Forward declare an STL container?

Question

Is it possible to forward declare an STL container in a header file? For example, take the following code:

#include <vector>

class Foo
{
private:
    std::vector<int> container_;
    ...
};

I want to be able to do something like this:

namespace std
{
    template <typename T> class vector;
}

class Foo
{
private:
    std::vector<int> container_;
    ...
};

Can this be done?

Answer 1

I don't think so because the compiler would have no way of knowing how much space to allocate for the container_ object. At best you could do:

std::vector<int> *container_;

and new it in the constructor, since the compiler knows the size of a pointer.

Answer 2

Declaring vector in the std namespace is undefined behavior. So, your code might work, but it also might not, and the compiler is under no obligation to tell you when your attempt won't work. That's a gamble, and I don't know that avoiding the inclusion of a standard C++ header is worth that.

See the following comp.std.c++.moderated discussion:

forward declaring std::vector. Works, but is it legal and standard compliant?

Answer 3

Apart from what the others said (except Adam Rosenfield), you may find it useful to know that there is a sanctioned way of forward-declaring iostreams and some related templates: http://msdn.microsoft.com/en-us/library/1af12yty%28VS.80%29.aspx . It would be cool if the standard had more such headers.