Question

I’ve noticed things like what follows in C++.

SomeClass obj = SomeClass();
int boo = obj["foo"];

What is this called and how can I do it?

example

class Boo {
public:
     int GetValue (string item) {
          switch (item) {
               case "foo" : return 1;
               case "apple" : return 2;
          }
          return 0;
     }
}

Boo boo = Boo();
int foo = boo.GetValue("foo");
// instead of that I want to be able to do
int foo = boo["foo"];

Answer 1

To use [], you’d overload operator[]:

class Boo {
public:
     int operator[](string const &item) {
          if (item == "foo")
              return 1;
          if (item == "apple")
              return 2;
          return 0;
     }
};

You might be interested to know std::map already provides pretty much what you seem to be looking for:

std::map<std::string, int> boo;

boo["foo"] = 1;
boo["apple"] = 2;

int foo = boo["foo"];

The obvious difference is that when/if you use this to look up a value that hasn’t previously been inserted, it’ll insert a new item with that key and the value 0.