Screenplay: Why Not To Use operator[]

Basic Operation (Looks Fine)

  • Insert two elements

    code/map-index-operator.cpp
    #include <map>
#include <iostream>

int main()
{
    std::map<int, std::string> my_map;

    my_map[1] = "one";
    my_map[2] = "two";

    std::cout << ':' << my_map[1] << ":\n";
    std::cout << ':' << my_map[2] << ":\n";

    return 0;
}

  • Fine: they are in

    $ ./cxx-stl-map-index-operator
:one:
:two:

And Elements That Have Never Been Inserted?

  • Now lets access an element that is not contained

    code/map-index-operator-access-notexist.cpp
    #include <map>
#include <iostream>

int main()
{
    std::map<int, std::string> my_map;
    std::cout << ':' << my_map[666] << ":\n";
    return 0;
}

  • It is in, but (apparently) with a default value

    $ ./cxx-stl-map-index-operator-access-notexist
::

Know It: operator[] Is Not Made For Read Access

  • Cannot use operator[] on a const map

    code/map-index-operator-nonconst.cpp
    #include <map>
#include <iostream>

int main()
{
    const std::map<int, std::string> my_map;
    std::cout << ':' << my_map[666] << ":\n";
    return 0;
}

    map-index-operator-nonconst.cpp:7:35: error: passing ‘const std::map<int, std::__cxx11::basic_string<char> >’ as ‘this’ argument discards qualifiers [-fpermissive]
    7 |     std::cout << ':' << my_map[666] << ":\n";
      |                                   ^

  • If an element exists with key, a non-const reference to it is returned

  • If none exists, on is created ⟶ default constructed

  • ⟶ May be overwritten