C execute function with list of arguments

Question

I need to evaluate function with list of arguments from array of arguments as in this example:

int compute(...) {
    int n;
    va_list params;
    va_start(params, n);
    // some custom computation with no "va" output
    va_end(params);
}

And some array of int (which is dynamic array, don't rely on fixed size):

int arr[10] = {0, 1, 3, 7, 8, 1, 3, 5, 7, 9};

And I need to call compute like JS function compute.apply(this, arr)

I'm implementing some library with C that's why I need it.

In C++ this is std::apply but I want the same in C.

Thanks

Hm. Variadic functions are probably not what you need here. If you want to operate on an array of n elements, write a function that takes that array and its length as parameters: int compute(const int *arr, size_t n) . — M Oehm
– M Oehm, Commented Oct 15, 2019 at 6:51
@GuillaumePetitjean I need something like std::apply from C++ in C — xercool
– xercool, Commented Oct 15, 2019 at 6:53
It's not whether you like it or not, it's whether C can do it or not. Also, you didn't demonstrate anything. How do you want to call your proposed compute function with the given array? — M Oehm
– M Oehm, Commented Oct 15, 2019 at 7:01
If your arrays are of a homogenous type, you can use the raw pointer + byte size approach of qsort like this. If your arrays can be heterogenous, you can create a variant type as a tagged union like this. Note how both approaches still must manage their memory explicitly. Is that what you are looking for? — M Oehm
– M Oehm, Commented Oct 15, 2019 at 8:39

Armali · Accepted Answer · 2019-10-17 13:18:18Z

1

In C++ this is std::apply but I want the same in C.

Since you want the same in C, you'll surely accept that you have to fulfill the same requirements, in particular, as a tuple supports std::get and std::tuple_size, their C equivalents. Now, as long as the arguments from the array are accessed in order from first to last, std::get can be implemented with va_…(), but just as the stdarg variable argument lists need some means of determining the amount of arguments (like a format string or an argument count), std::tuple_size cannot be implemented without such a means. You won't do without passing this information.

answered Oct 17, 2019 at 13:18

Armali

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2 Comments

xercool Over a year ago

Thank you for your answer. I decided just move project to C++ since I can combine C and C++ features. I also need to run a function in a separate thread for example... C++ is the best with this. But it looks like you're the only person who got my question. Idk why nobody else wants the same feature in C...

Armali Over a year ago

I can well imagine people wanting the same feature in C, it's just that C as it is doesn't provide a means of implicitly passing the needed size information, and there are high barriers for a language standard change.

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