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？

Question

嘿，所以我正在制作一个序列化函数，它采用基类指针'Joint'，提取联合后代的'type'，然后想要基于指针实际指向的任何类型的“关节”来实例化正确类型的“定义”。

• 但是我仍然收到有关基类联合的错误 包含后代类确实具有的函数，即使我 static_cast 将指针指向正确的类型。 我该如何制作 编译器意识到指针也被转换为新类型 有这个功能吗？

• 我也收到有关“typedef”的错误，该错误可能有所不同 基于联合*的“类型”，编译说它是 不明确的。 我如何告诉编译器无论如何，其中一个 if 语句将为 true？ （Jointdef 在 if 内部声明 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

您对 static_cast 的使用是问题所在。 static_cast 不会重新定义变量的类型：相反，它返回转换为您要求的类型的值。

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

另外，如果您的类使用多态性，请考虑使用dynamic_cast，它利用了 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，如果您喜欢这种事情，并且它在您的设计中有意义，您可以考虑使用多态方法来消除这种类型开关。关于为什么在这个网站和几乎所有地方都最好避免显式类型切换存在广泛的讨论。

---

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：这意味着它在编译阶段需要了解有关变量类型的所有信息。类型定义不能根据运行时条件而变化：它需要在编译阶段进行解析。

此外，符号（基本上，任何具有标识符的东西都是符号）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，您将无法从该 if 块外部访问它。

由于 typedef 在作用域内使用 typedef 生成符号，因此定义的类型在作用域结束之前都可用，并且不会再进一步​​。例如：

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