Casting a const reference to a const pointer to a const

Question

I am building an STL list. I made a decorator class (MyList) that is a list of a special class (ProtectMe). I want all of the items of the list to be const. So here's what I made:

#include <list> using namespace std; class ProtectMe{ private: int data_; public: ProtectMe(int data):data_(data){ } int data() const{return data_;} }; class MyList{ private: //A list of constant pointers to constant ProtectMes. list<const ProtectMe* const> guts_; public: void add(const ProtectMe& data){ guts_.push_front(&data); } };

I get the following compile error:

 error: ‘const _Tp* __gnu_cxx::new_allocator::address(const _Tp&) const [with _Tp = const ProtectMe* const]’ cannot be overloaded

I'm still scratching my head trying to decode where I went wrong. Why doesn't this code compile? What should I change?

guts_.push_front(const &data); Why keyword const in brackets ? Why do you wish list elements to be of pointers type ? — Mahesh
– Mahesh, Commented Jul 6, 2012 at 20:04
@Mahesh: No good reason. Just trying to make it compile. It obviously didn't help. I'll remove it... — User1
– User1, Commented Jul 6, 2012 at 20:06
Why not just list<ProtectMe> are you deriving from ProtectMe somewhere else??? — mathematician1975
– mathematician1975, Commented Jul 6, 2012 at 20:12

Yakov Galka · Accepted Answer · 2012-07-06 20:31:00Z

The value_type of standard containers must be CopyInsertable (or MoveInsertable) in order for the push_front to work. The value type of list<const ProtectMe* const> is constant, so it's not CopyInsertable.

† CopyInsertable means that

allocator_traits<A>::construct(m, p, v);

is well defined where p is a pointer to value_type, which by default calls placement new on p and thus requires it to be a non-const pointer.

Interesting..So, how would I have a list of const objects? As the name implies, I want ProtectMe to be read-only. Imagine there is a non-const method in ProtectMe...
@User1: list<const ProtectMe*> is enough, you won't be able to modify the pointed ProtectMes.

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