C++的强制类型转换，除了继承自C语言的写法（(目标类型)表达式）之外，还新增了4个关键字，分别是：static_cast、dynamic_cast、const_cast和reinterpret_cast。用法：xxx_cast<目标类型>(表达式)。由于后两者的使用频率较少，尤其是reinterpret_cast的风险性很高，所以就不展开讲了。这里主要将static_cast和dynamic_cast。
static_cast
解释
所谓static_cast，顾名思义，就是静态的转换，是在编译期间就能确定的转换。
主要用途
- 用于基本数据类型之间的转换。
#include <iostream>
using namespace std;
int main()
{
float f = 5.67;
auto i = static_cast<int>(f);
cout << i << endl; // 输出结果：5
return 0;
}
- 用于有转换关系的类之间的转换。
#include <iostream>
using namespace std;
class Src
{
public:
void foo()
{
cout << "This is Src" << endl;
}
};
class Dest
{
public:
/** 类型转换构造函数 */
Dest(const Src &from)
{
cout << "Converting from Src to Dest" << endl;
}
void foo()
{
cout << "This is Dest" << endl;
}
};
int main()
{
Src src;
auto dst = static_cast<Dest>(src); // 输出结果：Converting from Src to Dest
dst.foo(); // 输出结果：This is Dest
}
- 用于在类继承体系中指针或引用的上行转换（即派生类到基类的转换）。如果用作下行转换（即基类到派生类的转换），由于不会动态的检查指针或引用是否真正指向派生类对象，因此不安全，要用到稍后要讲的
dynamic_cast。#include <iostream>
using namespace std;
class Base
{
public:
void foo()
{
cout << "This is Base" << endl;
}
};
class Derived : public Base
{
public:
void foo()
{
cout << "This is Derived" << endl;
}
};
void test_upcast()
{
Derived derived;
Derived *pDerived = &derived;
auto pBase = static_cast<Base *>(pDerived);
pBase->foo(); // 输出结果：This is Base
}
void test_downcast()
{
Base base;
Base *pBase = &base;
auto pDerived = static_cast<Derived *>(pBase); // 不安全：pa并没有真正指向B类对象
pDerived->foo(); // 输出结果：This is Derived。这里虽然输出了结果，但是不安全
}
int main()
{
test_upcast();
test_downcast();
return 0;
}
dynamic_cast
解释
所谓dynamic_cast，顾名思义就是动态的转换，是一种能够在运行时检查安全性的转换。
使用条件：
- 基类必须有虚函数。
只能转引用或指针。
主要用途
用于继承体系中的上行或下行转换。上行转换跟static_cast是一样的；下行转换会在运行时动态判断。如果目标类型并没有指向对象的实际类型，那么：
- 指针的转换会返回
nullptr引用的转换会抛出
std::bad_cast异常#include <iostream>
using namespace std;
class Base
{
public:
virtual void foo()
{
cout << "This is Base" << endl;
}
};
class Derived : public Base
{
public:
void foo() override
{
cout << "This is Derived" << endl;
}
};
/** Derived * -> Base * */
void test_upcast_ptr()
{
Derived derived;
Derived *pDerived = &derived;
auto base = dynamic_cast<Base *>(pDerived); // 尝试将派生类指针转换为基类指针
if (base)
{
cout << "Derived * -> Base * was successful" << endl;
}
else
{
cout << "Derived * -> Base * failed" << endl;
}
}
/** Base * -> Derived * */
void test_downcast_ptr1()
{
Derived derived;
Base *pBase = &derived; // 基类指针指向派生类对象
auto pDerived = dynamic_cast<Derived *>(pBase); // 尝试将指向派生类对象的基类指针转换为派生类指针
if (pDerived)
{
cout << "Base * -> Derived * was successful" << endl;
}
else
{
cout << "Base * -> Derived * failed" << endl;
}
}
/** Base * -> Derived * */
void test_downcast_ptr2()
{
Base base;
Base *pBase = &base;
auto derived = dynamic_cast<Derived *>(pBase); // 尝试将指向基类对象的基类指针转换为派生类指针
if (derived)
{
cout << "Base * -> Derived * was successful" << endl;
}
else
{
cout << "Base * -> Derived * failed" << endl;
}
}
/** Derived & -> Base & */
void test_upcast_ref()
{
Derived derived;
Derived &refDerived = derived;
try
{
auto &base = dynamic_cast<Base &>(refDerived); // 尝试将派生类引用转换为基类引用
cout << "Derived & -> Base & was successful" << endl;
}
catch (bad_cast &)
{
cout << "Derived & -> Base & failed" << endl;
}
}
/** Base & -> Derived & */
void test_downcast_ref1()
{
Derived derived;
Base &refBase = derived; // 基类引用指向派生类对象
try
{
auto &refDerived = dynamic_cast<Derived &>(refBase); // 尝试将指向派生类对象的基类引用转换为派生类引用
cout << "Base & -> Derived & was successful" << endl;
}
catch (bad_cast &)
{
cout << "Base & -> Derived & failed" << endl;
}
}
/** Base & -> Derived & */
void test_downcast_ref2()
{
Base base;
Base &refBase = base;
try
{
auto &refDerived = dynamic_cast<Derived &>(refBase); // 尝试将指向基类对象的基类引用转换为派生类引用
cout << "Base & -> Derived & was successful" << endl;
}
catch (bad_cast &)
{
cout << "Base & -> Derived & failed" << endl;
}
}
int main()
{
test_upcast_ptr(); // Derived * -> Base * was successful
test_downcast_ptr1(); // Base * -> Derived * was successful
test_downcast_ptr2(); // Base * -> Derived * failed
test_upcast_ref(); // Derived & -> Base & was successful
test_downcast_ref1(); // Base & -> Derived & was successful
test_downcast_ref2(); // Base & -> Derived & failed
}
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