How to pass std::map as a default constructor parameter

Question

I haven't been able to figure this out. It's easy to create two ctors but I wanted to learn if there's an easy way to do this.

How can one pass a std::map as the default parameter to a ctor, e.g.

Foo::Foo( int arg1, int arg2, const std::map<std::string, std::string> = VAL)

I've tried 0, null, and NULL as VAL, none of the work because they are all of type int, g++ complains. What is the correct default to use here?

Or is this kind of thing not a good idea?

Answer 1

Since C++11 you can use aggregate initialization:

void foo(std::map<std::string, std::string> myMap = {});

Example:

#include <iostream>
#include <map>
#include <string>

void foo(std::map<std::string, std::string> myMap = {})
{
    for(auto it = std::cbegin(myMap); it != std::cend(myMap); ++it)
        std::cout << it->first << " : " << it->second << '\n';
}

int main(int, char*[])
{
    const std::map<std::string, std::string> animalKids = {
        { "antelope", "calf" }, { "ant", "antling" },
        { "baboon", "infant" }, { "bear", "cub" },
        { "bee", "larva" }, { "cat", "kitten" }
    };

    foo();
    foo(animalKids);

    return 0;
}

You can play around with this example at Godbolt.