Details
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Bug
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Resolution: Won't Do
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P3: Somewhat important
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None
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5.12.5
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None
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Linux/X11
Description
Using QtScript with gcc9 with -Wcast-function-type causes compiler warnings when compiling calls to
template<typename T> int qScriptRegisterMetaType( QScriptEngine *eng, QScriptValue (*toScriptValue)(QScriptEngine *, const T &t), void (*fromScriptValue)(const QScriptValue &, T &t), const QScriptValue &prototype = QScriptValue(), T * /* dummy */ = 0);
The warnings look like this
In file included from ../../../include/qt5/QtScript/QtScript:10, from src/scriptengine.h:9, from src/scriptengine.cpp:1: ../../../include/qt5/QtScript/qscriptengine.h: In instantiation of 'int qScriptRegisterMetaType(QScriptEngine*, QScriptValue (*)(QScriptEngine*, const T&), void (*)(const QScriptValue&, T&), const QScriptValue&, T*) [with T = QDocumentCursor]': src/scriptengine.cpp:454:127: required from here ../../../include/qt5/QtScript/qscriptengine.h:387:18: warning: cast between incompatible function types from 'QScriptValue (*)(QScriptEngine*, const QDocumentCursor&)' to 'QScriptEngine::MarshalFunction' \{aka 'QScriptValue (*)(QScriptEngine*, const void*)'} [-Wcast-function-type] 387 | eng, id, reinterpret_cast<QScriptEngine::MarshalFunction>(toScriptValue), | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ../../../include/qt5/QtScript/qscriptengine.h:388:9: warning: cast between incompatible function types from 'void (*)(const QScriptValue&, QDocumentCursor&)' to 'QScriptEngine::DemarshalFunction' \{aka 'void (*)(const QScriptValue&, void*)'} [-Wcast-function-type] 388 | reinterpret_cast<QScriptEngine::DemarshalFunction>(fromScriptValue), | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The warnings are caused by the fact that QtScript does
reinterpret_cast<QScriptEngine::MarshalFunction>(toScriptValue)
and
reinterpret_cast<QScriptEngine::DemarshalFunction>(fromScriptValue)
The type of toScriptValue is QScriptValue 
(QScriptEngine *, const T &t)
and the type of MarshalFunction is QScriptValue (QScriptEngine *, const void *)
The difference being that one function gets "const T &t" as its last argument and the other gets "const void *"
The same kind of issue exists with DemarshalFunction
Qt is able to get away with the reinterpret_cast because most (probably all) C++ compilers implement references internally as pointers, but nothing in the C++ standard guarantees that so it could break at some point. It is probably a good idea to add a bit of extra code and do explicitly the conversion between "const &" and "const void *". If done properly the extra code will be optimized away by the compiler.