Question

I was wondering which is the correct way?

_tcscpy(tchar_pointer, _tcslen(tchar_pointer), _T("Hello World"));

or

_tcscpy(tchar_pointer, _tcsclen(tchar_pointer), _T("Hello World"));

or

_tcscpy(tchar_pointer, ???, _T("Hello World"));

Answer 1

the tchar pointer are coming from external, and my side has no idea how large the buffer referred by the pointer is

If this is so, then none of these do what you want.

The way all the “safe” functions work is that you tell them how big the target buffer is.

You don’t know? You can’t use those functions.

int buffer_size = _tcslen(xxx) * sizeof(TCHAR)

This will not work. All it will guarantee is that the string copied is not longer than whatever is already in the buffer. If the buffer has not been initialized, it will fail; if the buffer begins with a '\0', nothing will be copied; and so forth.