How to write a QList into a binary file?

[Lawand]

How can I write a QList into a binary file since the QDataStream.operator<<() and QDataStream.operator>>() have no corresponding overload in which I can pass in a QList.

In the application I am writing, there is a Patient class which I have a QList of and I want to read it from a binary file at startup and manipulate it throughout the execution, then at the end, write it back into the same file.

Any advice?

[spirit]

it depends which type you use in template.

[Lawand]

Thanks for replying, the QList is of type Patient which is a class I defined, and I am getting an error when trying to write the list to a binary file, here is some code:

class Patient
{
public:
	Patient();
private:
	QString firstName;
	QString lastName;
};
 
int main()
{
	QList<Patient> patientList;
	QFile qFile("Patient_List.bin");
	if (qFile.open(QIODevice::ReadOnly))
	{
		QDataStream in(&qFile);
		in >> patientList;	//error here
	}
}

class Patient
{
public:
	Patient();
private:
	QString firstName;
	QString lastName;
};

int main()
{
	QList<Patient> patientList;
	QFile qFile("Patient_List.bin");
	if (qFile.open(QIODevice::ReadOnly))
	{
		QDataStream in(&qFile);
		in >> patientList;	//error here
	}
}

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[spirit]

implement your own QDataStrem:: operator << & >> for this type and everything should work.

[Lawand]

You mean something like this:

QDataStream& QDataStream::operator>>(Patient patient)
{
	*this >> patient.firstName;
	*this >> patient.lastName;
 
    return *this;
}
 
QDataStream& QDataStream::operator<<(Patient patient)
{
	*this << patient.firstName;
	*this << patient.lastName;
 
    return *this;
}

QDataStream& QDataStream::operator>>(Patient patient)
{
	*this >> patient.firstName;
	*this >> patient.lastName;

    return *this;
}

QDataStream& QDataStream::operator<<(Patient patient)
{
	*this << patient.firstName;
	*this << patient.lastName;

    return *this;
}

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[spirit]

almost, I would use these signatures.

QDataStream &operator<<(QDataStream &in, const Patient &patient)
{
...
}
 
QDataStream &operator>>(QDataStream &in, Patient &patient)
{
...
}

QDataStream &operator<<(QDataStream &in, const Patient &patient)
{
...
}

QDataStream &operator>>(QDataStream &in, Patient &patient)
{
...
}

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[Lawand]

Please bear with me ,
I defined those in the patient.cpp file:

QDataStream& QDataStream::operator<<(QDataStream &in, const Patient &patient)
{
	in << patient.firstName;
	in << patient.lastName;
 
    return in;
}
 
QDataStream& QDataStream::operator>>(QDataStream &out, Patient &patient)
{
	out >> patient.firstName;
	out >> patient.lastName;
 
    return out;
}

QDataStream& QDataStream::operator<<(QDataStream &in, const Patient &patient)
{
	in << patient.firstName;
	in << patient.lastName;

    return in;
}

QDataStream& QDataStream::operator>>(QDataStream &out, Patient &patient)
{
	out >> patient.firstName;
	out >> patient.lastName;

    return out;
}

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but I got these errors at the opening curly brace of operator<<:

Multiple markers at this line
	- non-inline function 'QDataStream& QDataStream::operator<<(QDataStream&, const 
	 Patient&)' is defined after prior declaration as dllimport: attribute ignored
	- prototype for `QDataStream& QDataStream::operator<<(QDataStream&, const Patient&)' 
	 does not match any in class `QDataStream'
	- `QDataStream& QDataStream::operator<<(QDataStream&, const Patient&)' must take 
	 exactly one argument

Multiple markers at this line
	- non-inline function 'QDataStream& QDataStream::operator<<(QDataStream&, const 
	 Patient&)' is defined after prior declaration as dllimport: attribute ignored
	- prototype for `QDataStream& QDataStream::operator<<(QDataStream&, const Patient&)' 
	 does not match any in class `QDataStream'
	- `QDataStream& QDataStream::operator<<(QDataStream&, const Patient&)' must take 
	 exactly one argument

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and these errors at the opening curly brace of operator>>:

Multiple markers at this line
	- `QDataStream& QDataStream::operator>>(QDataStream&, Patient&)' must take 
	 exactly one argument
	- prototype for `QDataStream& QDataStream::operator>>(QDataStream&, Patient&)' 
	 does not match any in class `QDataStream'

Multiple markers at this line
	- `QDataStream& QDataStream::operator>>(QDataStream&, Patient&)' must take 
	 exactly one argument
	- prototype for `QDataStream& QDataStream::operator>>(QDataStream&, Patient&)' 
	 does not match any in class `QDataStream'

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what's wrong?

[JhonJames]

Here:

//spirit's code:
QDataStream &operator<<(QDataStream &in, const Patient &patient)
 
 
//Your code:
QDataStream& QDataStream::operator<<(QDataStream &in, const Patient &patient)
             ^^^^^^^^^^^^^

//spirit's code:
QDataStream &operator<<(QDataStream &in, const Patient &patient)


//Your code:
QDataStream& QDataStream::operator<<(QDataStream &in, const Patient &patient)
             ^^^^^^^^^^^^^

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[spirit]

try this example
h - file

class Foo
{
public:
	Foo(int number = 0, const QString &name = QString()) 
		: m_number(number), m_name(name)
	{}
 
	void setNumber(int number) { m_number = number; }
	int number() const { return m_number; }
 
	void setName(const QString &name) { m_name = name; }
	QString name() const { return m_name; }
 
private:
	int m_number;
	QString m_name;
};
 
QDataStream &operator<<(QDataStream &out, const Foo &foo);
QDataStream &operator>>(QDataStream &in, Foo &foo);

class Foo
{
public:
	Foo(int number = 0, const QString &name = QString()) 
		: m_number(number), m_name(name)
	{}

	void setNumber(int number) { m_number = number; }
	int number() const { return m_number; }

	void setName(const QString &name) { m_name = name; }
	QString name() const { return m_name; }

private:
	int m_number;
	QString m_name;
};

QDataStream &operator<<(QDataStream &out, const Foo &foo);
QDataStream &operator>>(QDataStream &in, Foo &foo);

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cpp-file

QFile file("test.txt");
if (!file.open(QIODevice::WriteOnly))
	return;
 
QDataStream out(&file);
QList<Foo> res;
for (int i = 0; i < 11; ++i) {
	Foo f(i, tr("name%1").arg(i));
	res << f;
}
out << res;
file.close();
 
if (!file.open(QIODevice::ReadOnly))
	return;
 
QDataStream in(&file);	
res.clear();
in >> res;
 
for (int i = 0; i < res.size(); ++i) {
	const Foo f = res.at(i);
	qDebug() << f.number() << " " << f.name();
}

QFile file("test.txt");
if (!file.open(QIODevice::WriteOnly))
	return;

QDataStream out(&file);
QList<Foo> res;
for (int i = 0; i < 11; ++i) {
	Foo f(i, tr("name%1").arg(i));
	res << f;
}
out << res;
file.close();

if (!file.open(QIODevice::ReadOnly))
	return;

QDataStream in(&file);	
res.clear();
in >> res;

for (int i = 0; i < res.size(); ++i) {
	const Foo f = res.at(i);
	qDebug() << f.number() << " " << f.name();
}

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[Lawand]

Thank you JhonJames and spirit, writing and reading the List are working!

[eekhoorn12]

I have a similar kind of problem. I want to write a QList with custom classes to a binary file.

I want to write a

static QList<Server> serversList;

static QList<Server> serversList;

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to a binary file

I my server.cpp i have the following lines of code:

QDataStream& operator<<(QDataStream& out, const Server& server)
{
    out << server.serverAddres;
    ...
    out << server.serverUseSsl;
    return out;
}
 
QDataStream& operator>>(QDataStream& in,Server& server)
{
    in >> server.serverAddres;
    ...
    in >> server.serverUseSsl;
    return in;
}

QDataStream& operator<<(QDataStream& out, const Server& server)
{
    out << server.serverAddres;
    ...
    out << server.serverUseSsl;
    return out;
}

QDataStream& operator>>(QDataStream& in,Server& server)
{
    in >> server.serverAddres;
    ...
    in >> server.serverUseSsl;
    return in;
}

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In the file serverlist.cpp i do the operation:

if(!serverDataFile.open(QIODevice::ReadWrite))
    {
        return;
    }
 
    outputStream = new QDataStream(&serverDataFile);
    outputStream << listofServers;

if(!serverDataFile.open(QIODevice::ReadWrite))
    {
        return;
    }

    outputStream = new QDataStream(&serverDataFile);
    outputStream << listofServers;

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but when i compile i get the following error;

O:/projects/Post_Program/serverlist.cpp:44: error: no match for 'operator<<' in '((serverList*)this)->serverList::outputStream << serverList::listofServers'

O:/projects/Post_Program/serverlist.cpp:44: error: no match for 'operator<<' in '((serverList*)this)->serverList::outputStream << serverList::listofServers'

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[Lawand]

I think outputStream should be a QDataStream instead of QDataStream* for your code to work...

Or (if it has to be a QDataStream*) you gotta find another to call operator<<() such as:

(*outputStream) << listofServers

(*outputStream) << listofServers

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