Converting std::array to std::vector

Question

In the code below, the size of the function (foo) argument (std::vector) can be anything to make the function a generic one. However, sometimes the size container is known so std::array can be used. The problem is to convert the std::array to std::vector. What is the best way to solve this problem? Is it better to just use std::vector always in this case?

#include <iostream>
#include <array>
#include <vector>

using namespace std;

// generic function: size of the container can be anything
void foo (vector<int>& vec)
{
    // do something
}

int main()
{
   array<int,3> arr; // size is known. why use std::vector?
   foo (arr); // cannot convert std::array to std::vector

   return 0;
}

I assume that foo modifies the vector since it takes by mutable reference, is that correct? That's going to be very relevant to the answer. — Mooing Duck
– Mooing Duck, Commented Jul 22, 2014 at 17:49
For now you'll have to work around it, but in the future we'll write something like void foo(RangeOf<int>&); and it'll take any kind of range of ints. — Joseph Mansfield
– Joseph Mansfield, Commented Jul 22, 2014 at 17:50
This problem is why the standard library always takes begin and end iterators. That way it doesn't care which container you use. — Mark Ransom
– Mark Ransom, Commented Jul 22, 2014 at 17:55
// size is known. why use std::vector? -- why not use std::vector? — David Rodríguez - dribeas
– David Rodríguez - dribeas, Commented Jul 22, 2014 at 18:09

Alexander Gessler · Accepted Answer · 2014-07-22 17:49:59Z

16

Given that you pass in an array, foo does not seem to resize the container it gets passed in (however, it does seem to modify the elements since vec is passed non-const). It therefore does not need to know anything about the underlying container other than how to access elements.

Then, you can pass a pair of iterators (and make the iterator type a template argument), as many STL algorithms do.

answered Jul 22, 2014 at 17:49

Alexander Gessler

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3 Comments

Mooing Duck Over a year ago

Also, If you want to avoid making it a template, the parameters could be a int* first and a size_t count

Sarfaraz Nawaz Over a year ago

@MooingDuck: int *begin, int *end would be C++isher than int *first, size_t count.

Michael Over a year ago

Does the STL support the concept of a range? (a pair of iterators), or do I have to manually get begin(), end()?

Vlad from Moscow · Accepted Answer · 2014-07-22 17:50:53Z

0

The function accepts a vector by reference. So there is no any sense to pass an object of type std::array to the function. It could have a sense if the parameter of the function would be defined as const reference to a vector.

answered Jul 22, 2014 at 17:50

Vlad from Moscow

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