Preferred conversion from char (not char*) to std::string

std::string has a constructor that takes a number and a character. The character will repeat for the given number of times. Thus, you should use:

std::string str(1, ch);

To add to the answer, you can simply use initializer list

std::string str = {ch};

just use the overload that takes a char?

i.e. string(1, 'A')