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    <p>Think of it this way. This is perfectly valid:</p> <pre><code>void Type::print_self () const { std::cout &lt;&lt; *this &lt;&lt; std::endl; } </code></pre> <p>You aren't modifying the object itself, so the use of the <code>const</code> qualifier is perfectly valid. This method is modifying <code>std::cout</code>, but that doesn't count as far as the <code>const</code>ness of <code>Type::print_self()</code> goes.</p> <p>That said, <code>mutable Cache</code> looks to me to be a contradiction in terms unless you are only using that <code>Cache</code> element for local storage within <code>Render</code>. If you are truly using it as a cache it seems a bit dubious to qualify this element as <code>mutable</code>. Use it as a cache (e.g., across calls to <code>Render</code> rather than within a call to <code>Render</code>) and you have lied to both the compiler and to the user of the class.</p> <p><strong>Edit</strong><br> Per the comments made by the OP, the <code>Render</code> method truly is the graphical equivalent of <code>print_self()</code>. The 'real' state of the object (presumably not shown for the sake of constructing a minimal working example) presumably isn't modified by rendering. Designating <code>Render</code> as a <code>const</code> method is the right thing to do. If the <code>Cache</code> data member reason for being is to serve as speed bump that avoids the cost of constructing and destructing it with each call to <code>Render</code> there is nothing wrong with qualifying that <code>Cache</code> member as <code>mutable</code> (which is needed so that <code>Render</code> can remain <code>const</code>).</p>
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    1. CO`Render` is indeed the only one who uses the cache. I like your example, but let me extend it a bit. Imagine you've got `SnippetManager` that provides text snippets, with a method `GetSnippet( id )`. If you provide a snippet ID in `print_self()` that's unknown, the manager loads it. So `SnippetManager`'s _state_ is changed (not to the outside, because it will always return the same values); is it okay to declare `GetSnippet()` as const?
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    2. CO@Tank: If I was reviewing that `SnippetManager` code, I would look askance at `GetSnippet` being qualified as `const`. That said, I intentionally used `print_self` because I assumed that your `Render` method is the graphical equivalent of `print_self`. Your use of `const` and `mutable` here are probably fine if the cache is used only for rendering and you made it an instance variable as opposed to a local variable within `Render` for performance reasons. Do be ready to justify your use of `const` and `mutable` here during code review (assuming you have code reviews).
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    3. COThank you very much. As your answer & comments pointed me into the right direction, I set it as the accepted answer.
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