Another academic question - ANSI C standard on optional function parameters

Vaclav_

What is a current C ( ANSI C) standard on putting optional parameters / attributes into function? Or is there such a thing - ANSI standard _ anymore? "Popular" microprocessor compiler does not allow such thing and "workaround" is function overloading. Which IMHO in memory limited device adding more similar functions into code does not make much sense, unless you are memory chips salesman. Cheers Vaclav

Lost User

You could get around it by using the main pattern, where the first parameter tells the function how many items are in the array pointed to by the next; i.e.

int function(int count, void** somepointer)
{
// count tells how many items in the array pointed to by somepointer
// which may point to any type depending on requirements
}

CPallini

ANSI C only provides variable number of arguments, via the va_list, va_start, va_arg, va_end mechanism. See, for instance here[^].

THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

Vaclav_

Thanks, but I may have messed up my question. What I was looking for - is this syntax ANSI C standard function(int a, int b = 0) Where passing the second parameter in function call is optional. Cheers Vaclav

Lost User

No, that's C++; there is no overloading in C. See http://msdn.microsoft.com/en-us/library/cts394c0.aspx[^].

Vaclav_

Now I am totally lost. I thought overloading was having same function name with different types of variables. Such as function(int) function(void*) function(double) That works fine in Arduino C++ compiler.( And right now I have a bug in my code because I am using it). I am assuming Arduino compiler is C++, did not really try to find out. But since it works with classes....it must be C++ based. Cheers Vaclav

CPallini

Vaclav_Sal wrote:

I thought overloading was having same function name with different types of variables.
Such as
function(int)
function(void*)
function(double)

You thought right.

Vaclav_Sal wrote:

That works fine in Arduino C++ compiler

As it should: after all is a C++ compiler. You have to understand that C and C++ are two different programming languages. While C++ supports function overloading, C does not.

THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

Lost User

The following defintion is implied overloading since you can call it in one of two ways.

function(int a, int b = 0);

//
function(1, 2); // two parameters with explicit values

//
function(1); // one explicit and one implicit (0) parameter

Stefan_Lang

Since C doesn't allow overloading to start with, the only way to call a function with alternate sets of parameters is by thinking up a pattern on how you pass these parameters in a less rigid way. One way has already been mentioned: passing an array of void pointers. The disadvantage is that you have to figure out what is what, including type, and if more than one parameter is optional you need to additionally pass some information about which of the parameters were skipped. Pretty much the same goes for vararg lists. A slightly better approach that offers strict control over types and meaning of parameters is passing a struct that contains all possible parameters: write one function that allocates and/or initializes the struct with defaults, and in the code overwrite all values that you want different, then pass the struct to your processing function. If you have mandatory parameters without sensible defaults (e. g. array sizes), make sure to pass these to the intialization function.

GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

Vaclav_

OK, I did try this void TestFunctionA (int a, int b = 100) { a += 1; b += 10; Serial.print(b); } void TestFunctionB (int a) { a += 1; // b += 10; Serial.print(a); } and calls TestFunctionB (10); // int a, int b = 100) TestFunctionA(10,234); And the compiler said too many arguments in function call But if I change the b to float it compiles and it does makes sense to me. Partially. If the b parameter b is not passed, than the function A will be executed and compiled. Confusing, yes. But that was not the reason I asked. In "real" C compiler when a parameter is declared optional the code will compile without having to change all function calls to include the optional parameter. But I still do not know if that is ANY standard or just MS compiler specific. But Arduino compiler expects the "optional" parameter, so it is really not optional. Cheers Vaclav

Lost User

That does not make any sense at all, you are using two different functions.

Stefan_Lang

This doesn't compile as C code in VisualStudio 2010. If it did for you, you compiled as C++. Hint: in VS 2010 you can't create a C project directly, only a C/C++ project that will contain *.cpp files which by default will be compiled as C++. You can override that behaviour by explicitely forcing C compilation in the Compiler->Advanced settings tab of the project. Or you can put your C code in a *.C file (that you will have to explicitely create, of course).

GOTOs are a bit like wire coat hangers: they tend to breed in the darkness, such that where there once were few, eventually there are many, and the program's architecture collapses beneath them. (Fran Poretto)

Vaclav_

Whoops I was trying to demo a point and messed it up adding the A and B to the functions.

Lost User

It is always best to write and test your code, and then use copy and paste to show exactly what the issue may be.