forcing use of cbegin()/cend() in range-based for

Question

This question refers to:

When should I use the new ranged-for and can I combine it with the new cbegin/cend?

Based on that question, to force the use of

Answer 1

Update: std::as_const will be in C++17, in the <utility> header.

Prior to C++17, there's no built-in syntax for it; however, you can easily write a convenience wrapper:

template<typename T> constexpr const T &as_const(T &t) noexcept { return t; }
for (auto &v: as_const(container))

Note that this calls begin() const rather than cbegin() specifically; the Standard container general requirements specify that cbegin() and begin() const behave identically.

If your container treats non-const iteration specially, it might make sense for it itself to have a member function:

const Container &crange() const noexcept { return *this; }
for (auto &v: container.crange())

Answer 2

The range-based for loop never uses cbegin() or cend(). (Therefore there is no way to force it.) There are surprisingly many rumors to the contrary; some believe that cbegin() and cend() are used, but never try whether the same code would compile without begin() and end(). A trivial example follows. Presumably, only begin and end will be printed out, no matter how many const_casts are added.

#include <iostream>

class Iterable {
  struct Iterator {
    bool operator !=(const Iterator &) { return false; }
    int operator *(){ return 0; }
    Iterator& operator ++() { return *this; }
  };
public:
  Iterator cbegin() const noexcept {
    std::cout << "cbegin" << std::endl;
    return Iterator{};
  }
  Iterator cend() const noexcept {
    std::cout << "cend" << std::endl;
    return Iterator{};
  }
  Iterator begin() const noexcept {
    std::cout << "begin" << std::endl;
    return Iterator{};
  }
  Iterator end() const noexcept {
    std::cout << "end" << std::endl;
    return Iterator{};
  }
};

int main() {
  Iterable a;
  const Iterable b;
  for (auto i : a) {}
  for (auto i : b) {}
  for (const auto &i : a) {}
  for (const auto &i : b) {}
  return 0;
}

Answer 3

const auto& const_container = container;

for (const auto& v: const_container ) {