast from a prvalue of type “pointer to T1” to the type “pointer to T2” casts away constness.

[Note 3:

Some conversions which involve only changes in cv-qualification cannot be done using const_cast.

For instance, conversions between pointers to functions are not covered because such conversions lead to values whose use causes undefined behavior.

For the same reasons, conversions between pointers to member functions, and in particular, the conversion from a pointer to a const member function to a pointer to a non-const member function, are not covered.

— end note]

59)59)

const_cast is not limited to conversions that cast away a const-qualifier.