C/C++ Inline coercion of array of pointers (void**)

Question

I am using a library function that expects an array of pointers (void**) at some point, it works more or less like this.

void* args[] = { &var_a, &var_b, &var_c, ... };
someFunction(args);

After using it once, I would like to call the same function again, so what I do is create another variable like:

void* args_2[] = { &var_d, &var_e, &var_f, ... };
someFunction(args_2);

And so on ...

I would like to find a way to recycle the args symbol, so I don't have to do args_2, args_3, args_4 every time I call it; but when I try to reassign it like:

args = { &var_d, &var_e, &var_f, ... };

I get the following:

error: assigning to an array from an initializer list

I understand the error but I don't know how to avoid it or coerce this thing into the intended array of pointers type.

I know they are two different languages, but I am looking for a solution that works in both C and C++.

I could accept an answer that works in both scenarios. Also, @NathanOliver could you tell me more about what could be different? — Alex Morales
– Alex Morales, Commented Oct 25, 2019 at 12:24
Use a std::vector<void *>, that does support construction from an initializer list. Use as someFunction(args.data()); — Botje
– Botje, Commented Oct 25, 2019 at 12:28
If you want solutions for both C or C++, I suggest asking two separate questions, as the solutions are inherently different (C can do things C++ can't and vice versa). If you don't care, then I recommend just sticking with one language. Roll a dice if you must, but I suggest C++ as you can simply use std::vector — Max
– Max, Commented Oct 25, 2019 at 12:28
@almosnow well, no, that's a myth. kiran's answer is C-only, and mine is C++-only (even if you use typedef). What now? — Quentin
– Quentin, Commented Oct 25, 2019 at 12:41

Quentin · Accepted Answer · 2019-10-25 12:29:18Z

3

Why use a local variable at all, if you only need it once to call someFunction?

using args = void *[];

someFunction(args{&a, &b});
someFunction(args{&a, &b, &c});

Alternatively, C++ify this a bit more with a wrapper:

template <class... T>
decltype(auto) someFunction(T *... args) {
    void *args_[] { args... };
    return someFunction(args_);
}

someFunction(&a, &b);
someFunction(&a, &b, &c);

answered Oct 25, 2019 at 12:29

Quentin

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Alex Morales Over a year ago

Thanks Quentin, I'm just testing it quickly, will be back at this.

kiran Biradar · Accepted Answer · 2019-10-25 12:31:12Z

1

You could make use of compound literals as below with pointer to pointer.

void** args = (void *[]){ &var_a, &var_b, &var_c, ... };

args = (void *[]){ &var_d, &var_e, &var_f, ... };

answered Oct 25, 2019 at 12:31

kiran Biradar

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2 Comments

Alex Morales Over a year ago

Thank Kiran! In spirit I am looking for something like this, however, it does not compile: due to "error: taking address of temporary array". Which I understand as well, hmm, how could this be avoided?

kiran Biradar Over a year ago

@almosnow this might help you stackoverflow.com/questions/32941846/….

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