reinterpret_cast to the same type

Question

Consider following program:

struct A{};

int main()
{
    A a;
    A b = a;
    A c = reinterpret_cast<A>(a);
}

The compiler(g++14) throws an error about invalid cast from type 'A' to type 'A'. Why is casting to the same type invalid?

Answer 1

It is not allowed, because the standard says so.

There is a rather limited set of allowed conversion that you can do with reinterpret_cast. See eg cppreference. For example the first point listed there is:

1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. The resulting value is the same as the value of expression. (since C++11)

However, casting a custom type (no pointer!) to itself is not on the list. Why would you do that anyhow?

Answer 2

Because reinterpret_cast cannot be used for classes and structures, it should be used to reinterpret pointers, references and integral types. It is very well explained in cpp reference

So, in your case one possible valid expression would be reinterpret_cast<A*>(&a)