Question

嘿所以我正在创建一个带有基类指针'Joint'的序列化函数，提取它的联合后代的'type'，然后想要实例化正确类型的'定义'基于指针实际指向的任何类型的'关节'。

但是我仍然没有得到关于基类联合的错误包含后代类确实具有的函数，即使我 static_cast指向正确类型的指针。 我如何制作编译器意识到指针也是一种新的类型有功能吗？
我也遇到了'typedef'的错误，这可能会有所不同基于联合*的'类型'，编译说它是未定义。 我如何告诉编译器无论如何，其中一个if语句都是真的？（Jointdef在if中声明语句）

这是来源：

template<class Archive>
b2Joint* const preSave(Archive & ar, b2Joint* Joint) 
{
    int Type = Joint->GetType();
    ar & Type; // pulled out first so we know what kind of JointDef to instantiate

    if(Type == b2JointType::e_distanceJoint){
        b2DistanceJointDef JointDef;
        static_cast<b2DistanceJoint *>(Joint);
            JointDef.localAnchorA=      Joint->GetAnchorA();
            JointDef.localAnchorB=      Joint->GetAnchorB();
            JointDef.length=            Joint->GetLength();
            JointDef.frequencyHz=       Joint->GetFrequency();
    }
    if(Type == b2JointType::e_weldJoint){
        b2WeldJointDef JointDef;
        static_cast<b2WeldJoint *>(Joint);
            JointDef.localAnchorA=      Joint->GetAnchorA();
            JointDef.localAnchorB=      Joint->GetAnchorB();
            JointDef.referenceAngle=    Joint->GetReferenceAngle();     // function added
    }
    if(Type == b2JointType::e_gearJoint){ //TODO / NOTE: this must be loaded last since to be linked joints must first be made
        b2GearJointDef JointDef;          //             unless joints are in order of when they were created.......
        static_cast<b2GearJoint *>(Joint);
            JointDef.joint1=                Joint->GetJoint1;           //function added
            JointDef.joint2=                Joint->GetJoint2;           //function added
            JointDef.ratio=                 Joint->GetRatio();
    }
    if(Type == b2JointType::e_lineJoint){
        b2LineJointDef JointDef;
        static_cast<b2LineJoint *>(Joint);
            JointDef.localAnchorA=      Joint->GetAnchorA();
            JointDef.localAnchorB=      Joint->GetAnchorB();
            JointDef.enableLimit;       Joint->IsLimitEnabled();
            JointDef.localAxisA=        Joint->GetLocalAxisA()          //function made
            JointDef.lowerTranslation=  Joint->GetLowerLimit();
            JointDef.upperTranslation=  Joint->GetUpperLimit();
            JointDef.enableMotor=       Joint->IsMotorEnabled();
            JointDef.maxMotorForce=     Joint->GetMaxMotorForce();
            JointDef.motorSpeed=        Joint->GetMotorSpeed();
    }

    if(Type == b2JointType::e_mouseJoint){
        b2MouseJointDef JointDef;
        static_cast<b2MouseJoint *>(Joint);
            JointDef.target=            Joint->GetTarget();
            JointDef.maxForce=          Joint->GetMaxForce();
            JointDef.frequencyHz=       Joint->GetFrequency();
            JointDef.dampingRatio=      Joint->GetDampingRatio();
    }
    if(Type == b2JointType::e_prismaticJoint){
        b2PrismaticJointDef JointDef;
        static_cast<b2PrismaticJoint *>(Joint);
            JointDef.localAnchorA=      Joint->GetAnchorA();
            JointDef.localAnchorB=      Joint->GetAnchorB();
            JointDef.enableLimit;       Joint->IsLimitEnabled();
            JointDef.referenceAngle=    Joint->GetReferenceAngle();     //added function
            JointDef.localAxis1=        Joint->GetLocalAxis1();         //added function
            JointDef.maxMotorForce=     Joint->GetMaxMotorForce();      //added function
            JointDef.lowerTranslation=  Joint->GetLowerLimit();
            JointDef.upperTranslation=  Joint->GetUpperLimit();
            JointDef.enableMotor=       Joint->IsMotorEnabled();
            JointDef.motorSpeed=        Joint->GetMotorSpeed();
    }
    if(Type == b2JointType::e_pulleyJoint){
        b2PulleyJointDef JointDef;
        static_cast<b2PulleyJoint *>(Joint);
            JointDef.localAnchorA=      Joint->GetAnchorA();
            JointDef.localAnchorB=      Joint->GetAnchorB();
            JointDef.groundAnchorA=     Joint->GetGroundAnchorA();
            JointDef.groundAnchorB=     Joint->GetGroundAnchorB();
            JointDef.lengthA=           Joint->GetLength1();
            JointDef.lengthB=           Joint->GetLength2();
            JointDef.maxLengthA=        Joint->GetMaxPulleyLengthA(); //added function
            JointDef.maxLengthB=        Joint->GetMaxPulleyLengthB(); //added function
            JointDef.ratio=             Joint->GetRatio();
    }
    if(Type == b2JointType::e_revoluteJoint){
        b2RevoluteJointDef JointDef;
        static_cast<b2RevoluteJoint *>(Joint);
            JointDef.localAnchorA=      Joint->GetAnchorA();
            JointDef.localAnchorB=      Joint->GetAnchorB();
            JointDef.enableLimit;       Joint->IsLimitEnabled();
            JointDef.enableMotor=       Joint->IsMotorEnabled();
            JointDef.motorSpeed=        Joint->GetMotorSpeed();
            JointDef.maxMotorTorque=    Joint->GetMaxMotorTorque()  //added function
            JointDef.referenceAngle     Joint->GetReferenceAngle()  //added function
            JointDef.lowerAngle=        Joint->GetLowerLimit();
            JointDef.upperAngle=        Joint->GetUpperLimit();
    }
    else{ b2JointDef

    //if(Type == b2JointType::e_frictionJoint){         QUESTION: what is this... not in box2d guide...
    // base class JointDef data:
    JointDef.type=              Joint->GetType();
    JointDef.userData=          Joint->GetUserData();
    JointDef.bodyA=             Joint->GetBodyA();
    JointDef.bodyB=             Joint->GetBodyB();
    JointDef.collideConnected=  Joint->IsCollideConnected();        //added function

    ar & JointDef;
    return Joint;
}

Answer 1

您使用static_cast是个问题。 static_cast不会重新定义变量的类型：相反，它会返回投放到您要求的类型中的值。

Type* variable = static_cast<Type*>(variableToCast);

此外，如果您的类使用多态，请考虑使用dynamic_cast，而使用C ++的（不是非常强大的）运行时类型信息系统。

if (b2DistanceJointDef* def = dynamic_cast<b2JointTypeDef*>(Joint))
{
    // if you reach this block, this means Joint is actually a pointer to
    // a b2JointTypeDef object; and you may use the `def` variable to access
    // its members
}

更好的是，如果你喜欢这种东西，并且如果它在你的设计中有意义，你可以考虑使用多态方法来消除这种类型的开关。广泛讨论为什么它在这个网站上以及几乎在任何地方都能更好地避免显式类型切换。

对于typedef语句，您必须区分编译时信息和运行时信息。 C ++是一种静态类型语言：这意味着需要在编译阶段了解变量类型的所有内容。类型定义不能根据运行时条件而变化：它需要在编译阶段解决。

此外，符号（基本上，任何具有标识符的符号）都不能在其范围之外使用。因此，如果您在if块下声明任何内容，则无法从if阻止外部访问它。

由于typedef在范围内使用typedef生成符号，因此在范围结束之前使定义的类型可用，而不再进一步。例如：

if (foo)
{
    typedef int myInt;
    myInt var = 4; // works
}
myInt baz = 4; // fails

根据情况在if语句中声明不同的数据类型：如何关闭编译器？

1 个答案: