casting operators

the c++ casting operators

the four c++ casting operators are

• static_cast
• dynamic_cast
• reinterpret_cast
• const_cast

use static_cast

static_cast is a mechanism that can be used to convert pointers between related types, and perform explicit type conversions for standard data types that would otherwise happen automatically or implicitly.

casting a Derived* to a Base* is called upcasting while casting a Base* to a Derived* is called downcasting.

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Derived ObjFoo;
Base* ObjBaseFoo = &ObjFoo; 

however

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Derived ObjFoo;
Base* ObjBaseFoo = &ObjFoo; 
Derived* AnotherObjFoo = ObjBaseFoo; //error!!!!

using dynamic_cast and runtime type identification

the typical usage syntax of the dynamic_cast operator is

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destination_type* Dest = dynamic_cast<class_type*>(Source);
if(Dest)
    Dest->CallFunc();

e.g. using dynamic casting to tell whether a fish object is a tuna or a carp demolist13_1

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#include <iostream>
using namespace std;

class Fish
{
public:
    virtual void Swim()
    {
        cout << "fish swim in water " << endl;
    }
    virtual ~Fish() {}
};
class Tuna : public Fish
{
public:
    void Swim()
    {
        cout << "tuna swim " << endl;
    }

    void BecomeDinner()
    {
        cout << "Tuna Dinner " << endl;
    }
};

class Crap : public Fish
{
public:
    void Swim()
    {
        cout << "Crap Swim " << endl;
    }

    void Talk()
    {
        cout << "crap talk " << endl;
    }
};

void DetectFishType(Fish* ObjFish)
{
    Tuna*ObjTuna = dynamic_cast<Tuna*>(ObjFish);
    if(ObjTuna)
    {
        cout <<"detect tunna dinner "<<endl;
        ObjTuna->BecomeDinner();
    }
    Crap*ObjCrap = dynamic_cast<Crap*>(ObjFish);
    if(ObjCrap)
    {
        cout <<"detect crap talk "<<endl;
        ObjCrap->Talk();
    }

    cout<<"type check "<<endl;
    ObjFish->Swim();
}

int main()
{
    Crap MyLunch;
    Tuna MyDinner;

    DetectFishType(&MyLunch);
    cout<< endl;
    DetectFishType(&MyDinner);

    return 0;
}

Line 41-54 is used to test the nature of the input base class pointer of type Fish. the return value of dynamic_cast operation should always be checked for validity . It's NULL when the cast fails.

using reinterpret_cast

reinterpret_cast really does allow you to cast one object type to another, regardless of whether or not the types are related. syntax as seen in the following sample:

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Base* objBase = new Base();
Unrelated* notRelated = reinterpret_cast<Unrelated*>(objBase);

using const_cast

const_cast enables you to turn off the const access modifier to an object. A prevalent error like the follow sample:

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class Foo
{
public:
    void DisplayMembers();
};

void DisplayData (const Foo& ObjFoo)
{
    ObjFoo.DisplayMembers();//error!!
    //call to a non-const member using a const reference
}

we can make corrective changes to DisplayMembers(). The syntax for invoking DisplayMembers() in such a scenario is

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void DisplayData (const Foo& ObjFoo)
{
Foo& RefData= const_cast<Foo&>(ObjFoo);
RefData.DisplayMembers(); // Allowed!
}

const before or after function

来源：const在函数前后的区别

const放在函数后主要是限制类中的成员函数，const放在函数前是限制函数返回类型为指针时通过指针对返回值的修改。

1. const在函数前： const在函数前实际上是约束函数的返回的，表明返回的值不能做左值，实际上就是防止返回值直接被赋值。但是一般的变量无法修改函数返回值，其实是为了防止函数返回类型为指针类型时，通过指针对返回值进行修改。非指针的函数返回类型前加const是没有意义的。

2. const在函数后： const放在函数后通常是用在类中，限制类的成员函数不能对成员变量进行修改。同时，被函数后的const修饰的成员函数也不能调用其 他非const的成员函数。

Q： I need a Bird*, but have a Dog* at hand. The compiler does not allow me to use the pointer to the Dog object as a Bird*. However, when I use reinterpret_cast to cast the Dog* to Bird*, the compiler does not complain and it seems I can use this pointer to call Bird’s member function, Fly(). Is this okay? A: Again, definitely not. reinterpret_cast changed only the interpretation of the pointer, and did not change the object being pointed to (that is still a Dog). Calling a Fly() function on a Dog object will not give the results you are looking for, and could possibly cause an application failure.