9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.6 The constexpr and consteval specifiers [dcl.constexpr]

The constexpr specifier shall be applied only to the definition of a variable or variable template or the declaration of a function or function template.
The consteval specifier shall be applied only to the declaration of a function or function template.
A function or static data member declared with the constexpr or consteval specifier is implicitly an inline function or variable ([dcl.inline]).
If any declaration of a function or function template has a constexpr or consteval specifier, then all its declarations shall contain the same specifier.
[Note 1:
An explicit specialization can differ from the template declaration with respect to the constexpr or consteval specifier.
— end note]
[Note 2:
Function parameters cannot be declared constexpr.
— end note]
[Example 1: constexpr void square(int &x); // OK: declaration constexpr int bufsz = 1024; // OK: definition constexpr struct pixel { // error: pixel is a type int x; int y; constexpr pixel(int); // OK: declaration }; constexpr pixel::pixel(int a) : x(a), y(x) // OK: definition { square(x); } constexpr pixel small(2); // error: square not defined, so small(2) // not constant ([expr.const]) so constexpr not satisfied constexpr void square(int &x) { // OK: definition x *= x; } constexpr pixel large(4); // OK: square defined int next(constexpr int x) { // error: not for parameters return x + 1; } extern constexpr int memsz; // error: not a definition — end example]
A constexpr or consteval specifier used in the declaration of a function declares that function to be a constexpr function.
A function or constructor declared with the consteval specifier is called an immediate function.
A destructor, an allocation function, or a deallocation function shall not be declared with the consteval specifier.
The definition of a constexpr function shall satisfy the following requirements:
  • its return type (if any) shall be a literal type;
  • each of its parameter types shall be a literal type;
  • it shall not be a coroutine;
  • if the function is a constructor or destructor, its class shall not have any virtual base classes.
[Example 2: constexpr int square(int x) { return x * x; } // OK constexpr long long_max() { return 2147483647; } // OK constexpr int abs(int x) { if (x < 0) x = -x; return x; // OK } constexpr int constant_non_42(int n) { // OK if (n == 42) { static int value = n; return value; } return n; } constexpr int uninit() { struct { int a; } s; return s.a; // error: uninitialized read of s.a } constexpr int prev(int x) { return --x; } // OK constexpr int g(int x, int n) { // OK int r = 1; while (--n > 0) r *= x; return r; } — end example]
The definition of a constexpr constructor whose function-body is not = delete shall additionally satisfy the following requirements:
  • for a non-delegating constructor, every constructor selected to initialize non-static data members and base class subobjects shall be a constexpr constructor;
  • for a delegating constructor, the target constructor shall be a constexpr constructor.
[Example 3: struct Length { constexpr explicit Length(int i = 0) : val(i) { } private: int val; }; — end example]
The definition of a constexpr destructor whose function-body is not = delete shall additionally satisfy the following requirement:
  • for every subobject of class type or (possibly multi-dimensional) array thereof, that class type shall have a constexpr destructor.
For a constexpr function or constexpr constructor that is neither defaulted nor a template, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression, or, for a constructor, an evaluated subexpression of the initialization full-expression of some constant-initialized object ([basic.start.static]), the program is ill-formed, no diagnostic required.
[Example 4: constexpr int f(bool b) { return b ? throw 0 : 0; } // OK constexpr int f() { return f(true); } // ill-formed, no diagnostic required struct B { constexpr B(int x) : i(0) { } // x is unused int i; }; int global; struct D : B { constexpr D() : B(global) { } // ill-formed, no diagnostic required // lvalue-to-rvalue conversion on non-constant global }; constexpr int f(int x) { static int n = x; return n + x; // ill-formed, no diagnostic required // all calls reach the static variable declaration } — end example]
If the instantiated template specialization of a constexpr function template or member function of a class template would fail to satisfy the requirements for a constexpr function, that specialization is still a constexpr function, even though a call to such a function cannot appear in a constant expression.
If no specialization of the template would satisfy the requirements for a constexpr function when considered as a non-template function, the template is ill-formed, no diagnostic required.
An invocation of a constexpr function in a given context produces the same result as an invocation of an equivalent non-constexpr function in the same context in all respects except that
[Note 3:
Declaring a function constexpr can change whether an expression is a constant expression.
This can indirectly cause calls to std​::​is_­constant_­evaluated within an invocation of the function to produce a different value.
— end note]
The constexpr and consteval specifiers have no effect on the type of a constexpr function.
[Example 5: constexpr int bar(int x, int y) // OK { return x + y + x*y; } // ... int bar(int x, int y) // error: redefinition of bar { return x * 2 + 3 * y; } — end example]
A constexpr specifier used in an object declaration declares the object as const.
Such an object shall have literal type and shall be initialized.
In any constexpr variable declaration, the full-expression of the initialization shall be a constant expression ([expr.const]).
A constexpr variable shall have constant destruction.
[Example 6: struct pixel { int x, y; }; constexpr pixel ur = { 1294, 1024 }; // OK constexpr pixel origin; // error: initializer missing — end example]