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POQt QString cloning Segmentation Fault
 

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  1. POQt QString cloning Segmentation Fault
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    <p>I'm building my first Qt app using Qt Creator, and everything was going fine until I started getting a strange SIGSEGV from a line apparently harmless.</p> <p>This is the error:</p> <blockquote> <p>Program received signal SIGSEGV, Segmentation fault. 0x0804e2fe in QBasicAtomicInt::ref (this=0x0) at /usr/lib/qt/include/QtCore/qatomic_i386.h:120</p> </blockquote> <p>By backtracing the exception on <strong>gdb</strong>, I found that a simple getter is passing a NULL pointer to the clone constructor when I return my attribute.</p> <p>Backtrace output:</p> <blockquote> <p><code>(gdb) backtrace</code><br> <code>#0 0x0804e2fe in QBasicAtomicInt::ref (this=0x0) at /usr/lib/qt/include/QtCore/qatomic_i386.h:120</code><br> <code>#1 0x0804eb1b in QString (this=0xbfcc8e48, other=@0xbfcc8e80) at /usr/lib/qt/include/QtCore/qstring.h:712</code><br> <code>#2 0x0805715e in Disciplina::getId (this=0xbfcc8e7c) at disciplina.cpp:13</code><br> [...] </p> </blockquote> <p>Inspecting the pointer passed to the QString constructor:</p> <blockquote> <p>(gdb) x 0xbfcc8e80<br> 0xbfcc8e80: 0x00000000 </p> </blockquote> <p>And this is disciplina.cpp:13</p> <pre><code>QString Disciplina::getId() { return id; } </code></pre> <p>So, all points towards the copy constructor of QString receiving an empty pointer, which makes no sense to me. <strong>id</strong> was declared as a private QString.</p> <pre><code>private: QString id; </code></pre> <p>Well, I have no clue of what could be going on, and my debugging skills only go so far, so if anyone could throw an idea I'd be really glad.</p> <p>Thanks.</p> <p><strong>edit</strong></p> <p>More code, as requested.</p> <p><em>disciplina.h</em></p> <pre><code>#ifndef DISCIPLINA_H #define DISCIPLINA_H #include &lt;QString&gt; #include &lt;QMap&gt; #include "curso.h" #include "turma.h" class Curso; class Turma; class Disciplina { private: unsigned short int serie; QString id; QString nome; Curso* curso; QMap&lt;unsigned int, Turma*&gt; turmas; public: Disciplina(QString id, Curso* curso, QString nome, unsigned short int serie); QString getId(); const Curso getCurso(); QString getNome(); void setNome(QString nome); void addTurma(Turma* t, unsigned int id); QMap&lt;unsigned int, Turma*&gt; getTurmas(); }; #endif // DISCIPLINA_H </code></pre> <p><em>disciplina.cpp</em></p> <pre><code>#include "disciplina.h" Disciplina::Disciplina(QString id, Curso* curso, QString nome, unsigned short int serie) { this-&gt;id = id; this-&gt;curso = curso; this-&gt;nome = nome; this-&gt;serie = serie; } QString Disciplina::getId() { return id; } const Curso Disciplina::getCurso() { const Curso c(*this-&gt;curso); return c; } QString Disciplina::getNome() { return this-&gt;nome; } void Disciplina::setNome(QString nome) { this-&gt;nome = nome; } void Disciplina::addTurma(Turma* t, unsigned int id) { this-&gt;turmas.insert(id, t); } QMap&lt;unsigned int, Turma*&gt; Disciplina::getTurmas() { return this-&gt;turmas; } </code></pre> <p><em>Caller function (I broke it down for easier debugging)</em></p> <pre><code>Disciplina* MainWindow::getSelectedDisciplina() { if(ui-&gt;disciplinaTurma-&gt;count() &gt; 0 &amp;&amp; currentCurso) { QMap&lt;QString, Disciplina*&gt; qm(currentCurso-&gt;getDisciplinas()); QString key = ui-&gt;disciplinaTurma-&gt;itemText(ui-&gt;disciplinaTurma-&gt;currentIndex()); Disciplina* d = qm[key]; QMessageBox::information(this, d-&gt;getId(), d-&gt;getNome()); return d; } else return NULL; } </code></pre> <h2>Solved</h2> <p>The <code>Disciplina</code> object inserted into the map was getting out of scope and therefore deleted. Since, as Jacinto pointed out, Map created a vanilla value when you try to access a nonexistent key, it looked like the object was there.</p> <p>Thank you both <strong>Jacinto</strong> and <strong>sth</strong> for your help.</p>
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    1. USGabriel Gilini
    1. USGabriel Gilini
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      1. VTUpMod
    1. COSome more code, please.
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      1. USdirkgently
    2. COdovetailing off of my answer, what type is currentCurso->getDisciplinas()) ? If its not a QMap of the same type of key/value, then this doesn't work. Additionally, why create a value type copy of the map when you have access to the original? What you're doing just doesn't make a whole lot of sense to me.
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      1. POQt QString cloning Segmentation Fault
      1. USSan Jacinto
    3. COJust for debugging reasons. I broke down everything that happened there with some redundancy and stepped one line at a time.
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      1. USGabriel Gilini
 

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