Why does std::for_each on a map call the copy constructor?
c++, c++11, c++14, dictionary, lambda
Solution
The problem is that `std::map` has a `std::pair<const Key, Value>` as its internal value type. Rather than explicitly specifying this, Standard Library containers allow you to extract this from the container type:
In C++11 do (same as in C++98, but you would have to use a function object rather than a lambda inside `for_each`, and also use `typedef` instead of `using =`):
using value_type = std::map<int, A>::value_type;
std::for_each(begin(m), end(m), [](value_type const& a) { /* do nothing */ });
In C++14 do:
std::for_each(begin(m), end(m), [](auto const& a) { /* do nothing */ });
The use of `auto` inside the lambda is supported by Clang 3.4, Visual Studio 2013 November CTP, and GCC 4.9.
Problem
I have the following simple example, in which I want to call `std::for_each` on a collection of objects that are non-copyable: ``` class A { public: A() : x(0) {} A(const A&) = delete; private: int x; }; void func() { std::vector<A> v(10); std::map<int, A> m; // works as expected std::for_each(begin(v), end(v), [](const A& a) { /* do nothing */ }); // error calling copy constructor std::for_each(begin(m), end(m), [](const std::pair<int, A>& a) { /* do nothing */ }); } ``` If I put everything into a `std::vector`, it works as I expected, but when using a `std::map`, suddenly `std::for_each` wants to call the (deleted) copy constructor. Why? I would have assumed that I simply get a reference to the pair that is saved in the map, without any necessary copies.