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Structs.h doesn't work if .h is included in more than one .cpp?

I want to use this callbacks object in more than one .cpp, but VS gives "one or more detected multiplicable characters" if I include more than once.

#ifndef HEADER_H
#define HEADER_H

typedef struct {
void(__cdecl *callbackOne)(bool val);
void(__cdecl *callbackTwo)(bool val);
void(__cdecl *callbackThree)(bool val);
} Callbacks;
Callbacks callbacks;

#endif
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2 answers 


Callbacks callbacks;

This is the definition . Since it #include

works like a textual substitution, every source file that includes your header will have a (separate, different) instance of your structure.

Thus, when you merge the compiled object files, each one contains its own instance and corresponding symbol callbacks

, resulting in the described linker error.

In order to have one instance of your struct, you need to put the above definition in one source file.



To be able to use this instance from other source files, they must be able to reference it ("know its name"). What is the purpose of the ad ("give a name"):

extern Callbacks callbacks;

This is what you need to put in your title.

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Declare in header

extern Callbacks callbacks;

and define an object in some cpp file.



Callbacks callbacks;

Otherwise, you will have as many object definitions as there are cpp files that include this header.

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