Prevent an object to be casted to any but one type C++

Question

I defined this class:

template <class T>
class pure
{
public:
    pure(T const& attr) {
        this->attr = attr;
    }

    ~pure() {}

    T& operator=(T const& attr) {
        return attr;
    }

    operator T() {
        return this->attr;
    }

private:
    T attr;
};

This is a class that stores a value of type T. When I want to use an instance of it, I get the attr (by casting pure to T) instead of the instance (of pure) itself.

My goal here is to make a class that can't be casted to any type other than the type of attr, which is T.

If you don't your object being cast from `pure` to `T` don't offer a conversion operator for it. — thecoshman, Apr 03 '15 at 12:15
No, I want that my pure cannot be casted to any type **other than T** — Marc-Antoine Jacob, Apr 03 '15 at 12:18
seven downvotes? could any of them give some kind of feedback? — sp2danny, Apr 03 '15 at 14:03

score 7 · Accepted Answer · answered Apr 03 '15 at 12:16

If you have a C++11 capable compiler you could add e.g. the following member function:

template<typename U>
operator U() = delete;

That function is a generic casting operator, and marked as deleted (which is a new feature in C++11). Because it's marked as deleted, then casting to any other type than T will lead to a compiler error.

If you don't have a C++11 capable compiler, then you can add basically the same function declaration as a private function, and you will get another error:

private:
    template<typename U>
    operator U();

I have C++ 11 and your method worked very well, thanks a lot again ! I will mark your post as an answer ! Very good an simple, I like this delete keyword. — Marc-Antoine Jacob, Apr 03 '15 at 12:22

Prevent an object to be casted to any but one type C++

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