You can mark a conversion operator as ‘explicit’, then it will allow the converted type to be constructed from your user-defined type (that defines the conversion operator), but will not allow implicit assignment (also not copy-construction via ‘=').

class Foo { public: operator int() { return 5; } }; class Bar { public: explicit operator int() { return 10; } }; int main(int argc, char * argv[]) { int x; Foo foo; x = foo; // OK std::cout << x << std::endl; Bar bar; x = bar; // Error, assigning to 'int' from incompatible type 'Bar' int y(bar); // OK, allowed by 'explicit' std::cout << y << std::endl; int z = bar; // Also not allowed

}