In C++, how to iterate array in reverse using for_each? - c++11

Question

In C++11, using lambda/for_each, how do we iterate an array from end?
I tried the following, but both result in infinite loop:
for_each (end(A), begin(A), [](int i) {
....
});
for_each (A.rend(), A.rbegin(), [](int i) {
...
});
Any idea? Thanks.

Answer 1

You missed this ?
Flip your rbegin & rend
for_each (A.rbegin(), A.rend(), [](int i) {
...
});
Increasing reverse iterator moves them towards the beginning of the container

Answer 2

std::for_each( A.rbegin(), A.rend(), [](int i) { /*code*/ } ); is the simple solution.
I instead have written backwards which takes a sequence, extracts the begin and end iterator from it using the free begin and end functions (with std::begin and std::end using declarations nearby -- full ADL), creates reverse iterators around them, then returns a sequence with those two reverse iterators.
It is sort of neat, because you get this syntax:
for( int i : backwards(A) ) {
// code
}
which I find easier to read than std::for_each or manual for loops.
But I am a bit nuts.
Here is a minimal backwards. A full on solution handles adl and a few corner cases better.
template<class It, class C>
struct range_for_t{
It b,e;
C c; // for lifetime
It begin()const{return b;}
It end()const{return e;}
}
template<class It, class C>
range_for_t<It,C> range_for(It b,It e,C&& c){
return {std::move(b),std::move(e),std::forward<C>(c)};
}
template<class It>
range_for_t<It,int> range_for(It b,It e){
return {std::move(b),std::move(e)};
}
A simple range for range for only. Can be augmented with perfect forwarding.
Passing C as the container that may need lifetime extending. If passed as rvalue, copy is made, otherwise just reference. It is otherwise not used.
Next part is easy:
template<class It>
auto reverse_it(It it){
return std::reverse_iterator<It>(std::move(it));
}
template<class C>
auto backwards(C&&c){
using std::begin; using std::end;
auto b=begin(c), e=end(c);
return range_for(
reverse_it(e),reverse_it(b),
std::forward<C>(c)
);
}
That is untested but should work.
One important test is ensuring it works when you feed an rvalue vec like:
for(auto x:backwards(make_vec()))
works -- that is what the mess around storing C is about. It also assumes that moved container iterators have iterators who behave nicely.

Answer 3

Boost offers a feature named reversed, that can be used with C++ 11 range based for loop as describes Yakk in his answer:
for(int i : reverse(A))
{
// code
}
or
for(int i : A | reversed)
{
// code
}

Answer 4

Since C++20, there is a convenient adaptor for this:
#include <ranges>
...
for (auto& element: container | std::views::reverse)
For example:
#include <iostream>
#include <ranges>
#include <vector>
int main()
{
std::vector<int> container {1, 2, 3, 4, 5};
for (const auto& elem: container | std::views::reverse )
{
std::cout << elem << ' ';
}
}
// Prints 5 4 3 2 1
Try it here:
https://coliru.stacked-crooked.com/a/e320e5eec431cc87