C ++从指针构造

Question

I am trying to make my project complied with GCC to link with library compiled with MSVC (that I have no sources for). The problem is that this library also exists for linux x86, x86_64 and arm platforms compiled with GCC (and my project is crossplatform) and I do not want to write a lot of platform-dependent (and compiler-dependent) code. Also other libraries linked to project are designed to be compiled by GCC (like ffmpeg and vlc) and I have no possibility to migrate to MSVC. For that I've written C-style wrapper library (based on original headers).

Constructors and destructors for classes are done this way:

__declspec(dllexport) void* myClass_myClass()
{
    void* ptr = new myClass();
    return ptr;
}
__declspec(dllexport) void myClass_unmyClass(void* ptr)
{
    delete((myClass*)ptr);
}

This part is compiled with MSVC and linked to original library. As you see (and I can see it in dependency walker) I have c-style exports that I can link with GCC. It allocates memory, creates object and acts as designed.

Now I am writing second part of this wrapper that will contain classes and will be compiled with GCC and linked to my project. It should be based on original headers of library (to make sure it acts the same as original with both linux- and windows- versions of this library). And I am in doubts how to make constructor and destructor of classes. Let's say we have

class MyClass
{
public:
    MyClass()
    ~MyClass()
}

in original header.

How it can be defined in .cpp to bypass call to my c-style "consturctor" and "destructor" (so they will bypass calls to original constructors and destructors)? It would be great to assign pointer to this in constructor, but it is forbidden.

PS I have no internals of classes. They all have

private: myClassImpl* m_pImpl;

so I can't use copy constructor. Even if I could - I do not want to construct 2 times the same object.

Answer 1

Your wrapper class should be a handle class which calls myClass_myClass() in the constructor, stores the returned pointer as a member, and calls myClass_unmyClass() in the destructor on the stored pointer. All member functions in the original class should be copied in the wrapper, but just call the original functions (via wrapper "C" functions) on the stored pointer. Be careful with copy-constructors and copy-assignment operators -- the defaults won't do the right thing. You must either delete them or implement them to call appropriate wrapper "C" functions.