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    <p>No. However, most of the time they will be.</p> <p>While it's helpful to think of <code>const</code> as "thread-safe" and <code>mutable</code> as "(already) thread-safe", <code>const</code> is still fundamentally tied to the notion of promising "I won't change this value". It always will be.</p> <p>I have a long-ish train of thought so bear with me.</p> <p>In my own programming, I put <code>const</code> everywhere. If I have a value, it's a bad thing to change it unless I say I want to. If you try to purposefully modify a const-object, you get a compile-time error (easy to fix and no shippable result!). If you accidentally modify a non-const object, you get a runtime programming error, a bug in a compiled application, and a headache. So it's better to err on the former side and keep things <code>const</code>.</p> <p>For example: </p> <pre><code>bool is_even(const unsigned x) { return (x % 2) == 0; } bool is_prime(const unsigned x) { return /* left as an exercise for the reader */; } template &lt;typename Iterator&gt; void print_special_numbers(const Iterator first, const Iterator last) { for (auto iter = first; iter != last; ++iter) { const auto&amp; x = *iter; const bool isEven = is_even(x); const bool isPrime = is_prime(x); if (isEven &amp;&amp; isPrime) std::cout &lt;&lt; "Special number! " &lt;&lt; x &lt;&lt; std::endl; } } </code></pre> <p>Why are the parameter types for <code>is_even</code> and <code>is_prime</code> marked <code>const</code>? Because from an implementation point of view, changing the number I'm testing would be an error! Why <code>const auto&amp; x</code>? Because I don't intend on changing that value, and I want the compiler to yell at me if I do. Same with <code>isEven</code> and <code>isPrime</code>: the result of this test should not change, so enforce it.</p> <p>Of course <code>const</code> member functions are merely a way to give <code>this</code> a type of the form <code>const T*</code>. It says "it would be an error in implementation if I were to change some of my members". </p> <p><code>mutable</code> says "except me". This is where the "old" notion of "logically const" comes from. Consider the common use-case he gave: a mutex member. You <em>need</em> to lock this mutex to ensure your program is correct, so you need to modify it. You don't want the function to be non-const, though, because it would be an error to modify any other member. So you make it <code>const</code> and mark the mutex as <code>mutable</code>.</p> <p><em>None of this has to do with thread-safety.</em></p> <p>I think it's one step too far to say the new definitions replace the old ideas given above; they merely compliment it from another view, that of thread-safety. </p> <p>Now the point of view Herb gives that if you have <code>const</code> functions, they need to be thread-safe to be safely usable by the standard library. As a corollary of this, the only members you should really mark as <code>mutable</code> are those that are already thread-safe, because they are modifiable from a <code>const</code> function:</p> <pre><code>struct foo { void act() const { mNotThreadSafe = "oh crap! const meant I would be thread-safe!"; } mutable std::string mNotThreadSafe; }; </code></pre> <p>Okay, so we know that thread-safe things <em>can</em> be marked as <code>mutable</code>, you ask: should they be?</p> <p>I think we have to consider both view simultaneously. From Herb's new point of view, yes. They are thread safe so do not need to be bound by the const-ness of the function. But just because they <em>can</em> safely be excused from the constraints of <code>const</code> doesn't mean they have to be. I still need to consider: would it be an error in implementation if I did modify that member? If so, it needs to not be <code>mutable</code>!</p> <p>There's a granularity issue here: some functions may need to modify the would-be <code>mutable</code> member while others don't. This is like wanting only some functions to have friend-like access, but we can only friend the entire class. (It's a language design issue.)</p> <p>In this case, you should err on the side of <code>mutable</code>.</p> <p>Herb spoke just slightly too loosely when he gave a <code>const_cast</code> example an declared it safe. Consider:</p> <pre><code>struct foo { void act() const { const_cast&lt;unsigned&amp;&gt;(counter)++; } unsigned counter; }; </code></pre> <p>This is safe under most circumstances, except when the <code>foo</code> object itself is <code>const</code>:</p> <pre><code>foo x; x.act(); // okay const foo y; y.act(); // UB! </code></pre> <p>This is covered elsewhere on SO, but <code>const foo</code>, implies the <code>counter</code> member is also <code>const</code>, and modifying a <code>const</code> object is undefined behavior.</p> <p>This is why you should err on the side of <code>mutable</code>: <code>const_cast</code> does not quite give you the same guarantees. Had <code>counter</code> been marked <code>mutable</code>, it wouldn't have been a <code>const</code> object.</p> <p>Okay, so if we need it <code>mutable</code> in one spot we need it everywhere, and we just need to be careful in the cases where we don't. Surely this means all thread-safe members should be marked <code>mutable</code> then?</p> <p>Well no, because not all thread-safe members are there for internal synchronization. The most trivial example is some sort of wrapper class (not always best practice but they exist):</p> <pre><code>struct threadsafe_container_wrapper { void missing_function_I_really_want() { container.do_this(); container.do_that(); } const_container_view other_missing_function_I_really_want() const { return container.const_view(); } threadsafe_container container; }; </code></pre> <p>Here we are wrapping <code>threadsafe_container</code> and providing another member function we want (would be better as a free function in practice). No need for <code>mutable</code> here, the correctness from the old point of view utterly trumps: in one function I'm modifying the container <em>and that's okay because I didn't say I wouldn't</em> (omitting <code>const</code>), and in the other I'm not modifying the container <em>and ensure I'm keeping that promise</em> (omitting <code>mutable</code>).</p> <p>I think Herb is arguing the most cases where we'd use <code>mutable</code> we're also using some sort of internal (thread-safe) synchronization object, and I agree. Ergo his point of view works most of the time. But there exist cases where I simply <em>happen</em> to have a thread-safe object and merely treat it as yet another member; in this case we fall back on the old and fundamental use of <code>const</code>.</p>
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    1. CO+1, but I think one key phrase that is missing is that constness relates to a method's _observable_ behavior, not necessarily logical behavior.
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