Definitions and ODR (One Definition Rule)

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

• A function declaration without a function body:

int f(int); // declares, but does not define f

• Any declaration with an extern storage class specifier or with a language linkage specifier (such as extern "C"

extern const int a;     // declares, but does not define a
extern const int b = 1; // defines b

• Declaration of a non-inline(since C++17) static data member

struct S
{
    int n;               // defines S::n
    static int i;        // declares, but does not define S::i
    inline static int x; // defines S::x
};                       // defines S
 
int S::i;                // defines S::i

struct S
{
    static constexpr int x = 42; // implicitly inline, defines S::x
};
 
constexpr int S::x; // declares S::x, not a redefinition

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

struct S;             // declares, but does not define S
 
class Y f(class T p); // declares, but does not define Y and T (and also f and p)

enum Color : int; // declares, but does not define Color

• Declaration of a template parameter:

template<typename T> // declares, but does not define T

• A parameter declaration in a function declaration that isn't a definition:

int f(int x); // declares, but does not define f and x
 
int f(int x)  // defines f and x
{
    return x + a;
}

• A typedef declaration:

typedef S S2; // declares, but does not define S2 (S may be incomplete)

using S2 = S; // declares, but does not define S2 (S may be incomplete)

• A using-declaration:

using N::d; // declares, but does not define d

• An empty declaration (does not define any entities)
• A using-directive (does not define any entities)

extern template
f<int, char>; // declares, but does not define f<int, char>

• An explicit specialization whose declaration is not a definition:

template<>
struct A<int>; // declares, but does not define A<int>

An asm declaration does not define any entities, but it is classified as 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 or abstract class type

One Definition Rule

Only one definition of any variable, function, class type, enumeration type , concept (since C++20)

One and only one definition of every non-inline function or variable that is odr-used

For an inline functionor inline variable(since C++17), a definition is required in every translation unit where it is odr-used

For a class, a definition is required wherever the class is used in a way that requires it to be complete.

There can be more than one definition in a program of each of the following: class type, enumeration type, inline function, inline variable(since C++17), templated entity (template or member of template, but not full template specialization

• Each definition appears in a different translation unit.

• Each definition consists of the same sequence of tokens (typically, appears in the same header).
• Name lookup from within each definition finds the same entities (after overload resolution), except that:

• Constants with internal or no linkage may refer to different objects as long as they are not odr-used and have the same values in every definition.

• Overloaded operators, including conversion, allocation, and deallocation functions refer to the same function from each definition (unless referring to one defined within the definition).
• Corresponding entities have the same language linkage in each definition (e.g. the include file is not inside an extern "C" block).
• If a const object is constant-initialized in any of the definitions, it is constant-initialized in each definition.
• The rules above apply to every default argument used in each definition.
• If the definition is for a class with an implicitly-declared constructor, every translation unit where it is odr-used must call the same constructor for the base and members.

• If the definition is for a template, then all these requirements apply to both names at the point of definition and dependent names at the point of instantiation.

If all these requirements are satisfied, the program behaves as if there is only one definition in the entire program. Otherwise, the program is ill-formed, no diagnostic required.

Note: in C, there is no program-wide ODR for types, and even extern declarations of the same variable in different translation units may have different types as long as they are compatible struct S { int x; }; and the other .cpp file defines struct S { int y; }; , the behavior of the program that links them together is undefined. This is usually resolved with unnamed namespaces

Naming an entity

A variable is named by an expression if the expression is an identifier expression that denotes it.

A function is named by an expression or conversion in following cases:

• A function whose name appears as an expression or conversion (including named function, overloaded operator, user-defined conversion, user-defined placement forms of operator new
• An allocation or deallocation function for a class is named by a new expression
• A deallocation function for a class is named by a delete expression appearing in an expression.
• A constructor selected to copy or move an object is considered to be named by the expression or conversion even if copy elision takes place. Using a prvalue in some contexts does not copy or move an object, see mandatory elision. (since C++17)

A potentially evaluated expression or conversion odr-uses a function if it names it.

Potential results

The set of potential results of an expression E is a (possibly empty) set of identifier expressions that appear within E

• If E is an identifier expression, the expression E
• If E is a subscript expression (E1[E2]
• If E is a class member access expression of the form E1.E2 or E1.template E2 naming a non-static data member, the potential results of E1
• If E is a class member access expression naming a static data member, the identifier expression designating the data member is included in the set.
• If E is a pointer-to-member access expression of the form E1.*E2 or E1.*template E2 whose second operand is a constant expression, the potential results of E1
• If E is an expression in parentheses ((E1)), the potential results of E1
• If E is a glvalue conditional expression (E1 ? E2 : E3, where E2 and E3 are glvalues), the union of the potential results of E2 and E3
• If E is a comma expression (E1, E2), the potential results of E2
• Otherwise, the set is empty.

ODR-use (informal definition)

An object is odr-used if its value is read (unless it is a compile time constant) or written, its address is taken, or a reference is bound to it,

A reference is odr-used if it is used and its referent is not known at compile time,

A function is odr-used if a function call to it is made or its address is taken.

If an entity is odr-used, its definition must exist somewhere in the program; a violation of that is usually a link-time error.

struct S
{
    static const int x = 0; // static data member
    // a definition outside of class is required if it is odr-used
};
 
const int& f(const int& r);
 
int n = b ? (1, S::x) // S::x is not odr-used here
          : f(S::x);  // S::x is odr-used here: a definition is required

ODR-use (formal definition)

A variable x that is named by a potentially-evaluated expression expr that appears at a point P is odr-used by expr

• x is a reference that is usable in constant expressions at P
• x is not a reference and (until C++26) expr is an element of the set of potential results of an expression E, and any of the following conditions is satisfied:
  • E is a discarded-value expression
  • x is anon-volatile(since C++26) object that is usable in constant expressions at P

• E has non-volatile-qualified non-class type, and the lvalue-to-rvalue conversion is applied to it.

struct S { static const int x = 1; }; // applying lvalue-to-rvalue conversion
                                      // to S::x yields a constant expression
 
int f()
{
    S::x;        // discarded-value expression does not odr-use S::x
 
    return S::x; // expression where lvalue-to-rvalue conversion
                 // applies does not odr-use S::x
}

*this is odr-used if this appears as a potentially-evaluated expression (including the implicit this

A function is odr-used in following cases:

• A function is odr-used if it is named by (see below) a potentially-evaluated expression or conversion.
• A virtual member function is odr-used if it is not a pure virtual member function (addresses of virtual member functions are required to construct the vtable).
• A non-placement allocation or deallocation function for a class is odr-used by the definition of a constructor of that class.
• A non-placement deallocation function for a class is odr-used by the definition of the destructor of that class, or by being selected by the lookup at the point of definition of a virtual destructor.
• An assignment operator in a class T that is a member or base of another class U is odr-used by an implicitly-defined copy-assignment or move-assignment functions of U.
• A constructor (including default constructors) for a class is odr-used by the initialization that selects it.
• A destructor for a class is odr-used if it is potentially invoked.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

References