c++创建对象时一些小细节

1. 父类型指针 a = new 子类型; delete a时，父/子类的构造/析构函数时如何调用的？

直接看下面例子：

#include <memory>
#include <iostream>

using namespace std;

class BF{
    public:
        BF(){ cout << "base::base()
";fun();}
        virtual ~BF(){cout << "~base::base()
";fun();}
        virtual void fun(){cout << "base::fun() virtual
";}
};

class derive: public BF
{

public:
    derive(){cout << "derive::derive()
";fun();}
    ~derive(){cout << "~derive::derive()
";fun();}
    virtual void fun(){cout << "derive::fun() virtual
";}
};

int main()
{
    cout << "-------------------" << endl;
    BF *b = new BF();
    delete b;
    cout << "-------------------" << endl;
    derive *d = new derive();
    delete d;
    cout << "-------------------" << endl;
    BF *bd = new derive();
    delete bd;
    return 0;
    
}

运行结果：

-------------------
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

结论： 类型A指针 a = new 类型B 时，无论类型A如何，都是调用类型B的构造函数来初始化，delete a时也是调用类型B的析构函数

2. C++创建类对象时（无参）后不加括号与加括号的区别（参考：https://www.cnblogs.com/traeyee/p/4892410.html）

直接看例子：

#include<iostream>
using namespace std;

class C{
public:
    C(){
        cout<<"Hello C++
";
    };
    C(int m){
        cout<<"I love C++
";
    };
private:
    int n;
};

int main(){
    cout << "-------get parameter------" << endl;
    C a(1);
    cout << "-------get ()------" << endl;
    C b();
    cout << "-------nothing------" << endl;
    C c;
    return 0;
}

运行结果：

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

结论：

给参数时，调用相应的构造函数，在栈内存中实例化对象；

只有一个括号时……这尼玛是声明了一个返回该类对象的函数；

啥都没有只有对象名时，调用默认构造函数（如果没有定义默认构造函数则出错）。

3. C++中 new 一个对象 时加括号和不加括号的区别

#include<iostream>
using namespace std;

class C{
public:
    C(){
        cout<<"Hello C++
";
    };
    C(int m){
        cout<<"I love C++
";
    };
private:
    int n;
};

int main(){
    cout << "-------get parameter------" << endl;
    C* a = new C(1);
    cout << "-------get ()------" << endl;
    C* b = new C();
    cout << "-------nothing------" << endl;
    C* c = new C;
    return 0;
}

运行结果：

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

结论：

传入参数时，会调用相应的构造函数；没有参数时，无论是否加括号，都会调用默认构造函数