-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 24/01/16 02:19, Bart Smissaert wrote: > You didn't mention it but how arguments are called from VB6 is also > very important, that is ByVal or ByRef. I think that unless the > argument is going to be altered I have to pass them always ByVal.
Sort of. You need to look at the C interface. ByVal makes a copy of the value and passes that to the function. The function can do whatever it wants to the copy as it won't affect the caller's version. Byref passes a pointer to the value in memory. The called function has to dereference the pointer to get the value at that location. It can also modify the value at that location, affecting the caller. That ties in with your rule of thumb. But they are not interchangeable. Randomly specifying one or the other and seeing if it works is not a good idea. Sometimes you do the wrong one but can get lucky, or more likely crash/corrupt memory. As an analogy, it is the difference between handing you a photocopy of a document versus giving you a mailbox number that has a document inside. But realise that a mailbox is very different than what is inside, and it is especially the case that they can be very different sizes (eg it could be a big package inside the mailbox with a small number). C programmers will use ByRef if they want the item to be modified, but can also do so if the item is larger since a mailbox (pointer) number takes less space than the larger item. In C syntax an integer is written as 'int' while a pointer to an integer is written as 'int *' (the star is typically pronounced as pointer). Your rule should be ByVal when there are no '*' and ByRef when there are. The rules are non-obvious when you get more complicated combinations of types and pointers. Fortunately there is a site that turn them into English for you. http://www.cdecl.org/ Try the following: int x; int *x; Roger -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iEYEARECAAYFAlalMEQACgkQmOOfHg372QS1NACgkBqpEHb4q/XxAMgrfBDe/EMj 6+QAn2qDOgHITU8lrm68DiyIC62g06bb =I6gu -----END PGP SIGNATURE-----
