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POclear explanation sought: throw() and stack unwinding
 

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  1. POclear explanation sought: throw() and stack unwinding
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    <p>I'm not a programmer but have learned a lot watching others. I am writing wrapper classes to simplify things with a really technical API that I'm working with. Its routines return error codes, and I have a function that converts those to strings:</p> <pre><code>static const char* LibErrString(int errno); </code></pre> <p>For uniformity I decided to have member of my classes throw an exception when an error is encountered. I created a class:</p> <pre><code>struct MyExcept : public std::exception { const char* errstr_; const char* what() const throw() {return errstr_;} MyExcept(const char* errstr) : errstr_(errstr) {} }; </code></pre> <p>Then, in one of my classes:</p> <pre><code>class Foo { public: void bar() { int err = SomeAPIRoutine(...); if (err != SUCCESS) throw MyExcept(LibErrString(err)); // otherwise... } }; </code></pre> <p>The whole thing works perfectly: if <code>SomeAPIRoutine</code> returns an error, a try-catch block around the call to <code>Foo::bar</code> catches a standard exception with the correct error string in <code>what()</code>. </p> <p>Then I wanted the member to give more information:</p> <pre><code>void Foo::bar() { char adieu[128]; int err = SomeAPIRoutine(...); if (err != SUCCESS) { std::strcpy(adieu,"In Foo::bar... "); std::strcat(adieu,LibErrString(err)); throw MyExcept((const char*)adieu); } // otherwise... } </code></pre> <p>However, when <code>SomeAPIRoutine</code> returns an error, the <code>what()</code> string returned by the exception contains only garbage. It occurred to me that the problem could be due to <code>adieu</code> going out of scope once the <code>throw</code> is called. I changed the code by moving <code>adieu</code> out of the member definition and making it an attribute of the class <code>Foo</code>. After this, the whole thing worked perfectly: a try-call block around a call to <code>Foo::bar</code> that catches an exception has the correct (expanded) string in <code>what()</code>.</p> <p>Finally, my question: what exactly is popped off the stack (in sequence) when the exception is thrown in the if-block when the stack "unwinds?" As I mentioned above, I'm a mathematician, not a programmer. I could use a really lucid explanation of what goes onto the stack (in sequence) when this C++ gets converted into running machine code. </p>
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    1. USJerry Gagelman
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    1. COI think this should go on StackOverflow.com, not Programmers
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    2. CO`"I'm not a programmer"`: it looks like you are.
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      1. USBernd Elkemann
 

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