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POGeneral way to reset a member variable to its original value using the stack?
 

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  1. POGeneral way to reset a member variable to its original value using the stack?
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    <p>I came across a class instance function that needed to temporarily change a class instance variable, and then restore it when the function completed. The function had return statements all over the place, and before each return there was a restoring statement. That seemed messy to me, not to mention scary when a exception is thrown.</p> <p>As an improvement I came up with this generalization using a inner class definition. Here is a sample driver program (class restorer).</p> <pre><code>class Unwind { private: bool b_active_; ///&lt; the thing I want to be restored template&lt;typename T&gt; class restorer { T* ref_; T save_; public: restorer(T* perm) : ref_(perm), save_(*ref_) {}; ~restorer() { *ref_ = save_; } }; public: Unwind() : b_active_(false) {}; void a() { out("a in"); b(); out("a end"); } void b() { out("b in"); { restorer&lt;bool&gt; trust_in_the_stack(&amp;b_active_); // "restorer" created on the stack b_active_ = true; // change b_active_ only while "within" b() c(); out("b inner end"); } out("b end"); } void c() { out("c in"); d(); out("c end"); } void d() { out("d in"); cout &lt;&lt; "deepest" &lt;&lt; endl; out("d end"); } void out(const std::string&amp; msg) { std::cout &lt;&lt; msg &lt;&lt; ": " &lt;&lt; b_active_ &lt;&lt; std::endl; } }; int main() { Unwind u; u.a(); return 0; } </code></pre> <p>The output using g++ 4.2.3 (-Wall) was:</p> <pre> a in: 0 b in: 0 c in: 1 d in: 1 deepest d end: 1 c end: 1 b inner end: 1 b end: 0 a end: 0 </pre> <p>Which is what I expect at "b end".</p> <p>I felt that defining the class restorer inside the class Unwind helps to discourage misuse.</p> <p>My question is, is there a general and safer way to do this? I am worried about lifetime issues.</p> <p>Edit: Please assume that there are no threads, but "downstream" methods on the stack that change behavior based on this b_active_ flag.</p>
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    1. COI think it would be better to replace save_(*ref_) in the initialization list with save_(*perm). save_(*ref_) may bite You one day, when someone changes order of declarations in Your class.
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      1. USMaciej Hehl
    2. CO@Maciej H: Good point, thanks!
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