How to construct a std::vector or a boost::array from a C array without copying?

Question

Given a pointer to an array of char, is it possible to construct a std::vector or boost::array from it, and avoiding memory copying?

Thanks in advance!

You might be looking for the proposed std::array_view, or string_view since this is of char. — Qaz
– Qaz, Commented Jul 1, 2015 at 21:16

sehe · Accepted Answer · 2015-07-02 09:57:28Z

6

Because vectors own their own allocators and storage alike, there is no way (for non primitive elements construction from move_iterators could help a bit).

So assuming the goal is to get a true std::vector<char>& for the existing storage, you'll never succeed, not even with custom allocators¹.

If you want a string, you can use boost::string_ref (in utility/string_ref.hpp).

Otherwise, you could use a 1-dimensional multi_array_ref (from Boost Multi Array)

1. Using string_ref

This is definitely easiest:

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#include <boost/utility/string_ref.hpp>
#include <iostream>

using boost::string_ref;

int main() {
    char some_arr[] = "hello world";

    string_ref no_copy(some_arr);

    std::cout << no_copy;
}

2. `multi_array_ref`

This is more versatile, and works "better" if you are not geared towards a string interface.

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#include <boost/multi_array/multi_array_ref.hpp>
#include <iostream>

using ref = boost::multi_array_ref<char, 1>;
using boost::extents;

int main() {
    char some_arr[] = "hello world";

    ref no_copy(some_arr, extents[sizeof(some_arr)]);

    std::cout.write(no_copy.data(), no_copy.num_elements());
}

Both examples print

hello world

¹ specializing std::allocator<char> is too evil to consider and probably outright prohibited by the standard

edited Jul 2, 2015 at 9:57

answered Jul 1, 2015 at 21:20

sehe

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4 Comments

sehe Over a year ago

As a courtesy, here's the live stream of me answering this question (experiment)

sehe Over a year ago

@dyp I'd argue that there's no real value in using std::vector there then (the type will already be distinct due the custom allocator). But technically, that's a third option (depending on how narrowly you define std::vector<T> :))

sehe Over a year ago

@dyp Good luck passing that too void api2::foo(std::vector<char> const&); - that's why I said "I'd argue that there's no real value in using std::vector there". I think this is kind of the scenario the OP is facing. In fact, he should ask the library devs to fix the API to be less intrusive :)

sehe Over a year ago

@dyp I made this warning more explicit on your request.

Miki · Accepted Answer · 2015-07-02 09:35:15Z

1

An alternative without using boost would be std::reference_wrapper

#include <vector>
#include <iostream>
#include <functional>

using namespace std;

struct S
{
    S() : val(0) {}
    S(int val_) : val(val_) {}
    S(const S& other) : val(other.val) {
        cout << "copy" << endl;
    }

    int val;
};


int main()
{
    char a[] = "Hello";
    vector<reference_wrapper<char>> v(a, a+5);

    S vS[] = {S(1), S(2), S(3)};

    vector<S> v_copy(vS, vS + 3);
    vector<reference_wrapper<S>> v_nocopy(vS, vS+3);
}

Using struct Syou can see that objects are not copied to the vector. So this should work well also for char.

edited Jul 2, 2015 at 9:35

answered Jul 2, 2015 at 9:15

Miki

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2 Comments

sehe Over a year ago

This is nice. It's important to realize that effectively you work on a vector of char* now (from a memory viewpoint) and this could impact performance very badly. This is reason for me not to upvote it here, because I think the context is all about maximizing performance, by eliminating allocation/copying.

Miki Over a year ago

@sehe It sounds right. Just thought this was worth noting, at least for objects.

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How to construct a std::vector or a boost::array from a C array without copying?

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