c++ - 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，它接受一个基类指针'Joint'，提取它'type'的联合后代，然后想要根据指针真正指向的任何类型的“联合”实例化正确类型的“定义” .

但是，即使我 static_cast 指向正确类型的指针，我仍然收到关于不包含后代类所具有的函数的基类关节的错误。我如何让编译器意识到指针也被转换为具有该功能的新类型？
我也收到关于“typedef”的错误，这可能会根据关节*的“类型”而有所不同，编译时说它是未定义的。我如何告诉编译器无论如何，其中一个 if 语句都是正确的？（Jointdef 在 if 语句中声明）

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;
}

score 5 · Accepted Answer

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

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

此外，如果您的类使用多态性，请考虑使用dynamic_cast，它利用 C++ 的（不是很强大的）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，如果您喜欢这种东西，并且如果它在您的设计中有意义，您可能会考虑使用多态方法来消除这种类型切换。关于为什么在这个站点和几乎所有地方都避免显式类型切换更好的原因有广泛的讨论。

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：这意味着它需要在编译阶段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：它需要在编译阶段解决。

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，如果您在块下声明任何内容if，您将无法从该if块之外访问它。

由于typedefs 在作用域内使用 a 生成符号typedef使得定义的类型在作用域结束之前可用，并且不再可用。例如：

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

c++ - 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？

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