CODEWITHSUNDEEP
c++cinitialization
Luke
Luke

Reputation: 2486

Using C++ Library within C program

I am trying to use some software which has been written in C++ in a separate C program.

I have compiled the necessary components within a .lib file, and included some C code using

#ifdef __cplusplus
    extern "C" {
#endif

This code sort of wraps up the C++ object method calls within C callable functions. The objects are declared in file scope, and pointers to the objects are retreived initially and then later used to reference which object to call the method on. For example:

const void *GetHandle() {
    return (void *) &obj;
}

And then to call a method:

int GetInt(const void *handle) {
    Object *temp = (Object *) handle;
    return temp->getInt();
}

Is this a valid solution to the problem? At the moment, with just the GetHandle() methods in my C program, I get errors saying "error C2099: initializer is not a constant" when I try to assign their return values to some const void * variables.

EDIT

Fixed the problem. My ignorance of standard C bears itself here. I am used to Microchip C30 for dsPIC, which is a little different.

I was trying to both declare and initialize the pointers outside of any function. I did have them set as const but that's not really necessary, so I now just declare outside and then initialize in an init function. I also put extern "C" in front of all the functions in the cpp file, and it all compiles now. So, thanks to everyone for the help!

Upvotes: 1

Views: 211

Answers (3)

Ivan Tolstosheyev
Ivan Tolstosheyev

Reputation: 121

Your code have an error: you implicitly casting const void* handle to non-const Object* . Your approach is right - you should define all your wrapper function as extern "C" to force C-style linking, and also you must not forget about exceptions - you should catch them in wrappers to prevent them go through C stack.

Upvotes: 1

Shahbaz
Shahbaz

Reputation: 47493

This is a valid solution.

You might want to check the constness of your argument/return values/casts more carefully though.

For example, if the object itself is not const, then GetHandle should return void *, not const void *. Same way, if getInt is a const function, then GetInt correctly gets a const void *. However, it should also cast it to const Object *, not Object *.

For further diagnosis, you should provide the exact line you get the initializer is not a constant error (and the context where the function is called). My guess is that the error is related to some other part of your code, using these incorrectly cast values.

Upvotes: 1

Mahmoud Fayez
Mahmoud Fayez

Reputation: 3459

If I were you I would use the COM object to wrap all the C++ functionality please check this code project demonstration:

http://www.codeproject.com/Articles/13601/COM-in-plain-C

If you are not familiar with COM objects I think you can read this article first http://www.codeproject.com/Articles/633/Introduction-to-COM-What-It-Is-and-How-to-Use-It

Upvotes: 1

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