Correct way to work with vector of arrays

Question

Could someone tell what is the correct way to work with a vector of arrays?

I declared a vector of arrays (vector<float[4]>) but got error: conversion from 'int' to non-scalar type 'float [4]' requested when trying to resize it. What is going wrong?

You can't have a vector of native arrays, because they are neither assignable nor copyable. — Khaled Alshaya, Jan 06 '11 at 06:17

score 144 · Accepted Answer · answered Jan 06 '11 at 06:18

144

You cannot store arrays in a vector or any other container. The type of the elements to be stored in a container (called the container's value type) must be both copy constructible and assignable. Arrays are neither.

You can, however, use an array class template, like the one provided by Boost, TR1, and C++0x:

std::vector<std::array<double, 4> >

(You'll want to replace std::array with std::tr1::array to use the template included in C++ TR1, or boost::array to use the template from the Boost libraries. Alternatively, you can write your own; it's quite straightforward.)

answered Jan 06 '11 at 06:18

James McNellis

338,529
73
897
968

8

The second sentence is no longer true in C++11, but the rest is still correct. The problem in C++11 is that arrays are not [`Erasable`](http://en.cppreference.com/w/cpp/concept/Erasable), at least with the default allocator. – T.C. Jan 12 '16 at 03:22
1

For requirements on STL containers(post C++11), take a look at this thread, ["Has C++11 changed requirements for elements of STL containers, and how?".](https://stackoverflow.com/questions/13577881/has-c11-changed-requirements-for-elements-of-stl-containers-and-how) – JohnKoch Mar 18 '18 at 10:45
https://ideone.com/ENLC68 ? – Jason C Apr 21 '22 at 04:03

Nawaz · Answer 2 · 2011-01-06T06:21:23.667

11

Use:

vector<vector<float>> vecArray; //both dimensions are open!

edited Jan 06 '11 at 06:21

answered Jan 06 '11 at 06:15

Nawaz

341,464
111
648
831

18

vector of vector is not contiguous, in case that is required by the OP. – johnbakers May 14 '13 at 10:36

score 10 · Answer 3 · answered Oct 06 '14 at 11:37

10

There is no error in the following piece of code:

float arr[4];
arr[0] = 6.28;
arr[1] = 2.50;
arr[2] = 9.73;
arr[3] = 4.364;
std::vector<float*> vec = std::vector<float*>();
vec.push_back(arr);
float* ptr = vec.front();
for (int i = 0; i < 3; i++)
    printf("%g\n", ptr[i]);

OUTPUT IS:

6.28

2.5

9.73

4.364

IN CONCLUSION:

std::vector<double*>

is another possibility apart from

std::vector<std::array<double, 4>>

that James McNellis suggested.

answered Oct 06 '14 at 11:37

9

In your example arr is being dangerously cast to a pointer. If arr goes out of scope while vec still exists an undefined behavior (likely stack overflow) will occur. – Ruslan Yushchenko Dec 01 '14 at 11:07
5

this just made the vector non-copyable. – user3528438 Mar 26 '15 at 18:55

score 4 · Answer 4 · answered Jan 06 '11 at 06:17

4

Every element of your vector is a float[4], so when you resize every element needs to default initialized from a float[4]. I take it you tried to initialize with an int value like 0?

Try:

static float zeros[4] = {0.0, 0.0, 0.0, 0.0};
myvector.resize(newsize, zeros);

answered Jan 06 '11 at 06:17

Mark Ransom

286,393
40
379
604

This (after obvious minor fixes) doesn't compile, for the same reason as given in the accepted answer: it's not valid because plain arrays aren't valid container element types. 1st error from `g++`: `C:/msys64/mingw64/include/c++/10.2.0/bits/stl_uninitialized.h:281:63: error: static assertion failed: result type must be constructible from input type` – underscore_d May 28 '21 at 08:26

Correct way to work with vector of arrays

4 Answers4

Linked

Related