不能为我自己的类重载运算符 <<

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.