在C ++中传递成员函数指针 [英] Pass member function pointer in C++

问题描述

我试图传递一个函数指针（ QScriptEngine :: FunctionSignature （= QScriptValue（QScriptContext *，QScriptEngine *）））到另一个函数。但我需要传递的函数是一个类的成员函数。

我这样使用：

  class MyClass 
 {
 SomeVarType someVarINeedAccessTo; 
 
 QScriptValue print（QScriptContext * context，QScriptEngine * engine）
 {
 ... someVarINeedAccessTo ... 
} 
 
 void someFunction （）
 {
 QScriptEngine engine; 
 QScriptValue printFunction = engine.newFunction（print）; 
 engine.globalObject（）。setProperty（print，printFunction）; 
} 
};在这个例子中，我得到错误：
 p> 错误：没有匹配的函数调用QScriptEngine :: newFunction（<未解决的重载函数类型>）
注意：候选是：...  p> 
 
 
如何将打印函数传递给newFunction？
 
 
 
  EDIT： > 
 
 
我这样修改它：
  class MyClass 
 {
 public：
 ... 
 
 class TheEngine：public QScriptEngine 
 {
 public：
 MyClass * instance; 
}; 
 
 static QScriptValue print（QScriptContext * context，QScriptEngine * engine）; 
 
 void someFunction（）; 
 
 ... 
}; 
 
 Myclass :: someFunction（）
 {
 TheEngine engine; 
 
 ... 
 
 QScriptVaue printFunction = engine.createFunction（print）; 
 engine.globalObject（）。setProperty（print，printFunction）; 
 
 ... 
} 
 
 QScriptValue MyClass :: print（QScriptContext * context，QScriptEngine * engine）
 {
 TheEngine * eng = dynamic_cast< TheEngine *>（engine）; 
 eng-> instance-> doSomething（...）; 
 return engine-> undefinedValue（）; 
} 
  
 
 
解决方案
一个函数不是一个方法，所以签名是不同的。与其他语言相反，在C ++中一个方法不绑定到一个对象，所以它作为一个方法的签名，需要应用于一个对象。
 
 
 
你应该做的是包装一个合适的函数（或一个静态的）与良好的签名，并调用你想要的对象作为argumment传递的对象。
 
 
 
 like 
  MyClass：public QScriptEngine {
 static QScriptValue static_print（QScriptContext * context，QScriptEngine * engine）
 {
 MyClass * my_engine = dynamic_cast< MyClass>（engine）
 my_engine-> print（context）; 
} 
 
 //重新定义print以只接受一个参数。 
  
} 
 
 
 
传递your_class作为上下文的属性而不是引擎，但这是想法。你需要一个静态包装器的方法，在参数的某个地方的对象上应用你的函数。
 
I am trying to pass a function pointer (of type QScriptEngine::FunctionSignature (= QScriptValue (QScriptContext *, QScriptEngine *))) to an other function. But the function I need to pass is a member function of a class.

I use it like this:
class MyClass
{
    SomeVarType someVarINeedAccessTo;

    QScriptValue print(QScriptContext* context, QScriptEngine* engine)
    {
        ... someVarINeedAccessTo ...
    }

    void someFunction()
    {
        QScriptEngine engine;
        QScriptValue printFunction = engine.newFunction(print);
        engine.globalObject().setProperty("print", printFunction);
    }
};
With this example I get the error:

error: no matching function for call to QScriptEngine::newFunction(<unresolved overloaded function type>)
note: candidates are: ...

How can I pass the print function to newFunction?

EDIT:

I fixed it like this:
class MyClass
{
    public:
         ...

         class TheEngine : public QScriptEngine
         {
             public:
                  MyClass* instance;
         };

         static QScriptValue print(QScriptContext *context, QScriptEngine *engine);

         void someFunction();

         ...
 };

 Myclass::someFunction()
 {
     TheEngine engine;

     ...

     QScriptVaue printFunction = engine.createFunction(print);
     engine.globalObject().setProperty("print", printFunction);

     ...
 }

 QScriptValue MyClass::print(QScriptContext* context, QScriptEngine* engine)
 {
      TheEngine* eng = dynamic_cast<TheEngine*>(engine);
      eng->instance->doSomething(...);
      return engine->undefinedValue();
 }

解决方案
That doesn't work because a function is not a method and so the signature are different. As opposed to other language, in C++ a method is not bound to an object, so it as the signature of a method and need to be applied on a object.

What you should do is wrap a proper function which (or a static one) with the good signature and call the method you want on the object passed as argumment.

like 
MyClass : public QScriptEngine {
  static QScriptValue static_print(QScriptContext* context, QScriptEngine* engine)
  {
    MyClass *my_engine = dynamic_cast<MyClass>(engine)
    my_engine->print(context);
  }

// redefine print to take only one argument.
}

Maybe you would like instead to pass your_class as an attribute of the context rather than being the engine, but that's the idea. You need a static wrapper to your method with somewhere in the argument the object to apply your function on it.

                        
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