1.) You should not be using = in your if statement; that is assignment. You want ==, which is comparison.
2.) You are trying to compare a integer to a string of characters. You can't do that. In your original example, you were trying to compare a single character to a string. You can't do that either.
3.) Even if a were a string of characters, you can't use == to compare them. You would need to use strcmp(), or simply use std::strings (you have the header already) instead.