- Qt 6.6
- Qt QML
- Qt Quick Compiler
- QML script compiler
- Qt 6.6.1 Reference Documentation

QML type compiler

# QML script compiler

The QML script compiler compiles functions and expressions in QML and JavaScript files to a byte code that can be interpreted or Just-in-time compiled by the QML engine.

In addition, it compiles some functions and expressions in QML files into C++ code, within limitations set by the nature of JavaScript. It generates C++ code for functions that can be exhaustively analyzed. The following flow chart explains the compilation workflow.

## qmlsc and qmlcachegen

QML script compiler is available in two versions. One is qmlcachegen, which is a part of the Qt Quick Compiler. Another one is qmlsc, which is a part of the commercial-only add-on Qt Quick Compiler Extensions.

The base functionality of the QML script compiler is available in both the qmlsc tool and the qmlcachegen tool. qmlsc can generate more efficient C++ code when run in direct mode. It is enabled by passing `--direct-calls` via `QT_QMLCACHEGEN_ARGUMENTS` to qt_add_qml_module. In direct mode qmlsc expects that all C++ types used in your QML code can be made available to the generated code by simple C++ include statements. This means you have to link to private Qt APIs by passing Qt::QuickPrivate, Qt::QmlPrivate, etc to link_libraries() in CMake. qmlcachegen does not implement direct mode and ignores the –direct-calls option. Furthermore, qmlsc accepts a –static option that you can also pass via QT_QMLCACHEGEN_ARGUMENTS in qt_add_qml_module. In static mode, qmlsc ignores shadowing of properties. This is dangerous if your code actually shadows properties, but allows for more JavaScript code to be compiled to C++. qmlcachegen ignores the –static option.

## Limitations when compiling JavaScript to C++

Many JavaScript constructs cannot be efficiently represented in C++. The QML script compiler skips the C++ code generation for functions that contain such constructs and only generates byte code to be interpreted or run through the Just-in-time compiler. Most common QML expressions are rather simple: value lookups on QObjects, arithmetics, simple if/else or loop constructs. Those can easily be expressed in C++, and doing so makes your application run faster.

QML type compiler