Inside a template, some constructs have semantics which may differ from one instantiation to another. Such a construct depends on the template parameters. In particular, types and expressions may depend on the type and/or value of template parameters (as determined by the template arguments) and this determines the context for name lookup for certain names. An expressions may be type-dependent (that is, its type may depend on a template parameter) or value-dependent (that is, its value when evaluated as a constant expression ([expr.const]) may depend on a template parameter) as described in this subclause. In an expression of the form:
postfix-expression ( expression-listopt )
where the postfix-expression is an unqualified-id, the unqualified-id denotes a dependent name if
any of the expressions in the expression-list is a pack expansion,
any of the expressions or braced-init-lists in the expression-list is type-dependent, or
the unqualified-id is a template-id in which any of the template arguments depends on a template parameter.
If an operand of an operator is a type-dependent expression, the operator also denotes a dependent name. Such names are unbound and are looked up at the point of the template instantiation ([temp.point]) in both the context of the template definition and the context of the point of instantiation.
[ Example:
template<class T> struct X : B<T> {
typename T::A* pa;
void f(B<T>* pb) {
static int i = B<T>::i;
pb->j++;
}
};the base class name B<T>, the type name T::A, the names B<T>::i and pb->j explicitly depend on the template-parameter. — end example ]
In the definition of a class or class template, the scope of a dependent base class is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member. [ Example:
typedef double A;
template<class T> class B {
typedef int A;
};
template<class T> struct X : B<T> {
A a; // a has type double
};The type name A in the definition of X<T> binds to the typedef name defined in the global namespace scope, not to the typedef name defined in the base class B<T>. — end example ] [ Example:
struct A {
struct B { /* ... */ };
int a;
int Y;
};
int a;
template<class T> struct Y : T {
struct B { /* ... */ };
B b; // The B defined in Y
void f(int i) { a = i; } // ::a
Y* p; // Y<T>
};
Y<A> ya;The members A::B, A::a, and A::Y of the template argument A do not affect the binding of names in Y<A>. — end example ]
A name refers to the current instantiation if it is
in the definition of a class template, a nested class of a class template, a member of a class template, or a member of a nested class of a class template, the injected-class-name of the class template or nested class,
in the definition of a primary class template or a member of a primary class template, the name of the class template followed by the template argument list of the primary template (as described below) enclosed in <> (or an equivalent template alias specialization),
in the definition of a nested class of a class template, the name of the nested class referenced as a member of the current instantiation, or
in the definition of a partial specialization or a member of a partial specialization, the name of the class template followed by the template argument list of the partial specialization enclosed in <> (or an equivalent template alias specialization). If the nth template parameter is a parameter pack, the nth template argument is a pack expansion whose pattern is the name of the parameter pack.
The template argument list of a primary template is a template argument list in which the nth template argument has the value of the nth template parameter of the class template. If the nth template parameter is a template parameter pack, the nth template argument is a pack expansion whose pattern is the name of the template parameter pack.
A template argument that is equivalent to a template parameter (i.e., has the same constant value or the same type as the template parameter) can be used in place of that template parameter in a reference to the current instantiation. In the case of a non-type template argument, the argument must have been given the value of the template parameter and not an expression in which the template parameter appears as a subexpression. [ Example:
template <class T> class A {
A* p1; // A is the current instantiation
A<T>* p2; // A<T> is the current instantiation
A<T*> p3; // A<T*> is not the current instantiation
::A<T>* p4; // ::A<T> is the current instantiation
class B {
B* p1; // B is the current instantiation
A<T>::B* p2; // A<T>::B is the current instantiation
typename A<T*>::B* p3; // A<T*>::B is not the current instantiation
};
};
template <class T> class A<T*> {
A<T*>* p1; // A<T*> is the current instantiation
A<T>* p2; // A<T> is not the current instantiation
};
template <class T1, class T2, int I> struct B {
B<T1, T2, I>* b1; // refers to the current instantiation
B<T2, T1, I>* b2; // not the current instantiation
typedef T1 my_T1;
static const int my_I = I;
static const int my_I2 = I+0;
static const int my_I3 = my_I;
B<my_T1, T2, my_I>* b3; // refers to the current instantiation
B<my_T1, T2, my_I2>* b4; // not the current instantiation
B<my_T1, T2, my_I3>* b5; // refers to the current instantiation
};— end example ]
A dependent base class is a base class that is a dependent type and is not the current instantiation. [ Note: A base class can be the current instantiation in the case of a nested class naming an enclosing class as a base. [ Example:
template<class T> struct A {
typedef int M;
struct B {
typedef void M;
struct C;
};
};
template<class T> struct A<T>::B::C : A<T> {
M m; // OK, A<T>::M
};— end example ] — end note ]
A name is a member of the current instantiation if it is
An unqualified name that, when looked up, refers to at least one member of a class that is the current instantiation or a non-dependent base class thereof. [ Note: This can only occur when looking up a name in a scope enclosed by the definition of a class template. — end note ]
A qualified-id in which the nested-name-specifier refers to the current instantiation and that, when looked up, refers to at least one member of a class that is the current instantiation or a non-dependent base class thereof. [ Note: If no such member is found, and the current instantiation has any dependent base classes, then the qualified-id is a member of an unknown specialization; see below. — end note ]
An id-expression denoting the member in a class member access expression for which the type of the object expression is the current instantiation, and the id-expression, when looked up, refers to at least one member of a class that is the current instantiation or a non-dependent base class thereof. [ Note: If no such member is found, and the current instantiation has any dependent base classes, then the id-expression is a member of an unknown specialization; see below. — end note ]
[ Example:
template <class T> class A {
static const int i = 5;
int n1[i]; // i refers to a member of the current instantiation
int n2[A::i]; // A::i refers to a member of the current instantiation
int n3[A<T>::i]; // A<T>::i refers to a member of the current instantiation
int f();
};
template <class T> int A<T>::f() {
return i; // i refers to a member of the current instantiation
}— end example ]
A name is a dependent member of the current instantiation if it is a member of the current instantiation that, when looked up, refers to at least one member of a class that is the current instantiation.
A name is a member of an unknown specialization if it is
A qualified-id in which the nested-name-specifier names a dependent type that is not the current instantiation.
A qualified-id in which the nested-name-specifier refers to the current instantiation, the current instantiation has at least one dependent base class, and name lookup of the qualified-id does not find any member of a class that is the current instantiation or a non-dependent base class thereof.
An id-expression denoting the member in a class member access expression in which either
the type of the object expression is the current instantiation, the current instantiation has at least one dependent base class, and name lookup of the id-expression does not find a member of a class that is the current instantiation or a non-dependent base class thereof; or
the type of the object expression is dependent and is not the current instantiation.
If a qualified-id in which the nested-name-specifier refers to the current instantiation is not a member of the current instantiation or a member of an unknown specialization, the program is ill-formed even if the template containing the qualified-id is not instantiated; no diagnostic required. Similarly, if the id-expression in a class member access expression for which the type of the object expression is the current instantiation does not refer to a member of the current instantiation or a member of an unknown specialization, the program is ill-formed even if the template containing the member access expression is not instantiated; no diagnostic required. [ Example:
template<class T> class A {
typedef int type;
void f() {
A<T>::type i; // OK: refers to a member of the current instantiation
typename A<T>::other j; // error: neither a member of the current instantiation nor
// a member of an unknown specialization
}
};— end example ]
If, for a given set of template arguments, a specialization of a template is instantiated that refers to a member of the current instantiation with a qualified-id or class member access expression, the name in the qualified-id or class member access expression is looked up in the template instantiation context. If the result of this lookup differs from the result of name lookup in the template definition context, name lookup is ambiguous. [ Example:
struct A {
int m;
};
struct B {
int m;
};
template<typename T>
struct C : A, T {
int f() { return this->m; } // finds A::m in the template definition context
int g() { return m; } // finds A::m in the template definition context
};
template int C<B>::f(); // error: finds both A::m and B::m
template int C<B>::g(); // OK: transformation to class member access syntax
// does not occur in the template definition context; see [class.mfct.non-static]
— end example ]
A type is dependent if it is
a template parameter,
a member of an unknown specialization,
a nested class or enumeration that is a dependent member of the current instantiation,
a cv-qualified type where the cv-unqualified type is dependent,
a compound type constructed from any dependent type,
an array type whose element type is dependent or whose bound (if any) is value-dependent,
a function type whose exception specification is value-dependent,
a simple-template-id in which either the template name is a template parameter or any of the template arguments is a dependent type or an expression that is type-dependent or value-dependent or is a pack expansion [ Note: This includes an injected-class-name of a class template used without a template-argument-list. — end note ] , or
denoted by decltype(expression), where expression is type-dependent.
Except as described below, an expression is type-dependent if any subexpression is type-dependent.
this is type-dependent if the class type of the enclosing member function is dependent ([temp.dep.type]).
An id-expression is type-dependent if it contains
an identifier associated by name lookup with one or more declarations declared with a dependent type,
an identifier associated by name lookup with a non-type template-parameter declared with a type that contains a placeholder type,
an identifier associated by name lookup with one or more declarations of member functions of the current instantiation declared with a return type that contains a placeholder type,
an identifier associated by name lookup with a structured binding declaration whose brace-or-equal-initializer is type-dependent,
the identifier __func__, where any enclosing function is a template, a member of a class template, or a generic lambda,
a template-id that is dependent,
a conversion-function-id that specifies a dependent type, or
a nested-name-specifier or a qualified-id that names a member of an unknown specialization;
or if it names a dependent member of the current instantiation that is a static data member of type “array of unknown bound of T” for some T ([temp.static]). Expressions of the following forms are type-dependent only if the type specified by the type-id, simple-type-specifier or new-type-id is dependent, even if any subexpression is type-dependent:
simple-type-specifier ( expression-listopt ) ::opt new new-placementopt new-type-id new-initializeropt ::opt new new-placementopt ( type-id ) new-initializeropt dynamic_cast < type-id > ( expression ) static_cast < type-id > ( expression ) const_cast < type-id > ( expression ) reinterpret_cast < type-id > ( expression ) ( type-id ) cast-expression
Expressions of the following forms are never type-dependent (because the type of the expression cannot be dependent):
literal postfix-expression . pseudo-destructor-name postfix-expression -> pseudo-destructor-name sizeof unary-expression sizeof ( type-id ) sizeof ... ( identifier ) alignof ( type-id ) typeid ( expression ) typeid ( type-id ) ::opt delete cast-expression ::opt delete [ ] cast-expression throw assignment-expressionopt noexcept ( expression )
[ Note: For the standard library macro offsetof, see [support.types]. — end note ]
A class member access expression is type-dependent if the expression refers to a member of the current instantiation and the type of the referenced member is dependent, or the class member access expression refers to a member of an unknown specialization. [ Note: In an expression of the form x.y or xp->y the type of the expression is usually the type of the member y of the class of x (or the class pointed to by xp). However, if x or xp refers to a dependent type that is not the current instantiation, the type of y is always dependent. If x or xp refers to a non-dependent type or refers to the current instantiation, the type of y is the type of the class member access expression. — end note ]
Except as described below, an expression used in a context where a constant expression is required is value-dependent if any subexpression is value-dependent.
An id-expression is value-dependent if:
it is type-dependent,
it is the name of a non-type template parameter,
it names a static data member that is a dependent member of the current instantiation and is not initialized in a member-declarator,
it names a static member function that is a dependent member of the current instantiation, or
it is a constant with literal type and is initialized with an expression that is value-dependent.
Expressions of the following form are value-dependent if the unary-expression or expression is type-dependent or the type-id is dependent:
sizeof unary-expression sizeof ( type-id ) typeid ( expression ) typeid ( type-id ) alignof ( type-id ) noexcept ( expression )
[ Note: For the standard library macro offsetof, see [support.types]. — end note ]
Expressions of the following form are value-dependent if either the type-id or simple-type-specifier is dependent or the expression or cast-expression is value-dependent:
simple-type-specifier ( expression-listopt ) static_cast < type-id > ( expression ) const_cast < type-id > ( expression ) reinterpret_cast < type-id > ( expression ) ( type-id ) cast-expression
An expression of the form &qualified-id where the qualified-id names a dependent member of the current instantiation is value-dependent. An expression of the form &cast-expression is also value-dependent if evaluating cast-expression as a core constant expression succeeds and the result of the evaluation refers to a templated entity that is an object with static or thread storage duration or a member function.
A non-type template-argument is dependent if its type is dependent or the constant expression it specifies is value-dependent.
Furthermore, a non-type template-argument is dependent if the corresponding non-type template-parameter is of reference or pointer type and the template-argument designates or points to a member of the current instantiation or a member of a dependent type.
A template template-argument is dependent if it names a template-parameter or is a qualified-id that refers to a member of an unknown specialization.