如何在 C++ 中使用动态转换

Jinku Hu 2020年11月24日
如何在 C++ 中使用动态转换

本文将演示关于如何在 C++ 中使用动态转换的多种方法。

使用 dynamic_cast 将基类指针转换为派生指针

dynamic_cast 允许程序员在继承层次结构中转换对类的指针和引用。例如,基类指针可以被转换为派生类指针,允许程序员调用派生类成员函数。

dynamic_castRun-Time Type Information(RTTI)功能的一部分,它提供了一种在运行时而不是编译时访问对象类型的方法。

请注意,dynamic_cast 不能用于在 intfloat 等原始类型之间进行转换。此外,只有当基类包含至少一个虚拟成员函数时,才可以使用 dynamic_cast。在下面的例子中,我们声明一个新的 Base 类指针,并将其转换 Derived 类的新指针。由于 dynamic_cast 返回的是一个空指针,所以如果转换不成功,我们可以将表达式作为条件放在 if 语句中。

#include <iostream>
#include <typeinfo>

using std::cout;
using std::endl;

struct Base {
    virtual ~Base() = default;
};

struct Derived: public Base { };

int main()
{
    Base *bp = new Derived;
    if (Derived *dp = dynamic_cast<Derived*>(bp))
    {
        cout << "Successful cast - can use the pointer to 'Derived' object dp" << endl;
    }

    delete bp;
    return EXIT_SUCCESS;
}

输出:

Successful cast - can use the pointer to 'Derived' object dp

程序员可以安全地访问 if 段作用域中的对象,并根据需要调用 Derived 类成员方法。但要注意的是,由于我们是将 Derived 类的类型转换为 Base,在第 13 行,我们在 new 操作符后指定 Derived,因为下面的代码会产生不成功的转换。

#include <iostream>
#include <typeinfo>

using std::cout;
using std::endl;

struct Base {
    virtual ~Base() = default;
};

struct Derived: public Base { };

int main()
{
    Base *bp = new Base;
    if (Derived *dp = dynamic_cast<Derived*>(bp))
    {
        cout << "Successful cast - can use the pointer to 'Derived' object dp" << endl;
    }

    delete bp;
    return EXIT_SUCCESS;
}

RTTI 功能的另一部分是 typeid 操作符,它返回给定表达式的类型。可以利用它来比较多个表达式的类型,在接下来的代码示例中演示。注意,当使用 typeid 操作符时,必须包含 <typeinfo> 头文件。

#include <iostream>
#include <typeinfo>

using std::cout;
using std::endl;

struct Base {
    virtual ~Base() = default;
};

struct Derived: public Base { };

int main()
{
    Base *bp = new Derived;
    if (Derived *dp = dynamic_cast<Derived*>(bp))
    {
        cout << "Successful cast - can use the pointer to 'Derived' object dp" << endl;
        if (typeid(*bp) == typeid(*dp)) {
            cout << "bp and dp are of same type" << endl;
        }
    }

    delete bp;
    return EXIT_SUCCESS;
}

输出:

Successful cast - can use the pointer to 'Derived' object dp
bp and dp are of same type
Author: Jinku Hu
Jinku Hu avatar Jinku Hu avatar

Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.

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