Adding char to string in C++

I work with Eclipse & Arduino.

I want to add a char to a string. I tried to use append,insert ( yet these can not be resolved)
I tried to use += yet when i print the string it always have one char.Basically i deletes the string & writes only the new char i want to add in.
I tried moreover concat & it does the same thing.Also strcat gives me headache with the operands cause it needs a const char pointer & i want to add a char that changes.

It sounds like what you want (for convenience) is the String object class available with the Arduino library.