Call對象實例從C++通過腳本調用的函數

Question

我有一個類名爲水母誰使用Singleton設計模式：

jellyfish.h

#ifndef JELLYFISH_H 
#define JELLYFISH_H 

#include <QHash> 
#include <QScriptEngine> 

class QScriptValue; 

class Jellyfish : public QObject 
{ 

    public: 

     static Jellyfish * getInstance(); 
     static Jellyfish * instance; 

    private: 

     Jellyfish(); 

}; 

#ifndef 

jellyfish.cpp

Jellyfish * Jellyfish::instance = NULL; 

Jellyfish * Jellyfish::getInstance() 
{ 

    if (!Jellyfish::instance) 
    { 
     Jellyfish::instance = new Jellyfish(); 
    } 

    return Jellyfish::instance; 

} 

當我在main.cpp並進行測試，我沒有錯誤：

main.cpp

#include <QApplication> 

#include "jellyfish.h" 

class Jellyfish; 

int main(int argc, char *argv[]) 
{ 

    QApplication app(argc, argv); 

    Jellyfish *toto = Jellyfish::getInstance(); 
    Jellyfish *toto2 = Jellyfish::getInstance(); 
    Jellyfish *toto3 = Jellyfish::getInstance(); 

    return app.exec(); 

} 

但我想用一些靜態方法Jellyfish外部QScript文件：

jellyfish.h

private: 

    static QScriptValue set(QScriptContext *context, QScriptEngine *engine); 
    QScriptEngine *script_engine; 

jellyfish.cpp

Jellyfish::Jellyfish() 
{ 

    script_engine = new QScriptEngine; 

    /* ... */ 

    initScriptEngine(); 

} 

void Jellyfish::initScriptEngine() 
{ 

    QScriptValue object = script_engine->newQObject(this); 

    object.setProperty("set", script_engine->newFunction(set)); 

    script_engine->globalObject().setProperty("jellyfish", object); 

} 

QScriptValue Jellyfish::set(QScriptContext *context, QScriptEngine *engine) 
{ 

    // I have to load instance because I am in a static method. 
    // But this is where the application loop endlessly. 
    Jellyfish *jellyfish = Jellyfish::getInstance(); 
    return true; 

} 

而且finaly解析腳本：

jellyfish.set("line_numbers", true); 

問題

當我運行應用程序，getInstance()總是創建一個新的水母實例。但真正的問題是，在調試的輸出（qDebug() << "test";）我可以看到應用程序循環Jellyfish::getInstance();，直到我得到一個sgmentation fault。

有人能幫我理解嗎？

Answer 1

這是因爲你的水母構造函數遞歸調用自己！

調試器打破你的計劃，你會看到回溯這樣的：

... 
    Jellyfish::Jellyfish() 
    Jellyfish::getInstance() 
    Jellyfish::set 
    Jellyfish::initScriptEngine() 
    Jellyfish::Jellyfish() 
    Jellyfish::getInstance() 

的問題是，你的構造函數調用Jellyfish::getInstance()，但對象被創建時，Jellyfish::instance仍然是NULL：

Jellyfish * Jellyfish::getInstance() 
{ 

    if (!Jellyfish::instance) 
    { 
     // this line 
     // Jellyfish::instance = new Jellyfish(); 
     // is equivalent to 
     Jellyfish * temp = new Jellyfish(); // * 
     Jellyfish::instance = temp; // not NULL after construction 
    } 

    return Jellyfish::instance; 

} 

如果從構造函數中調用getInstance（），那麼它總是會導致無限遞歸（直到出現段錯誤）。唯一的一個解決方法是調用水母:: initScriptEngine（）不是從你的構造 - 但明確：

Jellyfish::getInstance()->initScriptEngine();