Gri*_*fin 0 c++ pointers if-statement base-class static-cast
嘿所以我正在创建一个序列化函数,它接受一个基类指针'Joint',提取它'type'的联合后代,然后想要根据指针实际指向的任何类型的'关节' 来实例化正确类型的'定义'. .
然而,我仍然得到关于基类关节不包含后代类确实具有的函数的错误,即使我static_cast指向正确类型的指针.我如何使编译器意识到指针正在被铸造一个具有该功能的新类型？
我也得到关于'typedef'的错误,它可以根据关节*的'type'而不同,编译说它是未定义的.我如何告诉编译器无论如何,其中一个if语句都是真的？(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;
}
你使用的static_cast是问题.static_cast不重新定义变量的类型:相反,它返回输入到您要求的类型的值.
Type* variable = static_cast<Type*>(variableToCast);
此外,如果您的类使用多态,请考虑使用,dynamic_cast而是使用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,如果你遇到这种事情,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转换.有一个广泛的讨论,为什么它在这个网站和几乎所有地方都更好,以避免明确的类型切换.
对于该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:需要在编译阶段解决它.
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,您将无法从该if块外部访问它.
由于typedefs使用typedef范围内的符号使得定义的类型可用,直到范围结束,并且不再进一步.例如:
if (foo)
{
typedef int myInt;
myInt var = 4; // works
}
myInt baz = 4; // fails