Initializing a 2-D array in C++14

Question

recently I've started to study C++'s standard library containers from the Deitel&Deitel book. Conceptually, it is quite clear to me, but I have a problem when trying to replicate a piece of code that includes a 2-D array initialization.

The initialization in the book is (pretty much) this one:

#include <iostream>
#include <array>
using namespace std;

int main(){

    array<array<int, 3>, 10> grades{{87, 96, 70},
        {68, 87, 90},
        {94, 100, 90},
        {100, 81, 82},
        {83, 65, 85},
        {78, 87, 65},
        {85, 75, 83},
        {91, 94, 100},
        {76, 72, 84},
        {87, 93, 73}};
  }

This shouldn't be problematic, but VisualStudioCode returns an error saying "too many initialization values". If instead, I simply initialize it in this way:

#include <iostream>
#include <array>
using namespace std;

int main(){

    array<array<int, 3>, 10> grades{87, 96, 70,
        68, 87, 90,
        94, 100, 90,
        100, 81, 82,
        83, 65, 85,
        78, 87, 65,
        85, 75, 83,
        91, 94, 100,
        76, 72, 84,
        87, 93, 73};
    }

it works, which is what I do expect since the book in a previous example initialized a 2-D array by linearizing all its elements. What I do not understand is why in the other case VSC returns that error, since it should work just fine.

score 0 · Answer 1 · answered Aug 07 '22 at 14:24

It is down to a typo. Thhis works, count the braces. Live demo here : https://godbolt.org/z/xqTWxEEov

#include <iostream>
#include <array>
// using namespace std; // <== unlearn to do this.

int main()
{
    std::array<std::array<int, 3>, 10> grades{{
        {87, 96, 70},
        {68, 87, 90},
        {94, 100, 90},
        {100, 81, 82},
        {83, 65, 85},
        {78, 87, 65},
        {85, 75, 83},
        {91, 94, 100},
        {76, 72, 84},
        {87, 93, 73}}};

        for(const auto entry : grades)
        {
           auto [v1,v2,v3] = entry;
           std::cout << v1 << ", " << v2 << ", " << v3 << "\n";
        }
  
        return 0;
  }

score 0 · Answer 2 · answered Aug 07 '22 at 14:30

The class template std::array is an aggregate that under the hood represents a structure with a data member of an array type..

The compiler considers in this initialization

array<array<int, 3>, 10> grades{{87, 96, 70},
    {68, 87, 90},
    {94, 100, 90},
    {100, 81, 82},
    {83, 65, 85},
    {78, 87, 65},
    {85, 75, 83},
    {91, 94, 100},
    {76, 72, 84},
    {87, 93, 73}};

the first list

{87, 96, 70}

as a whole initializer for the array grades.

To consider the lists as initializers for the inner elements of the type std::array<int, 3> you need to add one more pair of braces.as shown in the demonstration program below.

#include <iostream>
#include <array>

int main()
{
    std::array<std::array<int, 3>, 10> grades
    {
        {
            {87, 96, 70},
            {68, 87, 90},
            {94, 100, 90},
            {100, 81, 82},
            {83, 65, 85},
            {78, 87, 65},
            {85, 75, 83},
            {91, 94, 100},
            {76, 72, 84},
            {87, 93, 73}
        }
    };

    for ( const auto &row : grades )
    {
        for ( const auto &item : row )
        {
            std::cout << item << ' ';
        }
        std::cout << '\n';
    }
}

The program output is

As the inner element has the type std::array<int, 3> then by analogy you could add one more pair of braces around each initializer list like

std::array<std::array<int, 3>, 10> grades
{
    {
        { {87, 96, 70} },
        //...

Initializing a 2-D array in C++14

2 Answers2