How to pass std::map as a default constructor parameter

I haven't been able to figure this out. It's easy to create two ctors but I wanted to learn if there's an easy way to do this.

How can one pass a std::map as the default parameter to a ctor, e.g.

Foo::Foo( int arg1, int arg2, const std::map<std::string, std::string> = VAL)

I've tried 0, null, and NULL as VAL, none of the work because they are all of type int, g++ complains. What is the correct default to use here?

Or is this kind of thing not a good idea?