初始化Qb中的QPainter＃drawText seg错误

Question

Using Qt 5.7, Windows 7, MinGW 32-bit, the following program:

#include <QImage>
#include <QPainter>

int main () {
    QImage i(100, 100, QImage::Format_RGB888);
    QPainter p(&i);
    p.drawText(0, 0, "abc"); // line 7
}

Seg faults on the p.drawText call, giving the following stack trace, which ends with initializeDb :

1  initializeDb                  qfontdatabase.cpp 896  0x7930ed0 
2  QFontDatabase::findFont       qfontdatabase.cpp 2640 0x79361f6 
3  QFontDatabase::load           qfontdatabase.cpp 2795 0x7936b5e 
4  QFontPrivate::engineForScript qfont.cpp         215  0x79194ff 
5  QTextEngine::fontEngine       qtextengine.cpp   2094 0x793d24b 
6  QTextEngine::shapeText        qtextengine.cpp   1000 0x7938c0b 
7  QTextEngine::shape            qtextengine.cpp   1534 0x793b090 
8  QTextEngine::shapeLine        qtextengine.cpp   938  0x793884a 
9  QPainter::drawText            qpainter.cpp      5877 0x7a3dc91 
10 QPainter::drawText            qpainter.cpp      5700 0x7a3cfe6 
11 QPainter::drawText            qpainter.h        890  0x402a1e  
12 main                          main.cpp          7    0x4016b6  

Why is this happening and how do I make it not happen?

The .pro file, for completeness:

QT += core gui    
CONFIG += c++11
TARGET = untitled18
CONFIG += console
CONFIG -= app_bundle
TEMPLATE = app    
SOURCES += main.cpp

This is for a command line utility that generates images.

Note: Adding a QCoreApplication does not make a difference.

Answer 1

Looking at the Qt source code can help with problems like this.

Line 896 of qfontdatabase.cpp (where your stack trace shows the crash occurring at) looks like this:

QGuiApplicationPrivate::platformIntegration()->fontDatabase()->populateFontDatabase();   

.... so most likely either platformIntegration() or fontDatabase() is returning NULL for some reason.

Grepping through the source code we see that QGuiApplicationPrivate::platformIntegration() is defined here, at line 103 of gui/kernel/qguiapplication.h:

static QPlatformIntegration *platformIntegration()
{ return platform_integration; }

... so that method could definitely return NULL if the platform_integration variable hasn't been set yet to point to any valid object yet.

With just a bit more grepping around, we find that the only place where the platform_integration static variable is set is at line 1094 of gui/kernel/qguiapplication.cpp:

QGuiApplicationPrivate::platform_integration = QPlatformIntegrationFactory::create(name, arguments, argc, argv, platformPluginPath);

... which is part of a static function called init_platform(), which is called from QGuiApplicationPrivate::createPlatformIntegration(), which itself is called from various methods of the QGuiApplicationPrivate class.

But of course none of the QGuiApplicationPrivate methods can be called unless/until a QGuiApplicationPrivate object is created, which presumably won't happen unless/until you've created a QGuiApplication object.

So to sum up... it looks like Rinold is correct, you need to instantiate a QGuiApplication (or QApplication, which is a subclass of QGuiApplication) object first, before trying to use a QPainter to draw text.