Implicit conversion looses integer precision

ALL,

I'm trying to write a cross-platform code to compile on Windows, *nix and OSX. The first 2 compiles fine, but Xcode give following warning:

Implicit conversion looses integer precision "std::map<int, std::vector<Foo *>>::size_type (aka 'unsigned long') to std::__t::map<int, std::__t::vector<Foo *, std::__t::allocator<Foo *> >, std::__t::less<int>, std::__t::allocator<std::__t::pair<const int, std::__t::vector<Foo *, std::__t::allocator<Foo *> > > >::key_type (aka int)

Is it just a matter of writing an operator<() function in Foo?

TIA!!