Why is "this" NULL? C++ Segmentation fault [closed] - c++

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I'm building a Qt app and it's crashing because of a segmentation fault. After investigation, I found out that the cause of the segfault is that "this" is NULL and I try to access a member variable in the readInputFile(QString path) method.
In this line input += line;
I don't understand why this is happening. how can "this" be NULL ?
Here's where the object is created
void MainWindow::on_inpFileCheck_clicked()
{
if (ui->inpFileCheck->isChecked()) {
QString filePath = QFileDialog::getOpenFileName(this,tr("Open CSV file"), "/home", tr("CSV (*.csv)"));
myAlgo->readInputFile(filePath);
ui->inputEdit->clear();
ui->inputEdit->appendPlainText(myAlgo->getInput());
}
}
Here's the BaseAlgorithm header
#include "qstring.h"
#include "qmainwindow.h"
class BaseAlgorithm
{
public:
BaseAlgorithm();
QString readInputFile(QString);
int lenArr;
private:
QString input;
QString output;
};
And here's the implementation and where the problem happens
#include "basealgorithm.h"
#include "qfile.h"
#include "qtextstream.h"
BaseAlgorithm::BaseAlgorithm() {
numComparisons = 0;
input = "";
output = "";
intArr = NULL;
}
QString BaseAlgorithm::readInputFile(QString path) {
QFile inpFile(path);
if (inpFile.open(QIODevice::ReadOnly | QIODevice::Text)) {
QTextStream in(&inpFile);
while (!in.atEnd()) {
QString line = in.readLine();
input += line; // crash happens here
}
return input;
}
else {
return "ERROR";
}
}

Answer 1

In C++, it is perfectly possible to call a method via a null pointer to an object. As long as this is not actually dereferenced, the function would work fine.
UPDATE: The behavior is what is often exhibited by implementations, as this is usually treated as just another parameter in the compiled code. However, as #manni66 points out, the standard doesn't actually mandate the result of calling a method on a nullptr.

Answer 2

It is pretty obvious that this is NULL because myAlgo is NULL where it is called. So the question asked in the title here is answered.
Why is myAlgo NULL? We don't know because you didn't show that code.

Answer 3

I fixed it. The object was being created late.