C++ 四大型別轉換運算子詳解與面試題

四種轉換運算子

1. const_cast：移除/增加 const / volatile 資格。
2. dynamic_cast：執行期型別檢查，支援向下/橫向轉型（需多態基類別）。
3. reinterpret_cast：位元層級重新解釋，不改變位址（指標轉整數、不相關指標互轉）。
4. static_cast：編譯期靜態轉型，支援基本型別轉換、上行轉型、有轉換建構函式的類別轉換。

程式碼示範

/* cppprimer.cpp
 * Created on: 2014.2.10
 * Author: Spike
 */
#include <iostream>

struct S { S() : value(0) {} int value; };

void CastConst() {
    const S s;
    cout << "s.value = " << s.value << endl;
    S* ps = const_cast<S*>(&s);
    ps->value = 1;
    cout << "s.value = " << s.value << endl;
    S& rs = const_cast<S&>(s);
    rs.value = 2;
    cout << "s.value = " << s.value << endl;
}

struct B { virtual void f() { cout << "Base::f" << endl; } virtual ~B() {} };
struct B2 { virtual void g() { cout << "Base2::g" << endl; } virtual ~B2() {} };
struct D : public B, public B2 {
    virtual void f() { cout << "Derived::f" << endl; }
    virtual void g() { cout << "Derived::g" << endl; }
    virtual ~D() {}
};

void CastDynamic() {
    B* pB_D = new D;
    pB_D->f();
    D* pD_D = dynamic_cast<D*>(pB_D); // 向下轉型
    pD_D->g();
    B2* pB2_D = dynamic_cast<B2*>(pB_D); // 橫向轉型
    pB2_D->g();
    D* pD_Ds = static_cast<D*>(pB_D); // static_cast 也可向下，但無執行期檢查
    pD_Ds->g();
    // B2* pB2_Ds = static_cast<B2*>(pB_D); // 錯誤：static_cast 不能橫向轉型
    delete pB_D;
}

struct rA { int m_a; };
struct rB { int m_b; };
struct rC : public rA, public rB {};

void CastReinterpret() {
    int* i = new int(10);
    cout << "*i = " << *i << ", i = " << i << endl;
    double* d = reinterpret_cast<double*>(i);
    cout << "*d = " << *d << ", d = " << d << endl;

    rC c;
    cout << "&c = " << &c << endl
         << "reinterpret_cast<rB*> = " << reinterpret_cast<rB*>(&c) << endl
         << "static_cast<rB*> = " << static_cast<rB*>(&c) << endl
         << "reinterpret_cast<rA*> = " << reinterpret_cast<rA*>(&c) << endl
         << "static_cast<rA*> = " << static_cast<rA*>(&c) << endl;
    delete i;
}

int main() {
    cout << "=== const_cast ===\n"; CastConst();
    cout << "=== dynamic_cast ===\n"; CastDynamic();
    cout << "=== reinterpret_cast ===\n"; CastReinterpret();
}

輸出節錄

=== const_cast ===
s.value = 0
s.value = 1
s.value = 2
=== dynamic_cast ===
Derived::f
Derived::g
Derived::g
Derived::g
=== reinterpret_cast ===
*i = 10, i = 0x...
*d = 2.55917e-307, d = 0x...
&c = 0x...
reinterpret_cast<rB*> = 0x...
static_cast<rB*> = 0x... (位址不同，static_cast 會調整位址)
reinterpret_cast<rA*> = 0x...
static_cast<rA*> = 0x... (rA 為第一基類別，位址相同)

選擇指南

• 移除 const → const_cast。
• 多態基類別指標向下/橫向轉型 → dynamic_cast（需 RTTI）。
• 基本型別、enum、指標轉整數、不相關指標互轉 → reinterpret_cast。
• 其他靜態可知的安全轉換 → static_cast。