在 std::set 的定义中将 const_iterator 转换为 iterator&

Question

C++ STL uses a red-black tree to store data inside std::set and std::map . I noticed that set::iterator is actually a typedef of the const iterator of the red black tree:

//All code snippets taken from SGI STL. https://www.sgi.com/tech/stl/

typedef _Rb_tree<key_type, value_type, _Identity<value_type>, key_compare, _Alloc> _Rep_type;
typedef typename _Rep_type::const_iterator iterator;

This is reasonable because users are supposed not to modify the content of the set through an iterator. But set has to implement operations like insert and erase , which calls for a non-const iterator of the red-black tree. SGI STL uses a c-style cast to do this:

void erase(iterator __position) { 
  typedef typename _Rep_type::iterator _Rep_iterator;
  _M_t.erase((_Rep_iterator&)__position); 
}

I'm wondering:

1. Why is this cast safe? It is casting _Rep_type::const_iterator to _Rep_type::iterator& .
2. How to write the cast in C++ style? I've tried to do it : Neither static_cast nor const_cast will do the job. reinterpret_cast can compile, but I'm not sure if it does the same thing as the C-style cast.

Answer 1

iterator and _Rep_type::iterator are instances of the same class template, the former using const qualified types and the latter using the same, but non- const types. Something like this:

template <class T, class U>
struct B {};

using S = B<int&, int*>;
using Sconst = B<const int&, const int*>;

So for your questions:

1. It is safe because the two types have the exact same memory layout.
2. You cannot use static_cast because the compiler considers the types to be unrelated. You have to bring in the heavy artillery, reinterpret_cast :

int test() {
    S s;
    Sconst& sconst = reinterpret_cast<Sconst&>(s);
}

Answer 2

A C-Cast has no direct mapping to one specific C++ cast.

But a C-Cast can be mapped to one or more C++ casts.

When doing a C-Cast you can think of the compiler trying this set of C++ casts in order. The first one that is valid is what the C++ compiler will do: (see also C++ Standard , In N4917 7.6.3 Explicit type conversion (cast notation) [expr.cast])

1. const_cast
2. static_cast
3. static_cast followed by const_cast
4. reinterpret_cast
5. reinterpret_castfollowed by const_cast

So looking at your example, it looks like option 3 or 5 (but would need to try it with a compiler to be sure).

 (_Rep_iterator&)__position

 // Maps to:

 const_cast<_Rep_iterator&>(static_cast<_Rep_iterator const&>(__position))

 // or

 const_cast<_Rep_iterator&>(reinterpret_cast<_Rep_iterator const&>(__position))