declaring a const instance of a class

Your class is a POD (essentially because it doesn’t provide a default constructor). POD variables are not initialized upon declaration. That is, this:

foo x;

does not initialize x to a meaningful value. This has to be done separately. Now, when you declare it as const, this may never happen because you cannot assign to or change x any more.

Consider the equivalence to int:

int x; // legal
const int y; // illegal

As you have noticed, using std::string instead of foo compiles. That’s because std::string is not a POD. A simple solution to your dilemma is to provide a default constructor for foo:

class foo {
public:
    foo() { }
};

Now your const foo x; code compiles.