GPT答疑记录

https://stackoverflow.com/questions/332030/when-should-static-cast-dynamic-cast-const-cast-and-reinterpret-cast-be-used?

static_cast

1. 普通类型转换 int->float
2. pointer to void, void to pointer
3. 向上类型转换不是必须的, 但是向下转换只要不是虚拟继承就能使用

dynamic_cast

只用来处理多态下的指针和引用

class Animal
{
public:
    virtual void PrintName()
    {
        std::cout << "Animal\r\n";
    }
};

class Dog : public Animal
{
public:
    void PrintName() override
    {
        std::cout << "Dog\r\n";
    }
};

class Cat : public Animal
{
public:
    void PrintName() override
    {
        std::cout << "Cat\r\n";
    }
};

void TestCast()
{
    // Animal* animal = new Animal(); // not dog not cat
    // Animal* animal = new Dog(); // Dog not cat
    Animal* animal = new Cat(); // not dog Cat

    Dog* dog = dynamic_cast<Dog*>(animal);
    Cat* cat = dynamic_cast<Cat*>(animal);

    if (dog)
    {
        dog->PrintName();
    }
    else
    {
        std::cout << "not dog\r\n";
    }
    if (cat)
    {
        cat->PrintName();
    }
    else
    {
        std::cout << "not cat\r\n";
    }
}

const_cast

可以用来对变量移除和添加const, 其他的cast都无法做到 (static_cast可以添加const但是不能去除)

将const变量移除const是未定义的, 将非const的变量的const引用是安全的

int a = 1;
int* ap1 = &a;
const int* ap2 = static_cast<const int*>(ap1); // OK

int* ap3 = static_cast<int*>(ap2); // invalid static_cast from type 'const int*' to type 'int*'

int* ap4 = const_cast<int*>(ap2); // OK

• ex1.将const成员函数的this指针去除const来修改成员变量

  // https://www.geeksforgeeks.org/const_cast-in-c-type-casting-operators/
  #include <iostream> 
  using namespace std; 
  

class student
{
private:
int roll;
public:
// constructor
student(int r):roll(r) {}

// A const function that changes roll with the help of const_cast 
void fun() const
{ 
    ( const_cast <student*> (this) )->roll = 5; 
} 

int getRoll() { return roll; } 

};

int main(void)
{
student s(3);
cout << “Old roll number: “ << s.getRoll() << endl;

s.fun(); 

cout << "New roll number: " << s.getRoll() << endl; 

return 0; 

}

//$ Old roll number: 3
//$ New roll number: 5

- ex2.去除参数的const, 来调用非const参数的函数
```c++
#include <iostream> 
using namespace std; 

int fun(int* ptr) 
{ 
    return (*ptr + 10); 
} 

int main(void) 
{ 
    const int val = 10; 
    const int *ptr = &val; 
    int *ptr1 = const_cast <int *>(ptr); 
    cout << fun(ptr1); 
    return 0; 
}
//$ 20

reinterpret_cast

typedef unsigned short uint16;

// Read Bytes returns that 2 bytes got read. 

bool ByteBuffer::ReadUInt16(uint16& val)
{
  return ReadBytes(reinterpret_cast<char*>(&val), 2);
}

int b = 1;
char* bp1 = &b; //  cannot convert 'int*' to 'char*' in initialization
char* bp2 = static_cast<char*>(&b); // invalid static_cast from type 'int*' to type 'char*'

char* bp3 = reinterpret_cast<char*>(&b); // ok