CODEWITHSUNDEEP
c++callbackglobal-variablesglobal-object
user1478046
user1478046

Reputation:

How to pass/make an object/variable accessible inside a callback function which is called by a third party library?

How to make globalObject accessible by callbackFunction?

I am using a third party library library.h which has a method libraryFunction. The output of libraryFunction is a libraryFunctionOutput which is passed into the callback function callbackFunction.

How to pass another object (e.g. globalObject) for use inside the callback function callbackFunction?

Unfortunately the third party library is compiled so I cannot make any changes to it.

#include <stdio.h>
#include "library.h"

int callbackFunction(int libraryFunctionOutput):
    printf("%s, %d", libraryFunctionOutput, globalObject);
    return 1;

int main(int argc, char* argv[])
{
   int globalObject = 0;
   libraryFunction(callbackFunction);
}

In the documentation, the function is shown as:

int __stdcall libraryFunction(const char*    filename,
                              unsigned int   flags,
                              int            userID,
                              MathrelCallback callbackFunction);

The MathrelCallback struct is defined as the following:

struct MathrelCallback {MathrelHeader header;
                        MathrelInfo   data;
                        unsigned int Type;
};

Upvotes: 0

Views: 77

Answers (1)

Horstling
Horstling

Reputation: 2151

If the library really takes a function pointer (not a generalized Callable like std::function) and does not offer the ability to pass a context or user pointer to your callback, you have to make your global object global (with all its drawbacks):

#include <stdio.h>
#include "library.h"

static int globalObject = 0;

int callbackFunction(int libraryFunctionOutput)
{
    printf("%s, %d", libraryFunctionOutput, globalObject);
    return 1;
}

int main(int argc, char* argv[])
{
   libraryFunction(callbackFunction);
}

Upvotes: 1

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