Question

Say I have a map

map<string, string> m;

And do a

string x = m["asdf"];

without knowing if “asdf” is in the map or not. Then I won’t be able to reliably figure out if it was in the map or not by looking at x. Is that correct?

So I should have done a m.first(asdf) first to figure this out.

Answer 1

The map::operator[] inserts the key in the map with a default value, if not there.

If you do not want that:

std::map<string, string>::iterator it = m.find("asdf");
//if you use C++11:
//auto it = m.find("asdf");
if (it == m.end())
    std::cout << "Not found" << std::endl;
else
    std::cout << "The value is " << *it << std::endl;

The map::operator[] is useful for many things, particularly with references. For example:

std::string &x = m["asdf"];
x += "X";

That will add an "X" to the (possibly newly created) string in the map.