C++ type conversion

1 Type conversion names and syntax

C-style type casting (Type Cast) is very simple, no matter what type of conversion is:
TYPE b = (TYPE)a
C++-style type conversion provides 4 type conversion operators to deal with the application of different occasions.
　　　

　 static_cast       静态类型转换。如int转换成char
  reinterpreter_cast    重新解释类型 
　dynamic_cast       命名上理解是动态类型转换。如子类和父类之间的多态类型转换。
  const_cast，       字面上理解就是去const属性。
4种类型转换的格式：
    TYPE B = static_cast<TYPE> (a)  

2 General introduction to type conversion

1）static_cast<>()   静态类型转换，编译的时c++编译器会做类型检查；

Basic types can be converted but not pointer
types Use static_cast<>() for type conversion. Because the C++ compiler can generally pass the compilation check; if the implicit type conversion is not possible in the C language, the reinterpret_cast<>() can be used for forced type interpretation in C++. To sum up: static_cast<>() and reinterpret_cast<>() basically convert the casts in C language to overriding reinterpret_cast<>() which hardly guarantees portability. 4) dynamic_cast<>(), dynamic type conversion, safe conversion between base classes and subclasses; run-time type checking 5) const_cast<>(), remove read-only attributes of variables

3 case studies

static_cast usage and reinterpret_cast usage

void main01()
{
    double dPi = 3.1415926;

    //1静态的类型转换:  在编译的时 进行基本类型的转换 能替代c风格的类型转换 可以进行一部分检查
    int num1 = static_cast<int> (dPi); //c++的新式的类型转换运算符  
    int num2 = (int)dPi;                //c语言的 旧式类型转换 
    int num3 = dPi;                     //隐士类型转换
    cout << "num1:" << num1 << " num2:" << num2 << " num3:" << num3 << endl;


    char *p1 = "hello wangbaoming " ;
    int *p2 = NULL;
    p2 = (int *)p1;

    //2 基本类型能转换 但是不能转换指针类型
    //p2 = static_cast<int *> (p1); //“static_cast”: 无法从“char *”转换为“int *”

    //3 可以使用  reinterpret_cast 进行重新解释 
    p2 = reinterpret_cast<int *> (p1);
    cout << "p1 " << p1 << endl;
    cout << "p2 " << p2 << endl;

    //4 一般性的结论： c语言中  能隐式类型转换的 在c++中可以用 static_cast<>()进行类型转换  //C++编译器在编译检查一般都能通过
    //c语言中不能隐式类型转换的,在c++中可以用 reinterpret_cast<>() 进行强行类型 解释 

    system("pause");
    return ;
}

dynamic_cast usage and reinterpret_cast usage

class Animal 
{
public:
    virtual void  cry() = 0; 
};

class Dog : public Animal 
{
public:
    virtual void  cry()
    {
        cout << "wangwang " << endl;
    }

    void doSwim()  
    { 
        cout << "我要狗爬" << endl; 
    }
};


class Cat : public Animal
{
public:
    virtual void  cry()
    {
        cout << "miaomiao " << endl;
    }
    void doTree()  
    { 
        cout << "我要爬树" << endl; 
    }

};

class Book
{
public:
    void printP()
    {
        cout << price << endl;
    }

private:
    int price;

};

void ObjPlay(Animal *base)
{
    base->cry();
    Dog *pDog = dynamic_cast<Dog *>(base);
    if (pDog != NULL)
    {
        pDog->cry();
        pDog->doSwim();
    }

    Cat *pCat = dynamic_cast<Cat *>(base);
    if (pCat != NULL)
    {
        pCat->cry();
        pCat->doTree();
    }
}
void main02()
{
    Animal *base = NULL;

    //1 可以把子类指针赋给 父类指针 但是反过来是不可以的 需要 如下转换
    //pdog = base;  
    Dog *pDog = static_cast<Dog *> (base);

    //2 把base转换成其他 非动物相关的 err
    //Book *book= static_cast<Book *> (base);

    //3  reinterpret_cast //可以强制类型转换
    Book *book2= reinterpret_cast<Book *> (base);

    //4 dynamic_cast用法
    ObjPlay(new Cat());

    system("pause");
}

const_cast usage

//典型用法 把形参的只读属性去掉
void Opbuf(const char *p)
{
    cout << p << endl;
    char *p2 = const_cast<char*>(p);
    p2[0] = 'b';
    cout << p << endl;
}

void main()
{
    const char *p1 = "11111111111";

    char *p2 = "22222222";

    char *p3 = const_cast<char *>(p1);
    char buf[100] = "aaaaaaaaaaaa";

    Opbuf(buf);

    //要保证指针所执行的内存空间能修改才行 若不能修改 还是会引起程序异常
    //Opbuf("dddddddddddsssssssssssssss");

    system("pause");
}

4 Summary

Conclusion 1: Programmers need to know clearly: the variable to be converted, what type is before type conversion, and what type is after type conversion. What are the consequences after the conversion.
Conclusion 2: In general, type conversion is not recommended; avoid type conversion.