Check number of elements given to std::array compile time

Question

I have a performance critical code where I query N indices. How can I check compile-time with a static_assert whether exactly N indices are given, without sacrificing performance?

#include <array>

template<int N>
void test(const std::array<int, N>& indices)
{
    // static_assert:  has three elements.
    return;
}

int main()
{
    test<3>({1, 2, 3}); // OK
    test<3>({1, 2});    // Needs to crash, because 2 < 3

    test<2>({1, 2, 3}); // Crashes, because 3 > 2
    test<2>({1, 2});    // OK

    return 0;
}

It's ugly, but it kind of works: godbolt.org/z/1S11GE . The overload is ambiguous for the second case — Justin
– Justin, Commented Aug 8, 2019 at 21:27
Why use a std::array if you always want 3 items and you don't want a partial initializer? — Michael Chourdakis
– Michael Chourdakis, Commented Aug 8, 2019 at 21:28
@MichaelChourdakis Why not? That's exactly what std::array gives. — Justin
– Justin, Commented Aug 8, 2019 at 21:28
Why not change test() so it accepts three arguments of type int? That means the calling syntax doesn't need the {} for the initialiser (i.e. test(1,2,3) rather than test({1,2,3})) and a compilation error if you get the number of values wrong. You can also use a variadic template if needed. — Peter
– Peter, Commented Aug 8, 2019 at 21:35

NathanOliver · Accepted Answer · 2019-08-08 21:32:08Z

6

How can I check compile-time with a static_assert whether three indices are given

You can't. They array is of size 3 so it will always have 3 elements. When you do something like

test({1, 2});

The array initializes the first two elements with 1 and 2 and then zero-initializes the last element. This is how aggregate initialization works and you can't change it.

What you need to do is either add overloads for arrays of size 1 and 2 and delete them, or you could just change the function to have 3 parameters and then it must be called with 3 values.

answered Aug 8, 2019 at 21:32

NathanOliver

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Comments

Aykhan Hagverdili · Accepted Answer · 2019-08-08 22:15:31Z

4

Here's one way to do it:

#include <array>
#include <iostream>

template<int N, typename ...Args>
void test(Args... args)
{
    static_assert(sizeof...(args) == N);
    std::array<int, N> arr{ args... };
    for (auto&& elm : arr) {
        std::cout << elm << '\n';
    }
}

int main()
{
    test<3>(1, 2, 3); // OK
    //test<3>(1, 2);    // Crashes

    //test<2>( 1, 2, 3 ); // Crashes
    test<2>(1, 2);    // OK

    return 0;
}

Uses variadic templates instead.

answered Aug 8, 2019 at 22:15

Aykhan Hagverdili

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Comments

Yksisarvinen · Accepted Answer · 2019-08-08 21:38:05Z

2

A bit ugly, but should work - a wrapper for std::array:

class MyArray 
{
public:
    MyArray(int x, int y, int z): _array{x, y, z} 
    {};
private:
    std::array<int, 3> _array;
};

void test(const MyArray&) {
    //no need to check values here
}

You won't be able to create an object of this wrapper class with less than 3 arguments. See it online.

Of course, it won't work for general case you mentioned, but it should make it possible to differentiate certain classes.

answered Aug 8, 2019 at 21:38

Yksisarvinen

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2 Comments

Justin Over a year ago

You can make this work for the general case by having an N parameter like std::array, and a variadic constructor which tests that the number of parameters is N.

Yksisarvinen Over a year ago

@Justin I don't understand those well enough to use them correctly, but fortunately anyone can provide another answer here ;)

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