Need more input. What is the declaration for the xCards? If they are Cstyle strings then you can't assign one to the other and if they are STL string objects then they won't work with strcpy().
For Cstyle strings there is a strcat() function that will concatenate one string onto the other, assuming the first string has enough space to accomate the second string. Then, once you have concatenated B onto A you can copy A into C with strcpy.
For STL string objects you can use the + operator to concatenate B onto A and then use the assingment operator to assign the new, modified and longer A to C.