Definitions and ODR

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

C++ language

 

Basic Concepts

language keywords

phases of translation

comments

the main() function

names and identifiers

types

arithmetic types

objects

scope

object lifetime

storage duration and linkage

definitions and ODR

name lookup

memory model

 

Definitions are declarations that fully define the entity introduced by the declaration. Every declaration is a definition, except for the following:

1) Any declaration with an extern storage class specifier or with a language linkage specifier (such as extern "C") without an initializer

extern const int a; // declares, but doesn't define a
extern const int b = 1; // defines b

2) Function declaration without a function body

int f(int); // declares, but doesn't define f

3) Parameter declaration in a function declaration that isn't a definition

int f(int x); // declares, but doesn't define f and x
int f(int x) { // defines f and x
     return x+a;
}

4) Declaration of a static data member inside a class definition

struct S {    // defines S
    int n;        // defines S::n
    static int i; // declares, but doesn't define S::i
};
int S::i; // defines S::i

5) Declaration of a class name (by forward declaration or by the use of the elaborated type specifier in another declaration)

struct S; // declares, but doesn't define S
class Y f(class T p); // declares, but doesn't define Y and T (and also f and p)

6) Declaration of a template parameter

template<typename T> // declares, but doesn't define T

7) typedef declaration

typedef S S2; // declares, but doesn't define S2 (S may be incomplete)

• (C++11) alias-declaration

using S2 = S; // declares, but doesn't define S2 (S may be incomplete)

• (C++11) Opaque declaration of an enumeration.

enum Color : int; // declares, but doesn't define Color

• using-declaration

using N::d; // declares, but doesn't define d

• using-directive (does not define any entities)
• (C++11) static_assert declaration (does not define any entities)
• (C++11) Attribute declaration (does not define any entities)
• Empty declaration (does not define any entities)

Although asm declaration does not define any entities as well, it is a definition.

Where necessary, the compiler may implicitly define the default constructor, copy constructor, move constructor, copy assignment operator, move assignment operator, and the destructor.

If the definition of any object results in an object of incomplete type, the program is ill-formed.

One Definition Rule

Only one definition of any variable, function, class type, enumeration type, or template is allowed in any one translation unit.