STL’s Associative Containers - Learn C++ for Game Development - page 163

Values in an unordered_map are accessed a little differently. You will have to use either the [] operator
or the at method. Both are shown in Listing 16-11.
Listing 16-11. Using [] and at
string& myString = myUnorderedMap[2];
cout << "Found : " << myString << endl;
myString = myUnorderedMap.at(2);
cout << "Found : "<< myString << endl;
These methods differ in a very significant way. The [] operator can be used to insert or retrieve
values from an unordered_map . If the key 2 had not existed in myUnorderedMap , a new string would
have been added to the map and its reference returned. If the key did already exist, the current string
would have been returned. The at method never adds a new element to our unordered_map , instead
it throws an exception. Listing 16-12 shows how you can update the at call to handle exceptions.
Listing 16-12. Handling Exceptions
try
{
string& myString = myUnorderedMap.at(5);
cout << "Found : " << myString << endl;
}
catch (const std::out_of_range& outOfRange)
{
cout << "Out of range error: " << outOfRange.what() << endl;
}
Your program would have crashed with an unhandled exception error if you had called this code
without the try...catch block. Throwing your own exceptions is as easy as using the throw
keyword, as you can see in Listing 16-13.
Listing 16-13. Throwing Exceptions
void ExceptionExample()
{
if (true)
{
throw -1;
}
}
Catching an exception from this method is shown in Listing 16-14.