6 Basic concepts [basic]

6.1 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 attributes of these names.
A declaration may also have effects including:
A declaration is a definition unless
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
In some circumstances, C++ implementations implicitly define the default constructor, copy constructor, move constructor, copy assignment operator, move assignment operator, or destructor member functions.
end note
#include <string>

struct C {
  std::string s;    // std​::​string is the standard library class (Clause [strings])

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
A class name can also be implicitly declared by an elaborated-type-specifier ([dcl.type.elab]).
end note
A program is ill-formed if the definition of any object gives the object an incomplete type.
Appearing inside the braced-enclosed declaration-seq in a linkage-specification does not affect whether a declaration is a definition.