Game Development Reference
In-Depth Information
Chapter
16
STL's Associative Containers
All of the STL containers you have seen so far have stored individual data elements. In this chapter
you will be introduced to associative containers. These containers associate a key with the data
values that you are inserting. In the case of the
map
and
unordered_map
containers, you are required
to supply a value to use as the key, whereas the
set
and
unordered_set
containers automatically
generate keys from the data you supply. I'll explain the differences between each of these containers
as I work through some examples in this chapter, beginningwith the
set
container.
The STL
set
Container
You won't be surprised to find out that a
set
is a template class and you are required to specialize
the
set
container to store the types you require. Listing 16-1 gives an example of how you can
specialize the
set
template.
Listing 16-1. Specializing
set
using namespace std;
using MySet = set<int>;
Once you have specialized a
set
template you can initialize a container in the same manner as other
STL containers. Listing 16-2 shows how you can create and add values to a
set
.
Listing 16-2. Initializing and Adding to
set
Containers
MySet mySet = { 2, 1, 0 };
mySet.emplace(4);
mySet.insert(3);
There are two important aspects of Listing 16-2 to which you should pay particular attention. The first is
the
emplace
method used. This method has the same result as the
insert
method in that the value will be
added to the
set
. However,
emplace
is actually faster at achieving this than
insert
in some circumstances.
It achieves this speed improvement by avoiding a copy construction when using a
set
with class types.
You should always use
emplace
rather than
insert
with all containers when it is available.
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