string alpha, num, special;
Last time I checked,
is related to vectors, not stings. Probably the reason to not get the expected result.
Your code do not behave the way you expect, and you don't understand why !
There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.
Debugger - Wikipedia, the free encyclopedia
Mastering Debugging in Visual Studio 2010 - A Beginner's Guide
Basic Debugging with Visual Studio 2010 - YouTube
1.11 — Debugging your program (stepping and breakpoints) | Learn C++
The debugger is here to only show you what your code is doing and your task is to compare with what it should do.