问题
Take this files:
a.h
#ifndef A_H
#define A_H
char EL[] = "el";
#endif
a.cpp
#include "a.h"
b.h
#ifndef B_H
#define B_H
#include "a.h"
#endif
b.cpp
#include "b.h"
main.cpp
#include "b.h"
#include "a.h"
int main() { }
This is only an example, but I've really this problem:
g++ -c a.cpp
g++ -c b.cpp
g++ -c main.cpp
g++ -o main main.o a.o b.o
a.o:(.data+0x0): multiple definition of `EL'
main.o:(.data+0x0): first defined here
b.o:(.data+0x0): multiple definition of `EL'
main.o:(.data+0x0): first defined here
collect2: ld returned 1 exit status
why and how to solve?
回答1:
Include guards don't protect you against defining an object multiple times if you include the definition in multiple translation units!
As a solution, never define things in headers, but only declare them:
// header
extern char EL[2];
// TU
#include "header.h"
char EL[2] = "el";
// Other consumer
#include "header.h";
// can now use EL
(There are exceptions, of course; e.g. class definitions are fine (but class member function definitions are not (but inlined ones are)) -- beware.)
I should add that alternatively you can say static in your header file to make the definition private to each TU:
// header
static char EL[] = "EL"; // every TU gets a copy
(In C++0x you cannot use objects of static linkage as template parameters, though.)
来源：https://stackoverflow.com/questions/6803918/code-guards-fail