Strange problem

[calhal]

char *tmp=(char*)(txt.readLine().toStdString().c_str());
tab[i]=tmp;

char *tmp=(char*)(txt.readLine().toStdString().c_str());
tab[i]=tmp;

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I'm not a C++ expert but I think you should not do things like this. c_str() will give you a pointer internal object's storage containing null terminated string.
I'm not sure if the pointer you keep in tab[0] will be still valid when you are on next iteration after another

txt.readLine().toStdString().c_str()

txt.readLine().toStdString().c_str()

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.

Try what I wrote in a previous post - it's definitely more Qt-ish