Quel est l'effet de l'externe C dans C+

97

extern "C" void foo(int); extern "C" {    void g(char);    int i; } 

89

#ifdef __cplusplus extern "C" { #endif  // all of your legacy C code here  #ifdef __cplusplus } #endif 

extern "C" { #include "legacy_C_header.h" } 

72

void f() {} void g();  extern "C" {     void ef() {}     void eg(); }  /* Prevent g and eg from being optimized away. */ void h() { g(); eg(); } 

g++ -c -std=c++11 -Wall -Wextra -pedantic -o main.o main.cpp readelf -s main.o 

     8: 0000000000000000     7 FUNC    GLOBAL DEFAULT    1 _Z1fv      9: 0000000000000007     7 FUNC    GLOBAL DEFAULT    1 ef     10: 000000000000000e    17 FUNC    GLOBAL DEFAULT    1 _Z1hv     11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _GLOBAL_OFFSET_TABLE_     12: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _Z1gv     13: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND eg 

$ c++filt _Z1fv f() $ c++filt _Z1hv h() $ c++filt _Z1gv g() 

extern "C" {     // Overloading.     // error: declaration of C function ‘void f(int)’ conflicts with     void f();     void f(int i);      // Templates.     // error: template with C linkage     template <class C> void f(C i) { } } 

#include <cassert>  #include "c.h"  int main() {     assert(f() == 1); } 

#ifndef C_H #define C_H  /* This ifdef allows the header to be used from both C and C++   * because C does not know what this extern "C" thing is. */ #ifdef __cplusplus extern "C" { #endif int f(); #ifdef __cplusplus } #endif  #endif 

#include "c.h"  int f(void) { return 1; } 

g++ -c -o main.o -std=c++98 main.cpp gcc -c -o c.o -std=c89 c.c g++ -o main.out main.o c.o ./main.out 

main.cpp:6: undefined reference to `f()' 

#include <assert.h>  #include "cpp.h"  int main(void) {     assert(f_int(1) == 2);     assert(f_float(1.0) == 3);     return 0; } 

#ifndef CPP_H #define CPP_H  #ifdef __cplusplus // C cannot see these overloaded prototypes, or else it would get confused. int f(int i); int f(float i); extern "C" { #endif int f_int(int i); int f_float(float i); #ifdef __cplusplus } #endif  #endif 

#include "cpp.h"  int f(int i) {     return i + 1; }  int f(float i) {     return i + 2; }  int f_int(int i) {     return f(i); }  int f_float(float i) {     return f(i); } 

gcc -c -o main.o -std=c89 -Wextra main.c g++ -c -o cpp.o -std=c++98 cpp.cpp g++ -o main.out main.o cpp.o ./main.out 

main.c:6: undefined reference to `f_int' main.c:7: undefined reference to `f_float' 

60

#include <stdio.h>      // Two functions are defined with the same name //   but have different parameters  void printMe(int a) {   printf("int: %i\n", a); }  void printMe(char a) {   printf("char: %c\n", a); }      int main() {   printMe('a');   printMe(1);   return 0; }