Beware C++ Map's Lookup Operator

Imagine you are walking in the park on a beautiful day and suddenly … !

map:operator[] - Lookup or Insert

This is one of those quirks of the language C++:

map<int, ValueType> my_map;
if (my_map[42])
    // do something when key exists
else
    // do something when key does not exist

The above works counter intuitively since C++ creates a value object for the key 42 even when nothing is assigned since the object is created with a default constructor. So in, effect the predicate always evaluates to true even when clearly there is no such key in the map.

The right way to check for membership in map is to use map::find() or map::at() So the above code looks like:

if (my_map.find(42) != my_map.end())
    // do something when key exists
else
    // do something when key does not exist

Passing a key that’s not already mapped to map::at() throws an exception.

This might seem like a quirk of C++’s that needs fixing, but it is the same feature that allows the more intuitive functionality to exist in C++ in the first place:

map<int, ValueType> my_map;
my_map[42] = my_val_obj;

I’m making this a separate post as a note to self.

PS: Look up Item 24 on Scott Meyers’ Effective STL book for more details.