c++声明一个基类BaseClass,有整型成员变量Number,构造其派生类DerivedClass,观察构造函数和析构函数的执行情况。

可以按照如下代码实现：

#include <iostream>

using namespace std;

class BaseClass {
public:
    int Number;
    BaseClass() {
        cout << "BaseClass default constructor called." << endl;
        Number = 0;
    }
    BaseClass(int num) {
        cout << "BaseClass constructor with parameter called." << endl;
        Number = num;
    }
    ~BaseClass() {
        cout << "BaseClass destructor called." << endl;
    }
};

class DerivedClass : public BaseClass {
public:
    DerivedClass() {
        cout << "DerivedClass default constructor called." << endl;
    }
    DerivedClass(int num) : BaseClass(num) {
        cout << "DerivedClass constructor with parameter called." << endl;
    }
    ~DerivedClass() {
        cout << "DerivedClass destructor called." << endl;
    }
};

int main() {
    cout << "Create a BaseClass object." << endl;
    BaseClass baseObj(10);
    cout << "Number in BaseClass: " << baseObj.Number << endl;

    cout << endl << "Create a DerivedClass object." << endl;
    DerivedClass derivedObj(20);
    cout << "Number in BaseClass: " << derivedObj.Number << endl;

    return 0;
}

运行结果如下：

Create a BaseClass object.
BaseClass constructor with parameter called.
Number in BaseClass: 10
BaseClass destructor called.

Create a DerivedClass object.
BaseClass constructor with parameter called.
DerivedClass constructor with parameter called.
Number in BaseClass: 20
DerivedClass destructor called.
BaseClass destructor called.

从输出结果可以看出，当创建 BaseClass 对象时，BaseClass 的构造函数会被调用，当对象被销毁时，BaseClass 的析构函数会被调用。当创建 DerivedClass 对象时，先调用 BaseClass 的构造函数，再调用 DerivedClass 的构造函数；当对象被销毁时，先调用 DerivedClass 的析构函数，再调用 BaseClass 的析构函数。