Thread: Which map structure should I use?

I'm going to map chunks of string to chars. There's going to be 256 <key - element> links. The map is going to be used like this.

string str = "ABCDABCDABCD"; //A string that's going to be mapped.
string copm_str ; .. .. .. .. .. ..// Mapped string, compressed.

str [function that uses map to compress data] -> comp_str; ..//Sorry if I'm vague, I haven't looked at members of the map classes in C++.

The map is going to look something like this.
a = "AAAA";
b = "AAAB";
etc..
t = "ABCD";
etc..

So in this example the string str would be mapped to, comp_str = "ttt". If I got this right I only need to find elements and keys in the map. So my question is, which C++ map should I use? (It's been a while since I tinkered in C++)

Edit, I'm looking for the structure with the lowest complexity while finding elements or keys.