Question

I’m storing an std::map of std::shared_ptr<V> as value.

From one functions, I want to return a reference to this map, but with std::shared_ptr<const V> as value type, is that possible ?

Here is what I want to achieve:

#include <map>
#include <memory>

struct A {
    std::map<int, std::shared_ptr<int>> map;

    const std::map<int, std::shared_ptr<const int>>& ret1() const {
        return map;
    }
};

But this code fails to compile:

error: invalid initialization of reference of type 
'const std::map<int, std::shared_ptr<const int> >&' 
from expression of type 'const std::map<int, std::shared_ptr<int> >'

Thanks

Answer 1

A type T and a type const T are related but ultimately different types. If you don’t want the caller of ret1 to modify the values in your map, then either return a copy of the map, or accept a reference to a map as argument and copy the keys and values into that.