What would be equal of Java function with any number of parameters in C++?

Question

void anyNumberOfParamas(String... strings) {
    for (String str : strings) {
        // do something with string
    }
}

https://stackoverflow.com/questions/1579719/variable-number-of-parameters-in-function-in-c possible duplicate. ... exists in c, too. — Gladaed, Mar 13 '18 at 13:46
[Variadic functions](http://en.cppreference.com/w/cpp/utility/variadic), and since C++11 [variadic templates](https://en.wikipedia.org/wiki/Variadic_template), more general and powerful. — jdehesa, Mar 13 '18 at 13:48

Vittorio Romeo · Accepted Answer · 2018-03-13T13:52:14.273

4

You have two choices: a variadic template or a function taking std::initializer_list.

The first one looks like this:

template <typename... Ts>
void anyNumberOfParams(const Ts&... xs)
{
    (something(xs), ...);
}

With this approach, you get a different instantiation of anyNumberOfParams each you time you call it with different types. The "iteration" is done at compile-time.

You also need to constrain Ts... properly if you want to allow this function to only accept std::string.

The second one looks like this:

void anyNumberOfParams(std::initializer_list<std::string> xs)
{
    for(const auto& x : xs) something(x);
}

std::initializer_list is basically syntactic sugar over a const array of elements. In this case, the iteration is done at run-time and there is only a single instantiation of anyNumberOfParams.

Unfortunately, std::initializer_list has quirks you should be aware of (e.g. cannot move elements from it).

edited Mar 13 '18 at 13:52

answered Mar 13 '18 at 13:48

Vittorio Romeo

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Is there a benefit to taking a `std::initializer_list` by const reference? (Genuine question). – StoryTeller - Unslander Monica Mar 13 '18 at 13:49
Don't forget the `va_args` stuff too. Old-fashioned yes, but tried and tested. Not mentioning them any more is like not being allowed to talk about your ex-girlfriends. – Bathsheba Mar 13 '18 at 13:49
@StoryTeller: Why not ask the community? – Bathsheba Mar 13 '18 at 13:49
1

@Bathsheba - That's plan B. – StoryTeller - Unslander Monica Mar 13 '18 at 13:51
@StoryTeller: Ping me if you do. I'll up it. – Bathsheba Mar 13 '18 at 13:51
2

@StoryTeller initializer_list should probably not be taken by const reference, since it's always created by value. It's basically `struct { T const* data; std::size_t size; };`. If you look at any standard library functions or constructors that take an initializer_list, they do so by value – SJL Mar 13 '18 at 13:52
@Galik: then you have dynamic allocation. Use variadic templates instead. – Vittorio Romeo Mar 13 '18 at 13:57
@VittorioRomeo I just realized it wouldn't work anyway because you would need to pass by `const&` – Galik Mar 13 '18 at 13:58
@VittorioRomeo what about if I need to access two or more string at the same time, in Java: `strings[0]` `strings[1]` (access first two strings) – user924 Mar 13 '18 at 14:01
@user924: you can do it in many ways. In the variadic template case, you can use metaprogramming to extract the nth element from a pack. – Vittorio Romeo Mar 13 '18 at 14:02
@VittorioRomeo as I see the second solution won't work for me (in my case, for my task), __initializer_list doesn't provide subscripting.__ – user924 Mar 13 '18 at 14:05
Where is the third choice of variadic functions? They are fine for their uses. – SergeyA Mar 13 '18 at 14:25

What would be equal of Java function with any number of parameters in C++?

1 Answers1