Search code examples
c++reinterpret-cast

Why is converting from int to char* different from converting from std::string to char*?

I'm learning C++ and reading Andrei Alexandrescu's book on generic programming. He presents a templated class that can be used to convert between types:

template <class To, class From>
To safe_reinterpret_cast(From from)
{
    assert(sizeof(From) <= sizeof(To));
    return reinterpret_cast<To>(from);
}

This works fine for:

int i = 5;
char* p = safe_reinterpret_cast<char*>(i);

but fails for

std::string a("apple");
char* pp = safe_reinterpret_cast<char*>(a);

This is the error failure at compile time:

invalid cast from type 'std::basic_string<char>' to type 'char*'

Why does this cast fail?

Solution

  • Andrei Alexandrescu's infamous example only works for plain-old-data types.

    It does not work for pointers. The behaviour of casting unrelated pointer types is undefined.

    You can only reinterpret_cast to void*, and reinterpret_cast from void* back to the original pointer type.