·[solved]Can anyone tell me why it returns me "Segmentation fault"?

DidaHarp

I just want to get json data on the internet.And i use QNetworkAccessManager ,Qnetworkrequest,Qnetworkreply.So simple function i am going to define! But when i run it , it always returns me that! Damn C++ , i hate it!
here is my short and simple .h and .cpp file:
SearchByJson.h:
#include <QObject>
#include <QNetworkAccessManager>
#include <QNetworkReply>

class SearchByJson : public QObject
{
Q_OBJECT
public:
SearchByJson() {}
virtual ~SearchByJson() {}

Q_INVOKABLE QString startsearch();

private slots:
void getFinish();

private:
QNetworkAccessManager qnam;
QNetworkReply *reply;
};

SearchByJson.cpp:
#include "searchbyjson.h"
#include <QUrl>
#include <QNetworkRequest>
#include <QDebug>

QString SearchByJson::startsearch(){
reply = qnam.get(QNetworkRequest(QUrl("http://so.ard.iyyin.com/s/song_with_out?q=Apologize&page=1&size=3")));
connect(reply,SIGNAL(finished()),this,SLOT(getFinish()));
}

void SearchByJson::getFinish(){
if (reply->error() == QNetworkReply::NoError)
{
qDebug() << "error";
} else {
qDebug() << "Got it!";
}
}

alex_malyu

This should not even compile since QString SearchByJson::startsearch() does not return anything.
First thing to do when you get access violation is to find where in the code you get it.

for example if you have SearchByJson::getFinish() called when reply is NULL or does not point to valid object this may happen.

Rule of thumb in C++ - every pointer must be initialized.

SearchByJson() { reply=NULL; } // will make sure pointer is not going to point to invalid objects

Then check for nulls:

void SearchByJson::getFinish(){
Q_CHECK_PTR( reply ); // will abort and let you know where it did
if (reply->error() == QNetworkReply::NoError)
{
qDebug() << "error";
} else {
qDebug() << "Got it!";
}
}

on top the way you wrote startsearch you do now want it to be called
when reply is not not NULL, right?

QString SearchByJson::startsearch(){

Q_ASSERT( reply == NULL ); // this will assert 
reply = qnam.get(QNetworkRequest(QUrl("http://so.ard.iyyin.com/s/song_with_out?q=Apologize&page=1&size=3")));
connect(reply,SIGNAL(finished()),this,SLOT(getFinish()));
}

finally you probably would want to delete reply when it was finished:

void SearchByJson::getFinish(){
Q_CHECK_PTR( reply );
if (reply->error() == QNetworkReply::NoError)
{
qDebug() << "error";
} else {
qDebug() << "Got it!";
}
delete reply;
reply = NULL;
}

Edit: Added code markers - p3c0

DidaHarp

@alex_malyu YES! According to your guidance, I become calm and find the fault. And i feel better about C++. It's not so bad... It's very nice of you. Thank you !

code_fodder

@DidaHarp You can up-vote alex's post if you found it helpful. Also its really good practise to edit your post title and put [solved] at the beginning then it is marked at the top level as solved to everyone : )

like this:
[solved] Can anyone tell me why it returns me "Segmentation fault"?

DidaHarp

@code_fodder Alright!