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. A function or static data member declared with the constexpr specifier is implicitly an inline function or variable ([dcl.inline]). If any declaration of a function or function template has a constexpr specifier, then all its declarations shall contain the constexpr specifier. [ Note: An explicit specialization can differ from the template declaration with respect to the constexpr specifier. — end note ] [ Note: Function parameters cannot be declared constexpr. — end note ] [ Example:
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 specifier used in the declaration of a function that is not a constructor declares that function to be a constexpr function. Similarly, a constexpr specifier used in a constructor declaration declares that constructor to be a constexpr constructor.
The definition of a constexpr function shall satisfy the following requirements:
it shall not be virtual ([class.virtual]);
for a defaulted copy/move assignment, the class of which it is a member shall not have a mutable subobject that is a variant member;
its return type shall be a literal type;
each of its parameter types shall be a literal type;
its function-body shall be = delete, = default, or a compound-statement that does not contain
an asm-definition,
a goto statement,
an identifier label ([stmt.label]),
a try-block, or
a definition of a variable of non-literal type or of static or thread storage duration or for which no initialization is performed.
[ Example:
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 first(int n) {
static int value = n; // error: variable has static storage duration
return value;
}
constexpr int uninit() {
int a; // error: variable is uninitialized
return 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 shall satisfy the following requirements:
the class shall not have any virtual base classes;
for a defaulted copy/move constructor, the class shall not have a mutable subobject that is a variant member;
each of the parameter types shall be a literal type;
its function-body shall not be a function-try-block.
In addition, either its function-body shall be = delete, or it shall satisfy the following requirements:
either its function-body shall be = default, or the compound-statement of its function-body shall satisfy the requirements for a function-body of a constexpr function;
every non-variant non-static data member and base class subobject shall be initialized ([class.base.init]);
if the class is a union having variant members ([class.union]), exactly one of them shall be initialized;
if the class is a union-like class, but is not a union, for each of its anonymous union members having variant members, exactly one of them shall be initialized;
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:
struct Length {
constexpr explicit Length(int i = 0) : val(i) { }
private:
int val;
};
— end example ]
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 ([expr.const]), or, for a constructor, a constant initializer for some object ([basic.start.static]), the program is ill-formed; no diagnostic required. [ Example:
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
};
— 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 or constexpr constructor, that specialization is still a constexpr function or constexpr constructor, 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 or constexpr constructor when considered as a non-template function or constructor, the template is ill-formed; no diagnostic required.
A call to a constexpr function produces the same result as a call to an equivalent non-constexpr function in all respects except that
a call to a constexpr function can appear in a constant expression ([expr.const]) and
copy elision is mandatory in a constant expression ([class.copy]).
The constexpr specifier has no effect on the type of a constexpr function or a constexpr constructor. [ Example:
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]). [ Example:
struct pixel {
int x, y;
};
constexpr pixel ur = { 1294, 1024 };// OK
constexpr pixel origin; // error: initializer missing
— end example ]