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POHow to call constructor in gdb for pretty-printers
 

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    <p>When debugging with GDB I would like to set a convenience variable to a newly constructed value.</p> <p>I am programming using the Qt framework, so i would like to create a QString, but this is irrelevant to the question, since I would like to know how to do this with any class.</p> <p>I tried</p> <pre><code>(gdb) set $str = 'QString::QString("abc")' No symbol "QString::QString("abc")" in current context. (gdb) set $str = QString::QString("abc") Cannot resolve method QString::QString to any overloaded instance (gdb) set $str = QString("abc") A syntax error in expression, near `("abc")'. </code></pre> <p>then I tried using <code>set overload-resolution off</code>, resulting in the following:</p> <pre><code>set $str = QString::QString("abc") non-unique member `QString' requires type instantiation set $str = 'QString::QString(const char*)'("abc") Too few arguments in function call. </code></pre> <p>So I assumed a this pointer is needed:</p> <pre><code>(gdb) set $str = 'QString::QString(const char*)'(malloc(sizeof(QString)), "abc") (gdb) p $str $8 = void </code></pre> <p>Okay, constructors return void, that means I have to save the return value of malloc somewhere:</p> <pre><code>(gdb) set $pointer = malloc(sizeof(QString)) (gdb) p $pointer $9 = 6304560 (gdb) p/x $pointer $10 = 0x603330 (gdb) set $str = 'QString::QString(const char*)'($pointer, "abc") (gdb) p $str $11 = void (gdb) p $pointer $12 = 6304560 (gdb) p *((QString*)$pointer) $13 = "abc" </code></pre> <p>Okay, now this works as expected, however I want to use this code in gdb.parse_and_eval() for a python pretty printer. Now this will call malloc lots of times, creating a memory leak. So just call free()? Now something unexpected happens:</p> <pre><code>(gdb) call free($pointer) $14 = 0 (gdb) p *((QString*)$pointer) $15 = "abc" </code></pre> <p>The pointer still seems valid, which may of course be perfectly fine, since the memory has not been reused. However I am unsure whether this is okay because after allocating a few more memory blocks which fit exactly one QString the pointer value has still not been reused by malloc.</p> <p>Am I creating a big memory-leak if I use this in a pretty-printer, which may well be called lots of times during a debug session? Is there any easier solution of creating the desired result (i.e. using the Python API)?</p> <p>Also unrelated to this, why is free(3) giving me a return value of 0 while it is actually void?</p>
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