But I don't know why this problem is resolved.

Problem was "font.bold "

I wrote "true" on almost every "font.bold".

So, I only left a few, and I fixed most of them to "false."

it operated well.

Sorry for late reply

first.

int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
QGuiApplication app(argc, argv);

QSurfaceFormat format;
format.setSamples(8);
QSurfaceFormat::setDefaultFormat(format);

int fontID = QFontDatabase::addApplicationFont(":/font/NanumGothic.ttf");
qDebug() << "Font ID : " << fontID << QFontDatabase::applicationFontFamilies(fontID);

QFont defaultFont;

defaultFont.setFamily("NanumGothic");
defaultFont.setPixelSize(20);
defaultFont.setStyleStrategy(static_cast<QFont::StyleStrategy>(defaultFont.styleStrategy() | QFont::PreferNoShaping));
app.setFont(defaultFont);

CEventFilter cEF;
app.installEventFilter(&cEF);

CAppContainer appContainer;
appContainer.initialize();

appContainer.show();

return app.exec();

}

This is my main function.

you told me to write "QFontDatabase::addApplicationFont(":/your/qrc/path/NanumGothic.ttf");"

But I've already written it.

By any chance, is there a strange part of my main function?

If I made the character print from c++, it would be printed without any problem, but if I made the character print from QML, the above problem occurred.

So I think the font is applied to c++, but the font is not applied to QML.

But If i erase code related to the font in c++, QML totally doesn't print character.

So the font is not applied?

And

Now I'm trying to second way that you said(Appliying fonts in QML)

Do i have to make a qmldir file and put one sentence that "singleton Fonts 1.0 Fonts.qml"?

Thank for read my reply :)

// main.cpp
// register shipped font to application
QFontDatabase::addApplicationFont(":/your/qrc/path/NanumGothic.ttf");

You can also add the font from QML:

// qmldir
singleton Fonts 1.0 Fonts.qml

// Fonts.qml
pragma Singleton
import QtQuick 2.12

Item {
    property FontLoader nanumGothic: FontLoader {
        source: "qrc:/your/qrc/path/NanumGothic.ttf"
    }
    readonly property string indiFont: nanumGothic.name
}

// in any other QML
Text{
    text: "ABCDEF"
    font.family: Fonts.indiFont
}

I'm sorry

vxworks7
intel 8th
based on Qt 6.2.3
NanumGothic.ttf
through invokeMethod

in CPP
QMetaObject::invokeMethod(m_pOperPPIQmlInstance, "show_c2a_key", Q_ARG(QVariant, indi));

in QML
function show_c2a_key(indi)
{
if(indi === 1) {
c2a_key.text = "ABCDEFGHI"
}
}

and

Text{
id: c2a_key
x: 544
y: 430
color: "green"
text: ""
font.pixelSize: 16
font.bold: true
font.preferShaping: false
}

When the program runs, the terminal outputs the following statement:

QFontDatabase: Cannot find font directory /usr/local/Qt-5.15.3/lib/fonts.
Note that Qt no longer ships fonts. Deploy some (from https://dejavu-fonts.github.io/ for example) or switch to fontconfig.

Is there any connection?

What should i do

Provide more information.
On which platform?
On what hardware?
Which Qt version?
What font?
What are you using to print text?