Can`t overload the operator << for a my own class - c++

Question

I am trying to reduce code redundancy using the overload of an output operator to a stream instead of print functions.
//***.h
class MainWind : public QWidget
{
Q_OBJECT
public:
explicit MainWind(QWidget *parent = nullptr);
~MainWind();
MainWind *operator<<(const QString &str);
private:
Ui::MainWind *ui;
};
//***.cpp
MainWind *MainWind::operator<<(const QString &str)
{
ui->serverConsole->insertPlainText(str);
return this;
}
At this moment everything compiles successfully.
But when I try to use:
//other.cpp
MainWind *mainWind = new MainWind;
mainWind << QString("str");
I got this error:
ServerSocket.cpp:39: error: invalid operands to binary expression
('MainWind *' and 'QString')
qstring.h:1410: candidate function not viable: no known conversion from >'MainWind *' to 'QDataStream &' for 1st argument
...
And there are a lot of candidates for this position)
Or
//other.cpp
MainWind *mainWind = new MainWind;
mainWind <<"str";
I got this error:
ServerSocket.cpp:39: error: invalid operands to binary expression
('MainWind *' and 'const char [4]') ServerSocket.cpp:39: error:
invalid operands of types 'MainWind*' and 'const char [4]' to binary
'operator<<'
CurrentSession::inst().mainWind() << "str";
^
What could be the problem?
ADDITION TO THIS QUESTION:
Attempt to use:
//*.h
friend MainWind *operator<<(MainWind *out,QString &str);
//***.cpp
MainWind * operator<<(MainWind *out, QString &str)
{
out->ui->serverConsole->insertPlainText(str);
return out;
}
Compilation of previous code is successful.
According to the idea, if the first operand could not be a pointer, this code would not compile...
But when using this:
//other.cpp
MainWind *mW = new MainWind;
mW << "str";
Compilation go to error:
ServerSocket.cpp:37: error: invalid operands of types 'MainWind*' and 'const char [4]' to binary 'operator<<'
mW << "str";
^

Answer 1

You need to use *mainWind << QString("str");. The LHS has to be an object, not a pointer.
While at it, I strongly recommend changing the operator<< function to return a reference to the object, not a pointer.
MainWind& operator<<(const QString &str);
and the implementation to
MainWind& MainWind::operator<<(const QString &str)
{
ui->serverConsole->insertPlainText(str);
return *this;
}
That will allow you to chain the operator.
*mainWind << QString("str") << QString("Second string");

Answer 2

You overloaded << on MainWnd not MainWnd*.
*mainWind << QString("str");
Also you want QString const&

Answer 3

//other.cpp
MainWind *mainWind = new MainWind;
mainWind <<"str";
The reason is that mainWind << "str" looks for an operator<<() that accepts two arguments, the first of which is a MainWind *.
Whereas, you have defined a MainWind::operator<<() which is called with the first argument a MainWind &. There is no direct conversion from a MainWind * to a MainWind & (or to a MainWind). Hence the error message.
One way to get the code to compile is to change mainWind <<"str" to *mainWind << "str". The * dereferences the pointer, and produces a reference, which is what your operator<<() expects.
The catch is then that
*mainWind << "str" << "another str";
will not compile, since it is equivalent to
(*mainWind).operator<<("str") << "another str";
where (*mainWind).operator<<("str") returns a MainWind *. This causes the same problem (again) when trying to stream "another str".
The real fix is to change operator<<() so it returns a reference
// within the class definition of MainWind
MainWind &operator<<(const QString &str);
// definition of the operator<<()
MainWind &MainWind::operator<<(const QString &str)
{
ui->serverConsole->insertPlainText(str);
return *this;
}
and change the calling code to either
//other.cpp version 2
MainWind *mainWind = new MainWind;
*mainWind <<"str";
// this will work too
*mainWind << "str" << "another str";
// note reliance on cleaning up mainWind to avoid a memory leak
delete mainWind;
There is no other fix that would allow you to use mainWind << "str" since overloading non-member operator<<() is only permitted on class or enumerated types, not on pointers.