How to forbid the use of the default constructor in C++?

While the other answers are true, there is a new technique in C++11 for expressing that : deleted default constructor

It allows forbidding a function without depending on any other trick. It also clearly express your intention in the code.

class X
{ 
public:
    X(int) {}
    X() = delete; // will never be generated
};

int main()
{ 
    X x; // will not compile;
    X y(1); // will compile.
    return 0;
}