Does a For loop with Vector iterator copy values, making it inefficient?

Question

I'm using a for loop to go through all elements in a vector, and I've seen the slick code:

std::vector<int> vi;
// ... assume the vector gets populated
for(int i : vi) 
{
    // do stuff with i
}

However, from a quick test, it looks like it's copying the value from the vector into i each time (I tried modifying i in the for loop and the vector remains unchanged).

The reason I'm asking, is that I'm actually doing this with a vector of a large struct.

std::vector<MyStruct> myStructList;
for(MyStruct oneStruct : myStructList)
{
    cout << oneStruct;
}

So... is this a poor way of doing things, given the amount of memory copying? Is it more efficient to use traditional indexing?

for(int i=0; i<myStructList.size(); i++)
{
    cout << myStructList[i];
}

Thanks,

You could also just write `for(const MyStruct& oneStruct : myStructList)` to be sure to avoid unnecessary copies. — πάντα ῥεῖ, Apr 12 '21 at 22:43
See: https://stackoverflow.com/questions/2765999/what-is-a-reference-variable-in-c — Drew Dormann, Apr 12 '21 at 22:45
I believe starting in Clang 10, if there are actual "hidden" copies occurring in iterators, it now warns. — Michael Dorgan, Apr 12 '21 at 23:06
`for (const auto& i : myStructList)`. Really no reason to bother with the type. — sweenish, Apr 13 '21 at 00:56

score 3 · Answer 1 · answered Apr 12 '21 at 23:10

I tested that on Compiler Explorer and found that copying can actually be done even with -O3 optimization with gcc 10.3.

Here is my code for test:

#include <iostream>
#include <vector>
using std::cout;

struct MyStruct {
    int a[32];
};

std::ostream& operator<<(std::ostream& s, const MyStruct& m) {
    for (int i = 0; i < 32; i++) s << m.a[i] << ' ';
    return s;
}

std::vector<MyStruct> myStructList;

void test(void) {
    for(MyStruct oneStruct : myStructList)
    {
        cout << oneStruct;
    }
}

Here is a part of the result:

test():
        pushq   %r13
        pushq   %r12
        pushq   %rbp
        pushq   %rbx
        subq    $152, %rsp
        movq    myStructList(%rip), %r12
        movq    myStructList+8(%rip), %r13
        cmpq    %r13, %r12
        je      .L8
        leaq    144(%rsp), %rbp
.L11:
        movdqu  (%r12), %xmm0
        movdqu  16(%r12), %xmm1
        leaq    16(%rsp), %rbx
        movdqu  32(%r12), %xmm2
        movdqu  48(%r12), %xmm3
        movdqu  64(%r12), %xmm4
        movdqu  80(%r12), %xmm5
        movups  %xmm0, 16(%rsp)
        movdqu  96(%r12), %xmm6
        movdqu  112(%r12), %xmm7
        movups  %xmm1, 32(%rsp)
        movups  %xmm2, 48(%rsp)
        movups  %xmm3, 64(%rsp)
        movups  %xmm4, 80(%rsp)
        movups  %xmm5, 96(%rsp)
        movups  %xmm6, 112(%rsp)
        movups  %xmm7, 128(%rsp)
.L10:
        movl    (%rbx), %esi
        movl    $_ZSt4cout, %edi
        addq    $4, %rbx
        call    std::basic_ostream<char, std::char_traits<char> >::operator<<(int)
        movl    $1, %edx
        leaq    15(%rsp), %rsi
        movb    $32, 15(%rsp)
        movq    %rax, %rdi
        call    std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long)
        cmpq    %rbp, %rbx
        jne     .L10
        subq    $-128, %r12
        cmpq    %r12, %r13
        jne     .L11
.L8:
        addq    $152, %rsp
        popq    %rbx
        popq    %rbp
        popq    %r12
        popq    %r13
        ret

The lines between .L10: and jne .L11 corresponds to the operator<< function, and you can see large copying is done before that.

You should add & between MyStruct and oneStruct in the for loop to make it a reference and avoid unwanted data copies. Here is a part of a result with & added:

test():
        pushq   %r12
        pushq   %rbp
        pushq   %rbx
        subq    $16, %rsp
        movq    myStructList(%rip), %rbp
        movq    myStructList+8(%rip), %r12
        cmpq    %r12, %rbp
        je      .L8
        subq    $-128, %rbp
.L11:
        leaq    -128(%rbp), %rbx
.L10:
        movl    (%rbx), %esi
        movl    $_ZSt4cout, %edi
        addq    $4, %rbx
        call    std::basic_ostream<char, std::char_traits<char> >::operator<<(int)
        movl    $1, %edx
        leaq    15(%rsp), %rsi
        movb    $32, 15(%rsp)
        movq    %rax, %rdi
        call    std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long)
        cmpq    %rbp, %rbx
        jne     .L10
        leaq    128(%rbp), %rax
        cmpq    %rbp, %r12
        je      .L8
        movq    %rax, %rbp
        jmp     .L11
.L8:
        addq    $16, %rsp
        popq    %rbx
        popq    %rbp
        popq    %r12
        ret

Now you can see the large copying is eliminated and the pointer to the structure is directly used for execution of operator<<.

Does a For loop with Vector iterator copy values, making it inefficient?

1 Answers1