9 Declarations [dcl.dcl]

9.11 Linkage specifications [dcl.link]

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All functions and variables whose names have external linkage and all function types have a language linkage.
[Note 1: 
Some of the properties associated with an entity with language linkage are specific to each implementation and are not described here.
For example, a particular language linkage might be associated with a particular form of representing names of objects and functions with external linkage, or with a particular calling convention, etc.
— end note]
The default language linkage of all function types, functions, and variables is C++ language linkage.
Two function types with different language linkages are distinct types even if they are otherwise identical.
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Linkage between C++ and non-C++ code fragments can be achieved using a linkage-specification:
The string-literal indicates the required language linkage.
This document specifies the semantics for the string-literals "C" and "C++".
Use of a string-literal other than "C" or "C++" is conditionally-supported, with implementation-defined semantics.
[Note 2: 
Therefore, a linkage-specification with a string-literal that is unknown to the implementation requires a diagnostic.
— end note]
Recommended practice: The spelling of the string-literal should be taken from the document defining that language.
For example, Ada (not ADA) and Fortran or FORTRAN, depending on the vintage.
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Every implementation shall provide for linkage to the C programming language, "C", and C++, "C++".
[Example 1: complex sqrt(complex); // C++ language linkage by default extern "C" { double sqrt(double); // C language linkage } — end example]
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A module-import-declaration appearing in a linkage specification with other than C++ language linkage is conditionally-supported with implementation-defined semantics.
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Linkage specifications nest.
When linkage specifications nest, the innermost one determines the language linkage.
[Note 3: 
A linkage specification does not establish a scope.
— end note]
A linkage-specification shall inhabit a namespace scope.
In a linkage-specification, the specified language linkage applies to the function types of all function declarators and to all functions and variables whose names have external linkage.
[Example 2: extern "C" // f1 and its function type have C language linkage; void f1(void(*pf)(int)); // pf is a pointer to a C function extern "C" typedef void FUNC(); FUNC f2; // f2 has C++ language linkage and // its type has C language linkage extern "C" FUNC f3; // f3 and its type have C language linkage void (*pf2)(FUNC*); // the variable pf2 has C++ language linkage; its type // is “pointer to C++ function that takes one parameter of type // pointer to C function'' extern "C" { static void f4(); // the name of the function f4 has internal linkage, // so f4 has no language linkage; its type has C language linkage } extern "C" void f5() { extern void f4(); // OK, name linkage (internal) and function type linkage (C language linkage) // obtained from previous declaration. } extern void f4(); // OK, name linkage (internal) and function type linkage (C language linkage) // obtained from previous declaration. void f6() { extern void f4(); // OK, name linkage (internal) and function type linkage (C language linkage) // obtained from previous declaration. } — end example]
A C language linkage is ignored in determining the language linkage of class members, friend functions with a trailing requires-clause, and the function type of non-static class member functions.
[Example 3: extern "C" typedef void FUNC_c(); class C { void mf1(FUNC_c*); // the function mf1 and its type have C++ language linkage; // the parameter has type “pointer to C function'' FUNC_c mf2; // the function mf2 and its type have C++ language linkage static FUNC_c* q; // the data member q has C++ language linkage; // its type is “pointer to C function'' }; extern "C" { class X { void mf(); // the function mf and its type have C++ language linkage void mf2(void(*)()); // the function mf2 has C++ language linkage; // the parameter has type “pointer to C function'' }; } — end example]
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If two declarations of an entity give it different language linkages, the program is ill-formed; no diagnostic is required if neither declaration is reachable from the other.
A redeclaration of an entity without a linkage specification inherits the language linkage of the entity and (if applicable) its type.
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Two declarations declare the same entity if they (re)introduce the same name, one declares a function or variable with C language linkage, and the other declares such an entity or declares a variable that belongs to the global scope.
[Example 4: int x; namespace A { extern "C" int f(); extern "C" int g() { return 1; } extern "C" int h(); extern "C" int x(); // error: same name as global-space object x } namespace B { extern "C" int f(); // A​::​f and B​::​f refer to the same function extern "C" int g() { return 1; } // error: the function g with C language linkage has two definitions } int A::f() { return 98; } // definition for the function f with C language linkage extern "C" int h() { return 97; } // definition for the function h with C language linkage // A​::​h and ​::​h refer to the same function — end example]
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A declaration directly contained in a linkage-specification is treated as if it contains the extern specifier ([dcl.stc]) for the purpose of determining the linkage of the declared name and whether it is a definition.
Such a declaration shall not specify a storage class.
[Example 5: extern "C" double f(); static double f(); // error extern "C" int i; // declaration extern "C" { int i; // definition } extern "C" static void g(); // error — end example]
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[Note 4: 
Because the language linkage is part of a function type, when indirecting through a pointer to C function, the function to which the resulting lvalue refers is considered a C function.
— end note]
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Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent.
Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved.