使用命名空间隐藏内部类实现

Question

I am developing a library and a would like to provide my users a public interface separate from the real implementation that is hidden in a namespace. This way, I could change only the class HiddenQueue without changing myQueue that will be exposed to users only.

If I put the C++ code of HiddenQueue in the myQueue.cpp file the compiler complains saying _innerQueue has incomplete type. I thought that the linker was able to resolve this. What I am doing wrong here?

// myQueue.h
namespace inner{
    class HiddenQueue;
};

class myQueue{

public:
    myQueue();
);

private:

    inner::HiddenQueue _innerQueue;

};

///////////////////////////

// myQueue.cpp
namespace inner{
    class HiddenQueue{};
};

Answer 1

The compiler needs to know the exact memory layout of an object by looking at the header file it's defined in.

Your code says that class MyQueue has a member of type InnerQueue , which will be part of the memory layout of MyQueue objects. Therefore, to deduce the memory layout of MyQueue it needs to know the memory layout of InnerQueue . Which it does not, because you say "oh well, it's defined elsewhere".

What you are trying to do is closely related to "the PIMPL idiom "/"compiler firewall" technique.

To solve the problem, you must either include HiddenQueue.h in your header or declare _innerqueue as a pointer:

class myQueue {

public:
    myQueue();

private:

    inner::HiddenQueue* _pinnerQueue;
};

Using a pointer is possible because a pointer has a known memory size (dependent on your target architecture), therefore the compiler doesn't need to see the full declaration of HiddenQueue .

Answer 2

To be able to make a member of a class, you need to have a definition for it and not only a declaration. (A declaration is enough for a pointer or a reference to the class).

Answer 3

You need to provide pointer to _innetQueue rather then the object itself:

std::auto_ptr<inner::HiddenQueue> _innerQueue;

Search form pimpl ideom or d-pointer