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?

Answer 1

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.