1 繼承中的訪問權限問題

• 所有繼承方式, 子類都無法訪問父類的private成員.

那么用如下測試代碼嘗試一下:

#include <iostream> using namespace std;class father { public:int m_father_public_value; protected:int m_father_protected_value; private:int m_father_private_value; };class public_inherited_son : public father { public:void public_inherited_son_visit_father_public_value() {cout << m_father_public_value << endl;}void public_inherited_son_visit_father_protected_value() {cout << m_father_protected_value << endl;}void public_inherited_son_visit_father_private_value() {cout << m_father_private_value << endl;} };class protected_inherited_son: protected father { public:void protected_inherited_son_visit_father_public_value() {cout << m_father_public_value << endl;}void protected_inherited_son_visit_father_protected_value() {cout << m_father_protected_value << endl;}void protected_inherited_son_visit_father_private_value() {cout << m_father_private_value << endl;} };class private_inherited_son: private father { public:void private_inherited_son_visit_father_public_value() {cout << m_father_public_value << endl;}void private_inherited_son_visit_father_protected_value() {cout << m_father_protected_value << endl;}void private_inherited_son_visit_father_private_value() {cout << m_father_private_value << endl;} };int main() {return 0; }

編譯一下發現:

說明了子類不能訪問父類的私有成員.

• public繼承, 父類的public和protected成員還是子類的public和protected成員.

試著用public繼承的子類實例化的對象類外訪問父類的public和protected成員, 如下測試代碼:

int main() {public_inherited_son public_inherited_son_instance1;cout << public_inherited_son_instance1.m_father_public_value << endl;cout << public_inherited_son_instance1.m_father_protected_value << endl;return 0; }

編譯發現:

?

說明public繼承中父類的public和protected成員在子類中還是public和protected成員, 且protected成員在類外不能訪問.

• protected繼承, 父類的public和protected成員還是子類的protected成員.

試著用protected繼承的子類實例化的對象類外訪問父類的public和protected成員, 如下測試代碼:

int main() {protected_inherited_son protected_inherited_son_instance1;cout << protected_inherited_son_instance1.m_father_public_value << endl;cout << protected_inherited_son_instance1.m_father_protected_value << endl;return 0; }

編譯發現:

父類的public和protected成員都不能在類外訪問了, 說明都變成了protected成員.

• private繼承, 父類的public和protected成員成為子類的private成員.

就不測試了...

2 繼承后的子類和父類的析構順序問題

當繼承后的子類實例化對象之后, 其對象所占用的內存空間中會保留一份父類的對象空間. 所以子類實例化對象的過程中, 其父類的構造函數也會被調用, 析構過程同理.

用如下代碼測試一下其發生順序:

#include <iostream> using namespace std;class father { public:father() {cout << "calling father's constructor" << endl;}~father() {cout << "calling father's disconstructor" << endl;} };class public_inherited_son : public father { public:public_inherited_son() {cout << "calling son's constructor" << endl;}~public_inherited_son() {cout << "calling son's disconstructor" << endl;} };int main() {public_inherited_son public_inherited_son_instance1;return 0; }

結果為:

說明了子類實例化對象的過程中會先調用父類的構造函數, 然后是子類的構造函數. 同時也表明在棧上的話析構順序相反.

那么在堆區呢?

用如下代碼測試:

#include <iostream> using namespace std;class father { public:father() {cout << "calling father's constructor" << endl;}~father() {cout << "calling father's disconstructor" << endl;} };class public_inherited_son : public father { public:public_inherited_son() {cout << "calling son's constructor" << endl;}~public_inherited_son() {cout << "calling son's disconstructor" << endl;} };int main() {public_inherited_son *p_public_inherited_son_instance1 = new public_inherited_son;delete p_public_inherited_son_instance1;return 0; }

運行結果:

?

總結:

子類實例化對象的過程會先調用父類的構造函數, 然后是子類的構造函數. 析構函數的順序會反過來, 無論是棧上還是堆上.

2.1 調用父類構造函數想要傳遞參數方式

前面說到, 子類實例化對象時默認先調用父類構造函數, 再調用子類構造函數, 如果想要在調用父類構造函數時傳遞參數, 就需要顯示的調用父類構造函數, 即在子類構造函數初始化列表上顯示調用父類構造函數并傳遞參數.

2.2 C++11調用父類構造函數新方法

3 父類子類的同名成員處理方法

如果想要在子類對象訪問父類同名成員, 須在成員名前加上父類名稱作用域, 如下:

#include <iostream> using namespace std;class father { public:int member_value; };class public_inherited_son : public father { public:int member_value; };int main() {public_inherited_son public_inherited_son_instance1;public_inherited_son_instance1.member_value = 10;public_inherited_son_instance1.father::member_value = 1;cout << public_inherited_son_instance1.member_value << endl;cout << public_inherited_son_instance1.father::member_value << endl;return 0; }

結果如下:

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需注意: 如果子類中存在同名成員函數, 子類成員函數會隱藏掉父類中所有的同名成員函數(包括所有的重載函數), 如以下測試代碼:

#include <iostream> using namespace std;class father { public:void func1(int arg1) {cout << arg1 << endl;}void func1(int arg1, int arg2) {cout << arg1 << arg2 << endl;} };class public_inherited_son : public father { public:void func1(int arg1, int arg2, int arg3) {cout << arg1 << arg2 << arg3 << endl;} };int main() {public_inherited_son public_inherited_son_instance1;public_inherited_son_instance1.func1(1);public_inherited_son_instance1.func1(1, 2);return 0; }

編譯發現:

?

如果想要使用父類函數的話, 需要加上父類作用域, 如下:

#include <iostream> using namespace std;class father { public:void func1(int arg1) {cout << arg1 << endl;}void func1(int arg1, int arg2) {cout << arg1 << arg2 << endl;} };class public_inherited_son : public father { public:void func1(int arg1, int arg2, int arg3) {cout << arg1 << arg2 << arg3 << endl;} };int main() {public_inherited_son public_inherited_son_instance1;public_inherited_son_instance1.father::func1(1);public_inherited_son_instance1.father::func1(1, 2);return 0; }

結果為:

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總結

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