I use an STL map to store integers associated with string keys, like this:

 map<string, int> g_myMap;

 void Init() {
   g_myMap["a"] = 0;
   g_myMap["b"] = 1;
 }

 int Get(string key) {
   if (g_myMap.count[key] == 0) return -1;
   return g_myMap[key];
 }

So, the idea is that I return -1 if the specified key isn't in the map;
otherwise I return the int associated with the key.

Here's my question:  The caller almost always gives me a valid key, so
calling the count function to detect an invalid key is really inefficient.
Is there a more efficient way to catch the unusual case where the user calls
Get with a bogus key?

One thing I thought of is to somehow convince g_myMap that the default value
is -1 rather than 0.  That way I could rewrite Get as:

 int Get(string key) {
   int result = g_myMap[key];
   if (result == -1) <delete g_myMap[key]>
   return result;
 }

This would work but I don't know how to change the map so that it creates a
value of -1 rather than 0 if the key doesn't exist.

TIA  - Bob