class A {
    public:
        A( const char * name ){}
};

class X {
    public:
        X( A const&a ) {}
};

class Y {
    public:
        Y( A &a ) {}
};

A a( "1" );
X x1( a ); // Works

X x2( A( "2" ) ); // Works

Y y3( A( "3" ) ); // Error!

Car car( Person( "Mike" ), Person( "Sandra" ) );