[basic.def]

[Note 1:

A declaration can also have effects including:

(1.1)
a static assertion ([dcl.pre]),
(1.2)
controlling template instantiation ([temp.explicit]),
(1.3)
guiding template argument deduction for constructors ([temp.deduct.guide]),
(1.4)
use of attributes, and
(1.5)
nothing (in the case of an empty-declaration).

— end note]

Each entity declared by a declaration is also defined by that declaration unless:

(2.1)
it declares a function without specifying the function's body ([dcl.fct.def]),
(2.2)
it contains the extern specifier ([dcl.stc]) or a linkage-specification12 ([dcl.link]) and neither an initializer nor a function-body,
(2.3)
it declares a non-inline static data member in a class definition ([class.mem], [class.static]),
(2.4)
it declares a static data member outside a class definition and the variable was defined within the class with the constexpr specifier ([class.static.data]) (this usage is deprecated; see [depr.static.constexpr]),
(2.5)
it is an elaborated-type-specifier ([class.name]),
(2.6)
it is an opaque-enum-declaration ([dcl.enum]),
(2.7)
it is a template-parameter ([temp.param]),
(2.8)
it is a parameter-declaration ([dcl.fct]) in a function declarator that is not the declarator of a function-definition,
(2.9)
it is a typedef declaration ([dcl.typedef]),
(2.10)
it is an alias-declaration ([dcl.typedef]),
(2.11)
it is a namespace-alias-definition ([namespace.alias]),
(2.12)
it is a using-declaration ([namespace.udecl]),
(2.13)
it is a deduction-guide ([temp.deduct.guide]),
(2.14)
it is a static_assert-declaration ([dcl.pre]),
(2.15)
it is a consteval-block-declaration,
(2.16)
it is an attribute-declaration ([dcl.pre]),
(2.17)
it is an empty-declaration ([dcl.pre]),
(2.18)
it is a using-directive ([namespace.udir]),
(2.19)
it is a using-enum-declaration ([enum.udecl]),
(2.20)
it is a template-declaration ([temp.pre]) whose template-head is not followed by either a concept-definition or a declaration that defines a function, a class, a variable, or a static data member,
(2.21)
it is an explicit instantiation declaration ([temp.explicit]), or
(2.22)
it is an explicit specialization whose declaration is not a definition.

A declaration is said to be a definition of each entity that it defines.

[Example 1:

All but one of the following are definitions: int a; / defines a extern const int c = 1; / defines c int f(int x) { return x+a; } / defines f and defines x struct S { int a; int b; }; / defines S, S::a, and S::b struct X { / defines X int x; / defines non-static data member x static int y; / declares static data member y X(): x(0) { } / defines a constructor of X }; int X::y = 1; / defines X::y enum { up, down }; / defines up and down namespace N { int d; } / defines N and N::d X anX; / defines anX whereas these are just declarations: extern int a; / declares a extern const int c; / declares c int f(int); / declares f struct S; / declares S typedef int Int; / declares Int namespace N1 = N; / declares N1 extern X anotherX; / declares anotherX using N::d; / declares d

— end example]

[Note 2:

In some circumstances, C++ implementations implicitly define the default constructor ([class.default.ctor]), copy constructor, move constructor ([class.copy.ctor]), copy assignment operator, move assignment operator ([class.copy.assign]), or destructor member functions.

— end note]

[Example 2:

Given #include <string> struct C { std::string s; / std::string is the standard library class ([string.classes]) }; int main() { C a; C b = a; b = a; } the implementation will implicitly define functions to make the definition of C equivalent to struct C { std::string s; C() : s() { } C(const C& x): s(x.s) { } C(C&& x): s(static_cast<std::string&&>(x.s)) { } / : s(std::move(x.s)) { } C& operator=(const C& x) { s = x.s; return *this; } C& operator=(C&& x) { s = static_cast<std::string&&>(x.s); return *this; } / { s = std::move(x.s); return *this; } ~C() { } };

— end example]

[Note 3:

A class name can also be implicitly declared by an elaborated-type-specifier ([dcl.type.elab]).

— end note]

In the definition of an object, the type of that object shall not be an incomplete type ([basic.types.general]), an abstract class type ([class.abstract]), or a (possibly multidimensional) array thereof.

12)12)

Appearing inside the brace-enclosed declaration-seq in a linkage-specification does not affect whether a declaration is a definition.