Is There an Efficient Method of Checking Which Letter is Being Used?

  c++, letter, string

Lets say I have a string string str; and str has any number of letters in it, and I want to count how many of each letter is in the string. For example the word "Example" has 2 ‘e’, 1 ‘x’, 1 ‘a’, 1 ‘m’, 1 ‘p’, and 1 ‘l’. Is there a more efficient way of checking for each of these letters than:

for (int i = 0; i < str.length(); i++)
    if ( == 'a')
        //variable which keeps track of a ++
    //25 more of that for each other letter

It feels like there has to be a more efficient way of doing this, but I have no idea how. Please enlighten me.

Source: Windows Questions C++