c++ - Constructing a std::map from initializer_list error

Question

I'm trying to make a class constructor that will take an initializer list and init a map with it like this:

class Test {
    std::map<int, int> m_ints;
public:
    Test(std::initializer_list<std::pair<int, int>> init):
        m_ints(init)
    {}
};

But that results in a very long error message which I frankly don't understand. What do I need to change to make this work?

Answer 1

Declare the template argument of the std::initializer_list as having type std::pair<const int, int>

Here is a demonstrative program

#include <iostream>
#include <map>
#include <initializer_list>

class Test {
    std::map<int, int> m_ints;
public:
    Test(std::initializer_list<std::pair<const int, int>> init):
        m_ints(init)
    {}
};

int main()
{
    Test t = { { 1, 2 }, { 2, 3 } };

    return 0;
}

The corresponding constructor is declared the following way

map( initializer_list<value_type>,
     const Compare& = Compare(),
     const Allocator& = Allocator());

and value_type is defined like

typedef pair<const Key, T> value_type;

Thus you could define the constructor of your class also the following way

Test( std::initializer_list<std::map<int, int>::value_type> init ) :
      m_ints(init)
{}