meaning of array reference to 2d array.

[Aziz as]

Hello C++ coders ,
First of all, I am sorry for the poor subject for this post. I do not what this actually should be called. I have a piece of a syntax that I really could not understand.


int ia[3][4] = { 0,1,2,3,4,5,6,7,8,9,10,11 };
int(&row)[4] = ia[1];


If someone can be so kind and explain what this code does, I would appreciate that.

Thank you.

[keskiverto]

@gentleguy: That is no answer to:

explain what this code does

Lets try:

#include<iostream>

int main(){
    int ia[3][4] = { 0,1,2,3,4,5,6,7,8,9,10,11 };
    for ( auto r : ia ) {
        for ( int c=0; c<4; ++c ) {
            std::cout << " " << r[c];
        }
        // for ( auto e : r ) {   // ERROR, the r is not an array
        //    std::cout << " " << e;
        // }
        std::cout << '\n';
    }
    std::cout << '\n';

    int (&row)[4] = ia[1];
    for ( int c=0; c<4; ++c ) {
        std::cout << " " << row[c];
    }
    std::cout << "\n\n";
    
    for ( auto e : row ) { // OK, the row is an array
        std::cout << " " << e;
    }
    std::cout << "\n\n";
   
    row[2] = 42; // modifies ia
   
    for ( auto r : ia ) {
        for ( int c=0; c<4; ++c ) {
            std::cout << " " << r[c];
        }
        std::cout << '\n';
    }
	return 0;
}

The 'row' object looks like an array with known size. It is actually a reference to array; not a copy.

Usually (e.g. as function argument) array is decayed to a pointer, which does not know the size of array.


There is also an array pointer:

    int ia[3][4] = { 0,1,2,3,4,5,6,7,8,9,10,11 };
    int (*foo)[4] = ia;
    for ( int r=0; r<3; ++r ) {
        for ( auto e : foo[r] ) {
            std::cout << " " << e;
        }
        std::cout << '\n';
    }

In the pointer 'foo' a size (4) is retained, but it affects pointer math.
When we write auto bar = foo + 1;, the '+1' is actually +1*(4*sizeof(int)) bytes.