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How to use a range-based for loop

A nice extension in C++ 11 is range-based for loops. They are both readable and safer as you do not need to manually specify the boundary conditions.

There are a few ways to walk such containers. One is via a constant iterator e.g.:

By now you will have seen the following range-based for loop e.g.:

    for (auto const i : container) { }

Another, if you wish to modify the data as you go is:

    for (auto &i : container) { }

However it may be 'simpler' to always do the following and use a forward reference:

    for (auto &&i : container) { }

This handles corner cases for things like vectors of bitfields that you cannot have a reference to. Here is such a corner case. The iterator provided is actually a thing called a proxy iterator, vector::reference. So, this will not compile:

    #include <vector>
    int main() {
        std::vector<bool> v(8);
        for (auto& e : v) { e = true; }
    }

But this will!

    #include <vector>
    int main() {
        std::vector<bool> v(8);
        for (auto&& e : v) { e = true; }
    }

Here is a full example:

#include <algorithm>
#include <iostream>
#include <vector>

int main()
{
  // Create a vector of strings:
  std::initializer_list< std::string > init1 = {"elem1", "elem1"};
  std::vector< std::string >           vec1(init1);

  // Range based for loop iterator with a const:
  for (const auto &i : vec1) {
    std::cout << "vec1: walk " << i << std::endl;
  }

  // Range based for loop iterator with a modifiable reference:
  for (auto &i : vec1) {
    i += "+ stuff";
    std::cout << "vec1: walk " << i << std::endl;
  }

  // Range based for loop iterator with forward reference:
  for (auto &&i : vec1) {
    i += "+ more stuff";
    std::cout << "vec1: walk " << i << std::endl;
  }
}

To build:

cd range_based_for_loop
rm -f *.o example
clang -std=c++2a -Werror -g -O3 -fstack-protector-all -ggdb3 -Wall -c -o main.o main.cpp
clang  main.o -lstdc++  -o example
./example

Expected output:

# Create a vector of strings:

# Range based for loop iterator with a const:
vec1: walk elem1
vec1: walk elem1

# Range based for loop iterator with a modifiable reference:
vec1: walk elem1+ stuff
vec1: walk elem1+ stuff

# Range based for loop iterator with forward reference:
vec1: walk elem1+ stuff+ more stuff
vec1: walk elem1+ stuff+ more stuff