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
map<int, ValueType> my_map; if (my_map) // 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
42 even when nothing is assigned since the object is created with a
default constructor. So in, effect the predicate always evaluates to
even when clearly there is no such key in the map.
The right way to check for membership in map is to use
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
map<int, ValueType> my_map; my_map = 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
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