Is there a way to initialise a new struct variable that does not involve writing a constructor?

Question

I think that I vaguely recall that one of the newer c++ standards (maybe its c++11, or maybe 14?...17??) allows you to initialise a struct, whereby you can define a struct and then initialise it without having to write a constructor.

E.g.:

struct test
{
    int a;
    int b;
    std::string str;
};

int main()
{
    std::map<int, test> test_map;
    test_map[0] = test(1, 2, "test1"); // This is the line in question
    // Or it might be more like: test_map[0] = test{1, 2, "test1"};
    return 0;
}

I can't recall the name of this special initialisation (or if it even exists)!. So my questions are:

• Is there some new mechanism to achieve this without writing a constructor in the struct "test"?
• If so, what is it called (so I can read more about it).

If this "feature" does not exist then please put me out of my misery!, it could be that my imagination has made this up...

Answer 1

The "initialize without a constructor" is called aggregate initialization, it has always been a part of C++ since day 1. Unfortunately, some might say.

In C++98 you could write:

std::map<int, test> test_map;
test temp = { 1, 2, "test1" };
test_map[0] = temp;

C++11 added that you can use aggregate initialization in prvalues, so you do not need to declare the intermediate variable (and there are no extra copies):

std::map<int, test> test_map;
test_map[0] = { 1, 2, "test1" };

std::map<int, test> m2 = { {0, {1, 2, "test2"}} };    // and this