C++: std::array invalid array assignment [duplicate]

Question

I have the below code. What am I doing wrong that I can't simple assign the array myBar to the 1st index of foo? Using memcpy works but somehow I don't want to use this approach.

#include <array>
#include <string.h>
#include <stdio.h>
    
using bar = struct bar
{
    int  x;
    char y;
};
    
auto main() -> int
{
    std::array<bar [3], 4> foo;
    
    bar myBar [3] = { { 1, 'a'}, { 2, 'b'}, {3, 'c'} };
    
    //  Compiler error "Invalid array assigment" - comment it to get an executable file.
    foo [0] = myBar; 
    
    // This works and foo [0] has also the correct content
    memcpy( foo [0], myBar, sizeof(myBar) );
        
    for ( auto const & fooBar : foo [0] ) {
        printf( "x=%d - y=%c\n", fooBar.x, fooBar.y );
    }
    
    return 0;
}

Full working MRE:
https://onlinegdb.com/fdMYwcFNus

Answer 1

foo [0] = myBar; generates a compilation error because C arrays are non-assignable:
If you have 2 C arrays - x and y, you cannot use x = y;.
And in your case foo[0] and myBar are C arrays (not std::arrays).

A solution would be to use std::array consistently (which is recommended anyway), without mixing with C arrays:

#include <array>
#include <iostream>

struct bar {
    int  x;
    char y;
};

auto main() -> int {
    std::array<std::array<bar, 3>, 4> foo;
    std::array<bar, 3> myBar = { bar{ 1, 'a'}, bar{ 2, 'b'}, bar{3, 'c'} };
    foo[0] = myBar;
    for (auto const& fooBar : foo[0]) {
        std::cout << "x=" << fooBar.x << " - y=" << fooBar.y << "\n";
    }
}

Output:

x=1 - y=a
x=2 - y=b
x=3 - y=c

Live demo

Notes:

1. As you can see above, there's no need for a using statement (in C++, struct bar {...} is already a type).
2. I used the recommended C++ I/O instead of C's printf (from C++23, there is even a better alternative - std::print).

Answer 2

You can’t directly assign one C-style array to another in C++, that's why foo[0] = myBar; fails. Arrays don’t support the = operator. The reason memcpy() works is because it just does a raw memory copy, but that's not type-safe. A more C++-friendly fix is to use std::array for the inner array (so both the outer and inner parts are assignable), or use std::copy_n / std::to_array to copy the contents.