4 ms·

> C doesn't have optional parameters You are over-nitpicking. It's obvious that the GP meant that you can pass NULL by default if needed.

kazinator 7y ago
But when would that be needed? The argument is optional, and null is its default value. But the way the overloads are collapsed into a single function, there has to be a run-time check for that null. Using a null to indicate "argument is not there" is a C API trick. That parameter won't be optional in C.
- kragen 7y ago
  If you only have a K&R-style declaration visible, you can still call it with the wrong number of arguments, but (contrary to what you'd expect from doing the same thing in JS or Lua) most C implementations will instead pass some garbage data as the additional parameter, rather than NULL. Also, most C implementations will allow you to cast function pointers in a way that results in calling the function with the wrong number of arguments. If you're especially unlucky, that garbage data will happen to be NULL at first, but then become non-NULL when you recompile with debugging turned off, resulting in truly fascinating ways to spend your Friday and Saturday nights the month before shipping. (I know you know this, Kaz. I'm mentioning this for the benefit of those who haven't yet spent their Friday and Saturday nights this way.)
  - kazinator 7y ago
    If your compiler is allowing K&R declarations, or no declarations at all, without warnings, it's best to review the diagnostic options. If you have that nailed down early in your project, you won't have that frustrating afternoon later on.
    - kragen 7y ago
      That's true.
MaulingMonkey 7y ago
I'd agree if GP hadn't also used C++-specific optional parameters. Given the minefield that is vendor specific C++ish extensions to C, it seems worth a mention, even if I'm unaware of a compiler vendor that allows default parameters in your C code.
- cozzyd 7y ago
  It was just an illustration in C++ (the C version would just document it, obviously). There is no way to do this in a vendor specific way without some sort of name mangling that will hopelessly break the ABI...
  - a1369209993 7y ago
    Yes there is: void foo(void* p=0); // has exactly the same ABI as void bar(void* p); bar(); // error: missing parameter foo(); // shorthand for foo(0); // void(*)(void*) is a different type than void(*)(void*=0) // but they can be implicitly converted the same as // eg void* and char*
    - cozzyd 7y ago
      hmm I guess so!

kazinator 7y ago

But when would that be needed? The argument is optional, and null is its default value. But the way the overloads are collapsed into a single function, there has to be a run-time check for that null. Using a null to indicate "argument is not there" is a C API trick. That parameter won't be optional in C.

kragen 7y ago
If you only have a K&R-style declaration visible, you can still call it with the wrong number of arguments, but (contrary to what you'd expect from doing the same thing in JS or Lua) most C implementations will instead pass some garbage data as the additional parameter, rather than NULL. Also, most C implementations will allow you to cast function pointers in a way that results in calling the function with the wrong number of arguments. If you're especially unlucky, that garbage data will happen to be NULL at first, but then become non-NULL when you recompile with debugging turned off, resulting in truly fascinating ways to spend your Friday and Saturday nights the month before shipping. (I know you know this, Kaz. I'm mentioning this for the benefit of those who haven't yet spent their Friday and Saturday nights this way.)
- kazinator 7y ago
  If your compiler is allowing K&R declarations, or no declarations at all, without warnings, it's best to review the diagnostic options. If you have that nailed down early in your project, you won't have that frustrating afternoon later on.
  - kragen 7y ago
    That's true.

MaulingMonkey 7y ago

I'd agree if GP hadn't also used C++-specific optional parameters. Given the minefield that is vendor specific C++ish extensions to C, it seems worth a mention, even if I'm unaware of a compiler vendor that allows default parameters in your C code.

cozzyd 7y ago
It was just an illustration in C++ (the C version would just document it, obviously). There is no way to do this in a vendor specific way without some sort of name mangling that will hopelessly break the ABI...
- a1369209993 7y ago
  Yes there is: void foo(void* p=0); // has exactly the same ABI as void bar(void* p); bar(); // error: missing parameter foo(); // shorthand for foo(0); // void(*)(void*) is a different type than void(*)(void*=0) // but they can be implicitly converted the same as // eg void* and char*
  - cozzyd 7y ago
    hmm I guess so!