C++的強(qiáng)制類型轉(zhuǎn)換，除了繼承自C語言的寫法（(目標(biāo)類型)表達(dá)式）之外，還新增了4個(gè)關(guān)鍵字，分別是：static_cast、dynamic_cast、const_cast和reinterpret_cast。用法：xxx_cast<目標(biāo)類型>(表達(dá)式)。由于后兩者的使用頻率較少，尤其是reinterpret_cast的風(fēng)險(xiǎn)性很高，所以就不展開講了。這里主要將static_cast和dynamic_cast。

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static_cast

解釋

所謂static_cast，顧名思義，就是靜態(tài)的轉(zhuǎn)換，是在編譯期間就能確定的轉(zhuǎn)換。

主要用途

1. 用于基本數(shù)據(jù)類型之間的轉(zhuǎn)換。

#include 

using namespace std;

int main()
{
    float f = 5.67;
    auto i = static_cast(f);
    cout << i << endl;  // 輸出結(jié)果：5
    return 0;
}

2. 用于有轉(zhuǎn)換關(guān)系的類之間的轉(zhuǎn)換。

#include 

using namespace std;

class Src
{
public:
    void foo()
    {
        cout << "This is Src" << endl;
    }
};

class Dest
{
public:
    /** 類型轉(zhuǎn)換構(gòu)造函數(shù) */
    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(src);  // 輸出結(jié)果：Converting from Src to Dest
    dst.foo();  // 輸出結(jié)果：This is Dest
}

3. 用于在類繼承體系中指針或引用的上行轉(zhuǎn)換（即派生類到基類的轉(zhuǎn)換）。如果用作下行轉(zhuǎn)換（即基類到派生類的轉(zhuǎn)換），由于不會(huì)動(dòng)態(tài)的檢查指針或引用是否真正指向派生類對(duì)象，因此不安全，要用到稍后要講的dynamic_cast。

#include 

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(pDerived);
    pBase->foo();  // 輸出結(jié)果：This is Base
}

void test_downcast()
{
    Base base;
    Base *pBase = &base;
    auto pDerived = static_cast(pBase);  // 不安全：pa并沒有真正指向B類對(duì)象
    pDerived->foo();  // 輸出結(jié)果：This is Derived。這里雖然輸出了結(jié)果，但是不安全
}

int main()
{
    test_upcast();
    test_downcast();
    return 0;
}

dynamic_cast

解釋

所謂dynamic_cast，顧名思義就是動(dòng)態(tài)的轉(zhuǎn)換，是一種能夠在運(yùn)行時(shí)檢查安全性的轉(zhuǎn)換。

使用條件：

1. 基類必須有虛函數(shù)。
2. 只能轉(zhuǎn)引用或指針。

主要用途

用于繼承體系中的上行或下行轉(zhuǎn)換。上行轉(zhuǎn)換跟static_cast是一樣的；下行轉(zhuǎn)換會(huì)在運(yùn)行時(shí)動(dòng)態(tài)判斷。如果目標(biāo)類型并沒有指向?qū)ο蟮膶?shí)際類型，那么：

1. 指針的轉(zhuǎn)換會(huì)返回nullptr
2. 引用的轉(zhuǎn)換會(huì)拋出std::bad_cast異常

#include 

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(pDerived); // 嘗試將派生類指針轉(zhuǎn)換為基類指針
    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; // 基類指針指向派生類對(duì)象
    auto pDerived = dynamic_cast(pBase); // 嘗試將指向派生類對(duì)象的基類指針轉(zhuǎn)換為派生類指針
    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(pBase); // 嘗試將指向基類對(duì)象的基類指針轉(zhuǎn)換為派生類指針
    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(refDerived); // 嘗試將派生類引用轉(zhuǎn)換為基類引用
        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; // 基類引用指向派生類對(duì)象
    try
    {
        auto &refDerived = dynamic_cast(refBase); // 嘗試將指向派生類對(duì)象的基類引用轉(zhuǎn)換為派生類引用
        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(refBase); // 嘗試將指向基類對(duì)象的基類引用轉(zhuǎn)換為派生類引用
        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
}