[AdieuToLogic]

  to see if a string starts with another string in C++

As others have pointed out, while a person could use

  std::mismatch

C++'s std::basic_string type has a compare method. So, the Go example you presented:

  strings.HasPrefix(toCheck, prefix)

Could be expressed in Standard C++ as:

  toCheck.compare (0, prefix.length (), prefix) == 0;