如何使用带指针的转换构造函数？

Question

I have a class C that can be converted to class A and a function that takes an A* as an argument. I want to call it with a C* , but I can't seem to get a conversion constructor to work. I get: error: cannot convert 'C*' to 'A*' for argument '1' to 'void doSomething(A*)' . What am I doing wrong?

class C {
};

class A {
public:
    A(C* obj) {}
};

void doSomething(A* object);

int main()
{
    C* object = new C();
    doSomething(object);
}

Answer 1

Conversion constructors can only be defined for user defined types , in your case A . However, they do not apply to fundamental types as pointers like A* .

If doSomething was taking an A const& instead (or simply an A ) , then the conversion constructor would be invoked as you expect.

Answer 2

If you main requirement is to be able to call the existing doSomething function, then you can do this:

int main()
{
    C* object = new C();
    A a(object);
    doSomething(&a);
    // May need to delete object here -- depends on ownership semantics.
}

Answer 3

You probably mean that you want C to be a subclass of A:

class C : public A {
...
};