cppdraft_translate/cppdraft/dcl/constexpr.md

[dcl.constexpr]

# 9 Declarations [[dcl]](./#dcl)

## 9.2 Specifiers [[dcl.spec]](dcl.spec#dcl.constexpr)

### 9.2.6 The constexpr and consteval specifiers [dcl.constexpr]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L887)

The constexpr specifier shall be applied only to
the definition of a variable or variable template,
a structured binding declaration, or
the declaration of a function or function template[.](#1.sentence-1)

The consteval specifier shall be applied only to
the declaration of a function or function template[.](#1.sentence-2)

A function or static data member
declared with the constexpr or consteval specifier
on its first declaration
is implicitly an inline function or variable ([[dcl.inline]](dcl.inline "9.2.8 The inline specifier"))[.](#1.sentence-3)

If any declaration of a function or function template has
a constexpr or consteval specifier,
then all its declarations shall contain the same specifier[.](#1.sentence-4)

[*Note [1](#note-1)*:

An explicit specialization can differ from the template declaration
with respect to the constexpr or consteval specifier[.](#1.sentence-5)

— *end note*]

[*Note [2](#note-2)*:

Function parameters cannot be declared constexpr[.](#1.sentence-6)

— *end note*]

[*Example [1](#example-1)*: constexpr void square(int &x); // OK, declarationconstexpr int bufsz = 1024; // OK, definitionconstexpr struct pixel { // error: pixel is a typeint 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]](expr.const "7.7 Constant expressions")) so constexpr not satisfiedconstexpr void square(int &x) { // OK, definition x *= x;}constexpr pixel large(4); // OK, square definedint next(constexpr int x) { // error: not for parametersreturn x + 1;}extern constexpr int memsz; // error: not a definition — *end example*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L934)

A constexpr or consteval specifier
used in the declaration of a function
declares that function to be
a [*constexpr function*](#def:specifier,constexpr,function "9.2.6 The constexpr and consteval specifiers [dcl.constexpr]")[.](#2.sentence-1)

[*Note [3](#note-3)*:

A function or constructor declared with the consteval specifier
is an immediate function ([[expr.const]](expr.const "7.7 Constant expressions"))[.](#2.sentence-2)

— *end note*]

A destructor, an allocation function, or a deallocation function
shall not be declared with the consteval specifier[.](#2.sentence-3)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L946)

A function is [*constexpr-suitable*](#def:constexpr-suitable "9.2.6 The constexpr and consteval specifiers [dcl.constexpr]") if
it is not a coroutine ([[dcl.fct.def.coroutine]](dcl.fct.def.coroutine "9.6.4 Coroutine definitions"))[.](#3.sentence-1)

Except for instantiated constexpr functions,
non-templated constexpr functions shall be constexpr-suitable[.](#3.sentence-2)

[*Example [2](#example-2)*: constexpr int square(int x){ return x * x; } // OKconstexpr long long_max(){ return 2147483647; } // OKconstexpr int abs(int x) {if (x < 0) x = -x; return x; // OK}constexpr int constant_non_42(int n) { // OKif (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; } // OKconstexpr int g(int x, int n) { // OKint r = 1; while (--n > 0) r *= x; return r;} — *end example*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L987)

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

- [(4.1)](#4.1)

an invocation of a constexpr function
can appear in a constant expression ([[expr.const]](expr.const "7.7 Constant expressions")) and

- [(4.2)](#4.2)

copy elision is not performed in a constant expression ([[class.copy.elision]](class.copy.elision "11.9.6 Copy/move elision"))[.](#4.sentence-1)

[*Note [4](#note-4)*:

Declaring a function constexpr can change whether an expression
is a constant expression[.](#4.sentence-2)

This can indirectly cause calls to std​::​is_constant_evaluated within an invocation of the function to produce a different value[.](#4.sentence-3)

— *end note*]

[*Note [5](#note-5)*:

It is possible to write a constexpr function for which
no invocation satisfies the requirements of a core constant expression[.](#4.sentence-4)

— *end note*]

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L1010)

The constexpr and consteval specifiers have no
effect on the type of a constexpr function[.](#5.sentence-1)

[*Example [3](#example-3)*: 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*]

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L1023)

A constexpr specifier used in an object declaration
declares the object as const[.](#6.sentence-1)

Such an object
shall have literal type and
shall be initialized[.](#6.sentence-2)

A constexpr variable shall be constant-initializable ([[expr.const]](expr.const "7.7 Constant expressions"))[.](#6.sentence-3)

A constexpr variable that is an object,
as well as any temporary to which a constexpr reference is bound,
shall have constant destruction[.](#6.sentence-4)

[*Example [4](#example-4)*: struct pixel {int x, y;};constexpr pixel ur = { 1294, 1024 }; // OKconstexpr pixel origin; // error: initializer missingnamespace N {void f() {int x; constexpr int& ar = x; // OKstatic constexpr int& sr = x; // error: x is not constexpr-representable// at the point indicated below}// immediate scope here is that of N} — *end example*]