Defining a function that takes a std::array as a parameter

Question

Basic question here, with (hopefully) a simple answer: I'm trying to write a function whose first argument is a std::array, but with an arbitrary size.

void f(array<int> x) {
    //do stuff
}

isn't valid, because I need a size for array<int, #>. Is there a way to get around this?

Please pay particular attention to this section http://stackoverflow.com/a/4810672/2296458 — Cory Kramer, Jul 10 '14 at 17:30
@Cyber no, that is not a duplicate... did you even read the linked question and answer? The linked question is about C style arrays. — Theolodis, Jul 10 '14 at 17:32
@CSGregorian then go with [Quentin](http://stackoverflow.com/a/24682833/1941161)'s answer :) — Scis, Jul 10 '14 at 17:38

score 12 · Accepted Answer · answered Jul 10 '14 at 17:30

12

The only way is to make f a function template :

template <size_t N>
void f(array<int, N> x) {
    //do stuff
}

answered Jul 10 '14 at 17:30

Quentin

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score 12 · Answer 2 · edited May 23 '17 at 12:20

12

Quentins answer is correct if you do not need to be flexible at run-time.

However, if you need to be flexible at run-time I would recommend you to use an std::vector instead.

void f(vector<int> x) {
    //do stuff
}

Also, you might want to use a reference or const reference to avoid the copying of the array or vector, at least if you do not need the copy.

edited May 23 '17 at 12:20

Community

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answered Jul 10 '14 at 17:36

Theolodis

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score 5 · Answer 3 · edited Jul 10 '14 at 17:45

5

A second way around it, is using the function template (@Quentin) and delegate to a non template function:

#include <array>

void g(int* first, int* last) {
    //do stuff
}

template <size_t N>
inline void f(std::array<int, N> x) {
    g(x.begin(), x.end());
}

edited Jul 10 '14 at 17:45

Captain Obvlious

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answered Jul 10 '14 at 17:37

1

And what is the concrete benefit of the non template function? – Theolodis Jul 10 '14 at 17:39
2

@Theolodis: The implementation can be in a source file. – Jul 10 '14 at 17:42
1

@Theolodis _'And what is the concrete benefit ...?'_ The non template function is a bit more generic, and no longer depends on the compile time provided `size_t N`. – πάντα ῥεῖ Jul 10 '14 at 18:04
@πάνταῥεῖ ok, I didn't think of that... Actually it could even be used with the vector solution I proposed too. – Theolodis Jul 10 '14 at 18:08
@Theolodis _'Actually it could even be used with the vector solution ...'_ Yup, exactly. You've got the point. The `std::vector` variant could be even a specialization for `N=0` or so. – πάντα ῥεῖ Jul 10 '14 at 18:20
1

Or you could just call `g()` directly without bothering with `f()`. :) – ppl Jul 10 '14 at 21:05

Defining a function that takes a std::array as a parameter

3 Answers3