c++創建對象時一些小細節

1. 父類型指針 a = new 子類型; delete a時,父/子類的構造/析構函數時如何調用的?

直接看下面例子:

 1 #include <memory> 2 #include <iostream> 3  4 using namespace std; 5  6 class BF{ 7     public: 8         BF(){ cout << "base::base()\n";fun();} 9         virtual ~BF(){cout << "~base::base()\n";fun();}10         virtual void fun(){cout << "base::fun() virtual\n";}11 };12 13 class derive: public BF14 {1517 public:18     derive(){cout << "derive::derive()\n";fun();}19     ~derive(){cout << "~derive::derive()\n";fun();}20     virtual void fun(){cout << "derive::fun() virtual\n";}21 };22 23 int main()24 {25     cout << "-------------------" << endl;26     BF *b = new BF();27     delete b;28     cout << "-------------------" << endl;29     derive *d = new derive();30     delete d;31     cout << "-------------------" << endl;32     BF *bd = new derive();33     delete bd;34     return 0;35     36 }

運行結果:

-------------------
base::base()
base::fun() virtual
~base::base()
base::fun() virtual
-------------------
base::base()
base::fun() virtual
derive::derive()
derive::fun() virtual
~derive::derive()
derive::fun() virtual
~base::base()
base::fun() virtual
-------------------
base::base()
base::fun() virtual
derive::derive()
derive::fun() virtual
~derive::derive()
derive::fun() virtual
~base::base()
base::fun() virtual

結論: 類型A指針 a = new 類型B 時,無論類型A如何,都是調用類型B的構造函數來初始化,delete a時也是調用類型B的析構函數

2. C++創建類對象時(無參)后不加括號與加括號的區別(參考:https://www.cnblogs.com/traeyee/p/4892410.html)

直接看例子:

 1 #include<iostream> 2 using namespace std; 3  4 class C{ 5 public: 6     C(){ 7         cout<<"Hello C++\n"; 8     }; 9     C(int m){10         cout<<"I love C++\n";11     };12 private:13     int n;14 };15 16 int main(){17     cout << "-------get parameter------" << endl;18     C a(1);19     cout << "-------get ()------" << endl;20     C b();21     cout << "-------nothing------" << endl;22     C c;23     return 0;24 }

運行結果:

-------get parameter------
I love C++
-------get ()------
-------nothing------
Hello C++

結論:

給參數時,調用相應的構造函數,在棧內存中實例化對象;

只有一個括號時……這尼瑪是聲明了一個返回該類對象的函數;

啥都沒有只有對象名時,調用默認構造函數(如果沒有定義默認構造函數則出錯)。

3. C++中 new 一個對象 時加括號和不加括號的區別

 1 #include<iostream> 2 using namespace std; 3  4 class C{ 5 public: 6     C(){ 7         cout<<"Hello C++\n"; 8     }; 9     C(int m){10         cout<<"I love C++\n";11     };12 private:13     int n;14 };15 16 int main(){17     cout << "-------get parameter------" << endl;18     C* a = new C(1);19     cout << "-------get ()------" << endl;20     C* b = new C();21     cout << "-------nothing------" << endl;22     C* c = new C;23     return 0;24 }

運行結果:

-------get parameter------
I love C++
-------get ()------
Hello C++
-------nothing------
Hello C++

結論:

傳入參數時,會調用相應的構造函數;沒有參數時,無論是否加括號,都會調用默認構造函數