C++类型转换dynamic_cast/reinterpret_cast/static_cast/const_cast

C++中类型转换一般这样写

double double_age = 13.9;
int int_age = int(age);
// int int_age =  (int)double_age; //也可以这样写
// int int_age =  (int)(double_age); //也可以这样写
// int int_age =  ((int)(double_age)); //当然也可以这样写，只要您不觉得累
1
2
3
4
5

但是今天讲的是强制类型转换
https://cplusplus.com/doc/tutorial/typecasting/

dynamic_cast (expression)
reinterpret_cast (expression)
static_cast (expression)
const_cast (expression)

dynamic_cast

如果父类指针强制转换为子类，dynamic_cast转换判断出不安全，会获得空指针。

// dynamic_cast
#include 
#include 
using namespace std;

class Base { virtual void dummy() {} };
class Derived: public Base { int a; };

int main () {
  try {
    Base * pba = new Derived;
    Base * pbb = new Base;
    Derived * pd;

    pd = dynamic_cast<Derived*>(pba);
    if (pd==0) cout << "Null pointer on first type-cast.\n";

    pd = dynamic_cast<Derived*>(pbb);
    if (pd==0) cout << "Null pointer on second type-cast.\n";

  } catch (exception& e) {cout << "Exception: " << e.what();}
  
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22

reinterpret_cast

二进制内存形式的强制转换

class A { /* ... */ };
class B { /* ... */ };
A * a = new A;
B * b = reinterpret_cast(a);
1
2
3
4

static_cast

显示类型转换

class Base {};
class Derived: public Base {};
Base * a = new Base;
Derived * b = static_cast<Derived*>(a)
1
2
3
4

const_cast

const_cast可以用于const关键字的去除

// const_cast
#include 
using namespace std;

void print (char * str)
{
  cout << str << '\n';
}

int main () {
  const char * c = "sample text";
  print ( const_cast<char *> (c) );
  return 0;
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14