QPrinter, QPrintDialog giving errors not encountered in code examples

In the imageviewer example, QPainter and QPrintDialog objects are defined and used as following:

#ifndef QT_NO_PRINTER
QPrinter printer;
#endif

and

QPrintDialog dialog(&printer, this);

A QPainter object is then initialized with QPrinter (printer).

When I tried to use the same code in my function, it looks like:

void imageviewer::print()
{
...
#ifdef QT_NO_开发者_开发问答PRINTER

QPrinter printer(this);             //ERROR 1
QPrintDialog dialog(&printer, this);//ERROR 2 and 3

if (dialog.exec())                  //ERROR 4
{
    //do the painting
}

#endif
}

The errors are:

1. variable 'QPrinter printer' has initializer but incomplete type
2. 'QPrintDialog' was not declared in this scope
3. Expected ';' before 'dialog'
4. 'dialog' was not declared in this scope

What I am not able to understand is why are these errors arising when I am using them in my code, but not in the example.

As a friend pointed out, I made sure that I used the right #include files and made sure that 'printer' and 'dialog' were not touched anywhere else in the example.

You are using #ifdef QT_NO_PRINTER in your code but the example uses #ifndef QT_NO_PRINTER

Pay attention to difference between if not defined and if defined

If your code compiles then it means that you have QT_NO_PRINTER in your project. Without a printer you can not print !

QPrinter printer(this); 

This is declaring a function (see https://en.wikipedia.org/wiki/Most_vexing_parse).

You need to write:

QPrinter printer = QPrinter(this);

or:

QPrinter printer((this));