How to append a char to a std::string?

y += d;

I would use += operator instead of named functions.

Use push_back():

std::string y("Hello worl");
y.push_back('d')
std::cout << y;

To add a char to a std::string var using the append method, you need to use this overload:

std::string::append(size_type _Count, char _Ch)

Edit : Your're right I misunderstood the size_type parameter, displayed in the context help. This is the number of chars to add. So the correct call is

s.append(1, d);

not

s.append(sizeof(char), d);

Or the simpliest way :

s += d;