6 Basics [basic]

6.2 Declarations and definitions [basic.def]

A declaration may introduce one or more names into a translation unit or redeclare names introduced by previous declarations.

If so, the declaration specifies the interpretation and semantic properties of these names.

A declaration may 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).

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 or a linkage-specification 19 ([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 (this usage is deprecated; see [depr.static.constexpr]),
(2.5)
it is introduced by an elaborated-type-specifier ([class.name]),
(2.6)
it is an opaque-enum-declaration,
(2.7)
it is a template-parameter,
(2.8)
it is a parameter-declaration in a function declarator that is not the declarator of a function-definition,
(2.9)
it is a typedef declaration,
(2.10)
it is an alias-declaration ([dcl.typedef]),
(2.11)
it is a using-declaration,
(2.12)
it is a deduction-guide,
(2.13)
it is a static_assert-declaration,
(2.14)
it is an attribute-declaration,
(2.15)
it is an empty-declaration,
(2.16)
it is a using-directive,
(2.17)
it is a using-enum-declaration,
(2.18)
it is a template-declaration 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.
(2.19)
it is an explicit instantiation declaration ([temp.explicit]), or
(2.20)
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

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
namespace N1 = N;               // defines N1
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
extern X anotherX;              // declares anotherX
using N::d;                     // declares d

— end example

]

[Note

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

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

A class name can also be implicitly declared by an elaborated-type-specifier .

— end note

]

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

19)

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

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