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copied!<p>It's quite simple. Say I have a vector:</p> <pre><code>std::vector<int> vec; </code></pre> <p>I fill it with some data. Then I want to get some iterators to it. Maybe pass them around. Maybe to <code>std::for_each</code>:</p> <pre><code>std::for_each(vec.begin(), vec.end(), SomeFunctor()); </code></pre> <p>In C++03, <code>SomeFunctor</code> was free to be able to <em>modify</em> the parameter it gets. Sure, <code>SomeFunctor</code> could take its parameter by value or by <code>const&</code>, but there's no way to <em>ensure</em> that it does. Not without doing something silly like this:</p> <pre><code>const std::vector<int> &vec_ref = vec; std::for_each(vec_ref.begin(), vec_ref.end(), SomeFunctor()); </code></pre> <p>Now, we introduce <code>cbegin/cend</code>:</p> <pre><code>std::for_each(vec.cbegin(), vec.cend(), SomeFunctor()); </code></pre> <p>Now, we have syntactic assurances that <code>SomeFunctor</code> cannot modify the elements of the vector (without a const-cast, of course). We explicitly get <code>const_iterator</code>s, and therefore <code>SomeFunctor::operator()</code> will be called with <code>const int &</code>. If it takes it's parameters as <code>int &</code>, C++ will issue a compiler error.</p> <hr> <p>C++17 has a more elegant solution to this problem: <a href="http://en.cppreference.com/w/cpp/utility/as_const" rel="noreferrer"><code>std::as_const</code></a>. Well, at least it's elegant when using range-based <code>for</code>:</p> <pre><code>for(auto &item : std::as_const(vec)) </code></pre> <p>This simply returns a <code>const&</code> to the object it is provided. </p>

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