Should you forward declare classes that are part of a library?

[Gringoliath]

Is it ok to declare classes that are part of a library that your using or should you only forward declare classes that you made?

For example:

namespace std
{
    class system_clock; // Part of the C++ standard library.
}


namespace sf
{
    class RenderWindow; // Part of the SFML library.
}

[mbozzi]

Is it ok to declare classes that are part of a library that your using or should you only forward declare libraries that you made?

Generally, you should only forward-declare names defined in your own code. This is because forward declarations often rely on implementation details that you shouldn't depend on.

Specifically, your first example is forbidden; the second is merely a bad idea.

In general, the documented interface of a function or function may not match the implementation - a thing that's documented as a function might be a class; a thing that looks like a class might actually be a class template. There might be default arguments involved, or candidates that you don't know about. Any reliance on these details immediately makes your code non-portable between different implementations of the same library.

The first case is wrong for two reasons:
1. it is undefined behavior to add to namespace std, except template specializations when certain specific conditions are met.
2. std::string is not a class, but an alias of a template specialization, whose implementation you can't depend on.

I have no idea whether the second one is correct, but it's surely bad practice. Just include the file you need.

[Gringoliath]

Thanks for the help!

[Peter87]

The biggest advantage of using forward declarations is to avoid circular dependencies, something that is not a problem when other libraries.