7 Expressions [expr]

7.7 Constant evaluation [expr.const]

7.7.2 Core constant expressions [expr.const.core]

1
#
An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following:
  • (1.1)
    this ([expr.prim.this]), except
  • (1.2)
    a control flow that passes through a declaration of a block variable ([basic.scope.block]) with static ([basic.stc.static]) or thread ([basic.stc.thread]) storage duration, unless that variable is usable in constant expressions;
    [Example 1: constexpr char test() { static const int x = 5; static constexpr char c[] = "Hello World"; return *(c + x); } static_assert(' ' == test()); — end example]
  • (1.3)
    an invocation of a non-constexpr function;63
  • (1.4)
    an invocation of an undefined constexpr function;
  • (1.5)
    an invocation of an instantiated constexpr function that is not constexpr-suitable;
  • (1.6)
    an invocation of a virtual function ([class.virtual]) for an object whose dynamic type is constexpr-unknown;
  • (1.7)
    an expression that would exceed the implementation-defined limits (see [implimits]);
  • (1.8)
    an operation that would have undefined or erroneous behavior as specified in [intro] through [cpp];64
  • (1.9)
    an lvalue-to-rvalue conversion unless it is applied to
    • (1.9.1)
      a glvalue of type cv std​::​nullptr_t,
    • (1.9.2)
      a non-volatile glvalue that refers to an object that is usable in constant expressions, or
    • (1.9.3)
      a non-volatile glvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;
  • (1.10)
    an lvalue-to-rvalue conversion that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;
  • (1.11)
    an lvalue-to-rvalue conversion that is applied to an object with an indeterminate value;
  • (1.12)
    an invocation of an implicitly-defined copy/move constructor or copy/move assignment operator for a union whose active member (if any) is mutable, unless the lifetime of the union object began within the evaluation of E;
  • (1.13)
    in a lambda-expression, a reference to this or to a variable with automatic storage duration defined outside that lambda-expression, where the reference would be an odr-use ([basic.def.odr], [expr.prim.lambda]);
    [Example 2: void g() { const int n = 0; [=] { constexpr int i = n; // OK, n is not odr-used here constexpr int j = *&n; // error: &n would be an odr-use of n }; } — end example]
    [Note 1: 
    If the odr-use occurs in an invocation of a function call operator of a closure type, it no longer refers to this or to an enclosing variable with automatic storage duration due to the transformation ([expr.prim.lambda.capture]) of the id-expression into an access of the corresponding data member.
    [Example 3: auto monad = [](auto v) { return [=] { return v; }; }; auto bind = [](auto m) { return [=](auto fvm) { return fvm(m()); }; }; // OK to capture objects with automatic storage duration created during constant expression evaluation. static_assert(bind(monad(2))(monad)() == monad(2)()); — end example]
    — end note]
  • (1.14)
    a conversion from a prvalue P of type “pointer to cv void” to a type “cv1 pointer to T”, where T is not cv2 void, unless P is a null pointer value or points to an object whose type is similar to T;
  • (1.15)
    a reinterpret_cast ([expr.reinterpret.cast]);
  • (1.16)
    pointer arithmetic ([expr.add]) where one (possibly converted) operand points to the first element of an array of unknown bound and the other (possibly converted) operand is of integral type with non-zero value;
  • (1.17)
    a modification of an object ([expr.assign], [expr.post.incr], [expr.pre.incr]) unless it is applied to a non-volatile lvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;
  • (1.18)
    an invocation of a destructor ([class.dtor]) or a function call whose postfix-expression names a pseudo-destructor ([expr.call]), in either case for an object whose lifetime did not begin within the evaluation of E;
  • (1.19)
    a new-expression ([expr.new]), unless either
    • (1.19.1)
      the selected allocation function is a replaceable global allocation function ([new.delete.single], [new.delete.array]) and the allocated storage is deallocated within the evaluation of E, or
    • (1.19.2)
      the selected allocation function is a non-allocating form ([new.delete.placement]) with an allocated type T, where
      • (1.19.2.1)
        the placement argument to the new-expression points to an object whose type is similar to T ([conv.qual]) or, if T is an array type, to the first element of an object of a type similar to T, and
      • (1.19.2.2)
        the placement argument points to storage whose duration began within the evaluation of E;
  • (1.20)
    a delete-expression ([expr.delete]), unless it deallocates a region of storage allocated within the evaluation of E;
  • (1.21)
    a call to an instance of std​::​allocator<T>​::​allocate ([allocator.members]), unless the allocated storage is deallocated within the evaluation of E;
  • (1.22)
    a call to an instance of std​::​allocator<T>​::​deallocate ([allocator.members]), unless it deallocates a region of storage allocated within the evaluation of E;
  • (1.23)
    a construction of an exception object, unless the exception object and all of its implicit copies created by invocations of std​::​current_exception or std​::​rethrow_exception ([propagation]) are destroyed within the evaluation of E;
  • (1.24)
    a throw-expression ([expr.throw]) with no operand, unless there is a currently handled exception whose exception object was constructed within the evaluation of E;
  • (1.25)
    an await-expression ([expr.await]);
  • (1.26)
    a yield-expression ([expr.yield]);
  • (1.27)
    a three-way comparison ([expr.spaceship]), relational ([expr.rel]), or equality ([expr.eq]) operator where the result is unspecified;
  • (1.28)
    a dynamic_cast ([expr.dynamic.cast]) or typeid ([expr.typeid]) expression on a glvalue that refers to an object whose dynamic type is constexpr-unknown;
  • (1.29)
    a dynamic_cast ([expr.dynamic.cast]) expression, typeid ([expr.typeid]) expression, or new-expression ([expr.new]) that would throw an exception where no definition of the exception type is reachable;
  • (1.30)
    an expression that would produce an injected declaration (see below), unless E is the corresponding expression of a consteval-block-declaration ([dcl.pre]);
  • (1.31)
    an asm-declaration ([dcl.asm]);
  • (1.32)
    an invocation of the va_arg macro ([cstdarg.syn]);
  • (1.33)
    a non-constant library call ([defns.nonconst.libcall]); or
  • (1.34)
    a goto statement ([stmt.goto]).
    [Note 2: 
    A goto statement introduced by equivalence ([stmt]) is not in scope.
    For example, a while statement ([stmt.while]) can be executed during constant evaluation.
    — end note]
2
#
It is implementation-defined whether E is a core constant expression if E satisfies the constraints of a core constant expression, but evaluation of E has runtime-undefined behavior.
3
#
It is unspecified whether E is a core constant expression if E satisfies the constraints of a core constant expression, but evaluation of E would evaluate
4
#
[Example 4: int x; // not constant struct A { constexpr A(bool b) : m(b?42:x) { } int m; }; constexpr int v = A(true).m; // OK, constructor call initializes m with the value 42 constexpr int w = A(false).m; // error: initializer for m is x, which is non-constant constexpr int f1(int k) { constexpr int x = k; // error: x is not initialized by a constant expression // because lifetime of k began outside the initializer of x return x; } constexpr int f2(int k) { int x = k; // OK, not required to be a constant expression // because x is not constexpr return x; } constexpr int incr(int &n) { return ++n; } constexpr int g(int k) { constexpr int x = incr(k); // error: incr(k) is not a core constant expression // because lifetime of k began outside the expression incr(k) return x; } constexpr int h(int k) { int x = incr(k); // OK, incr(k) is not required to be a core constant expression return x; } constexpr int y = h(1); // OK, initializes y with the value 2 // h(1) is a core constant expression because // the lifetime of k begins inside h(1) — end example]
5
#
For the purposes of determining whether an expression E is a core constant expression, the evaluation of the body of a member function of std​::​allocator<T> as defined in [allocator.members], is ignored.
6
#
For the purposes of determining whether E is a core constant expression, the evaluation of a call to a trivial copy/move constructor or copy/move assignment operator of a union is considered to copy/move the active member of the union, if any.
[Note 3: 
The copy/move of the active member is trivial.
— end note]
7
#
For the purposes of determining whether E is a core constant expression, the evaluation of an id-expression that names a structured binding v ([dcl.struct.bind]) has the following semantics:
  • (7.1)
    If v is an lvalue referring to the object bound to an invented reference r, the behavior is as if r were nominated.
  • (7.2)
    Otherwise, if v names an array element or class member, the behavior is that of evaluating e[i] or e.m, respectively, where e is the name of the variable initialized from the initializer of the structured binding declaration, and i is the index of the element referred to by v or m is the name of the member referred to by v, respectively.
[Example 5: #include <tuple> struct S { mutable int m; constexpr S(int m): m(m) {} virtual int g() const; }; void f(std::tuple<S&> t) { auto [r] = t; static_assert(r.g() >= 0); // error: dynamic type is constexpr-unknown constexpr auto [m] = S(1); static_assert(m == 1); // error: lvalue-to-rvalue conversion on mutable // subobject e.m, where e is a constexpr object of type S using A = int[2]; constexpr auto [v0, v1] = A{2, 3}; static_assert(v0 + v1 == 5); // OK, equivalent to e[0] + e[1] where e is a constexpr array } — end example]
8
#
During the evaluation of an expression E as a core constant expression, all id-expressions, splice-expressions, and uses of *this that refer to an object or reference whose lifetime did not begin within the evaluation of E are treated as referring to a specific instance of that object or reference whose lifetime and that of all subobjects (including all union members) includes the entire constant evaluation.
For such an object that is not usable in constant expressions, the dynamic type of the object is constexpr-unknown.
For such a reference that is not usable in constant expressions, the reference is treated as binding to an unspecified object of the referenced type whose lifetime and that of all subobjects includes the entire constant evaluation and whose dynamic type is constexpr-unknown.
[Example 6: template <typename T, size_t N> constexpr size_t array_size(T (&)[N]) { return N; } void use_array(int const (&gold_medal_mel)[2]) { constexpr auto gold = array_size(gold_medal_mel); // OK } constexpr auto olympic_mile() { const int ledecky = 1500; return []{ return ledecky; }; } static_assert(olympic_mile()() == 1500); // OK struct Swim { constexpr int phelps() { return 28; } virtual constexpr int lochte() { return 12; } int coughlin = 12; }; constexpr int how_many(Swim& swam) { Swim* p = &swam; return (p + 1 - 1)->phelps(); } void splash(Swim& swam) { static_assert(swam.phelps() == 28); // OK static_assert((&swam)->phelps() == 28); // OK Swim* pswam = &swam; static_assert(pswam->phelps() == 28); // error: lvalue-to-rvalue conversion on a pointer // not usable in constant expressions static_assert(how_many(swam) == 28); // OK static_assert(Swim().lochte() == 12); // OK static_assert(swam.lochte() == 12); // error: invoking virtual function on reference // with constexpr-unknown dynamic type static_assert(swam.coughlin == 12); // error: lvalue-to-rvalue conversion on an object // not usable in constant expressions } extern Swim dc; extern Swim& trident; constexpr auto& sandeno = typeid(dc); // OK, can only be typeid(Swim) constexpr auto& gallagher = typeid(trident); // error: constexpr-unknown dynamic type — end example]
63)63)
Overload resolution ([over.match]) is applied as usual.
64)64)
This includes, for example, signed integer overflow ([expr.pre]), certain pointer arithmetic ([expr.add]), division by zero ([expr.mul]), or certain shift operations ([expr.shift]).