CODEWITHSUNDEEP
c++
Chethan Ravindranath
Chethan Ravindranath

Reputation: 2061

Static_cast refuses to cast pointers to a class even with user defined conversion operator defined

I'm learning about c++ typecasting. I have this piece of code

class Y 
{
private:
    int m_y;    
};

class X
{
public:
    operator Y() { return Y(); }
private:
    int m_x;    
};

int _tmain(int argc, _TCHAR* argv[])
{
    X x;
    Y y;
    y = static_cast<Y>(x); // calls the user defined conversion operator

    Y* yPtr;
    yPtr = static_cast<Y*>(&x); // error C2440: 'static_cast' : cannot convert from 'X *' to 'Y *'.
                                // Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
}

Why doesn't C++ return a pointer to an object of Y converted from the object x using the user defined conversion operator?

Upvotes: 2

Views: 537

Answers (2)

eerorika
eerorika

Reputation: 238461

statc_casting the object works because you create a temporary of the object using the converting constructor.

But when you static_cast a pointer, you're making a temporary copy of the pointer. Pointers don't have converting constructors.

You cannot static_cast unrelated pointers to another. Not even if they have converting constructors.

Even if it were allowed, consider what would happen if it worked like static_casting the object: A temporary would be created using the converting constructor, but instead of assigning the temporary to a variable, you'd be assigning the address of the temporary to a pointer. But since the temporary does not exist after the expression, the pointer would immediately be pointing to invalid memory.

Upvotes: 1

TartanLlama
TartanLlama

Reputation: 65770

Although an object of a given type can be converted to another type, you can't necessarily cast their pointers.

Converting an X to a Y in this example says "take this X and make me a Y from it". An X is not a Y, but it can be converted to one.

Converting an X* to a Y* says "this is a pointer to X and now I'm saying it's a pointer to Y". That doesn't make sense, because an X is not a Y, so you can't treat it like one until you convert the object.

Upvotes: 1

Related Questions