Const pointer to const reference C++

Question

How can I store const references or addresses to const someType& objects?

#include<iostream>
#include<list>

const int &foo( int& a) 
{
   return a;
}

int main()
{
    int a = 5;
    const int& p = foo(a);

    std::list<const int&> _list;
    _list.push_back(p);

    return 0;
}

When using std containers - the containers must be able to move the objects around. This means that you can not have containers of references. Const or otherwise. — UKMonkey, Oct 25 '17 at 09:41
the stored object should meet the requirements of `Assignable`. Since reference are initialized only once at declaration, you can have them in any containers . — P0W, Oct 25 '17 at 09:44
if you need reference-like objects in a container, `std::reference_wrapper` might be for you — sp2danny, Oct 25 '17 at 09:44
Possible duplicate of [STL containers with reference to objects](https://stackoverflow.com/questions/7707842/stl-containers-with-reference-to-objects) — underscore_d, Oct 25 '17 at 09:50
or https://stackoverflow.com/questions/922360/why-cant-i-make-a-vector-of-references or any of the many other threads you could have found if you had searched first. Also, nowhere does your example actually try to form a pointer to a reference - not that doing so is valid anyway. — underscore_d, Oct 25 '17 at 09:50

frogatto · Answer 1 · 2017-10-25T10:03:29.103

3

Your code doesn't compile. The problem is here:

std::list<const int&> _list;

When you declare a std::list<T>. T must meet the requirements of CopyAssignable and CopyConstructible. But neither a const nor a reference is assignable.

Apart from your problem, solutions suggesting std::reference_wrapper are not good in my opinion. Since when an object gets deleted, its reference inside that list will dangle. The best idea would be using smart pointers.

edited Oct 25 '17 at 10:03

answered Oct 25 '17 at 09:47

frogatto

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Nice remark about using `std::reference_wrapper`, definitely something to keep in mind! – AVH Oct 25 '17 at 13:45

AVH · Answer 2 · 2017-10-25T13:47:16.560

As commented by UKMonkey you can not store constant references directly in the std containers. However, there's std::cref which can be stored in such containers and wraps a constant reference. Note that you need C++11 for it.

Edit: Do heed Hi I'm Frogatto's warning about using this though. Unless you have a very compelling reason to use std::cref, it's probably better to stick to the alternative he mentions (smart pointers).

Const pointer to const reference C++

2 Answers2