How do I save a lambda for later callback?

Question

How do I fix the code below to store a lambda so I can invoke it at a later time?

The error I currently get is field 'm_callback' has incomplete type.

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

        template< typename FuncT > 
        void setCallback( FuncT&& callback )
        {
            m_callback = callback;
        }

    private:
        auto m_callback;   // this line is broken
};

int main(int argc, char** argv)
{
    Foo foo;

    foo.setCallback( [] (int x){ return true; } );

    return 0;
}

Answer 1

The auto keyword can't be used liked that. I recommend using something like this:

#include <functional>
std::function<bool (int)> m_callback;

This is done from Visual Studio 2010.

Answer 2

The auto keyword can only be used in conjunction with an initalization expression.

So... this works:

auto callback = [](int x){ return x == 0; };

... but this doesn't:

auto callback;
callback = [](int x){ return x == 0; };

I would recommend that you use something like function with a specific signature to represent a callback.