assigning new array to reference variable-CodePudding

void byReference(int (&p)[3]){
    int q[3] = {8, 9, 10};
    p = q;
}

I want to write function where i can reassign the p with new array. I am not sure if we can do that.

My goal : i want to change the original array, like we do swapping of two number by call-by reference.

Edit:

my working solution :

void byReference(int*& p){
    int* q = new int[2];
    q[0] = 8;
    q[1] = 9;
    
    p = q;
}

int main(){
    int *x = new int[2];
    x[0] = 1;
    x[1] = 2;

    byReference(x);
    
    return 0;
}

CodePudding user response：

In c it is recomended to use std::array for fixed size arrays, and std::vector for a dynamic size arrays.

Both of them can be passed by refernce, to be modified by a function. This requires the function to declare that the argument is passed by refernce using the & symbol.

See the example below:

#include <array>
#include <vector>

// Get an array (with a fixed size of 3) by refernce and modify it:
void ModifyStdArray(std::array<int, 3> & a) {
    a = std::array<int, 3>{8, 9, 10};
    // or:
    a[0] = 8;
    a[1] = 9;
    // etc.
}

// Get a vector by refernce and modify it:
void ModifyStdVector(std::vector<int> & v) {
    v = std::vector<int>{ 1,2,3,4 };
    // or:
    v.clear();
    v.push_back(1);
    v.push_back(2);
    // etc.
}

int main()
{
    std::array<int, 3> a1;
    // Pass a1 by reference:
    ModifyStdArray(a1);
    // Here a1 will be modified.

    std::vector<int> v1;
    // Pass v1 by reference:
    ModifyStdVector(v1);
    // Here v1 will be modified.
}

CodePudding user response：

You cannot copy arrays by assignment. You can use std::copy

void byReference(int(&p)[3]) {
    int q[3] = { 8, 9, 10 };
  //  p = q;
    std::copy(&q[0], &q[3], &p[0]);
}

....

int a[3] = { 1, 2, 3 };
byReference(a);

// a now 8,9,10

CodePudding user response：

You can do it like this:

#include <iostream>
#include <array>
#include <algorithm>

// with std::array
void byReference( std::array<int, 3>& p )
{
    const std::array<int, 3> q { 8, 9, 10 };
    std::copy_n( std::begin( q ), std::size( p ), std::begin( p ) ); // copy q to p
}

// with C-style arrays
void byReference( int (&p)[ 3 ] )
{
    const int q[ 3 ] { 8, 9, 10 };
    std::copy_n( std::begin( q ), std::size( p ), std::begin( p ) ); // copy q to p
}


int main( )
{
    std::array<int, 3> arr;
    // int arr[ 3 ]; // or this one

    byReference( arr );

    std::cout << "The array is: ";
    std::copy_n( std::begin( arr ), std::size( arr ),
                 std::ostream_iterator<int>( std::cout, " " ) ); // copy the elements of arr to
                                                                 // the stdout (aka print them)
    std::cout << '\n';
}

Output:

The array is: 8 9 10