Difference between string.empty and string[0] == '\0'

Question

Suppose we have a string

std::string str; // some value is assigned

What is the difference between str.empty() and str[0] =

Answer 1

empty() is not implemented as looking for the existence of a null character at position 0, its simply

bool empty() const
{
    return size() == 0 ;
}

Which could be different