C++程序员经常会面对的问题就是内存泄漏，如果操作不当发生内存泄漏，那么将会是毁灭性的Bug。在继承里，大部分基类中的析构函数通常会声明为虚函数，这样会避免指针占用的堆内存得不到释放，从而造成内存泄漏的情况。

#include <iostream>using namespace std;class People {
public:People(char *name_parameters, int age_parameters);People(const People &people);~People();virtual void display();char *get_name();int get_age();private:char *name;int age;
};People::People(char *name_parameters, int age_parameters) : name(name_parameters), age(age_parameters) {cout << "people class constructed is created" << endl;
}People::People(const People &people) { cout << "people copy constructed is used" << endl; }People::~People() { cout << "people destructed is used" << endl; }int People::get_age() { return this->age; }char *People::get_name() { return this->name; }void People::display() { cout << "name:" << name << ",age:" << age << endl; }class Teacher : public People {
public:Teacher(char *name, int age, int salary);~Teacher();void display();int get_salary();private:int salary;
};Teacher::Teacher(char *name, int age, int salary) : People(name, age), salary(salary) {cout << "Teacher class constructed is created" << endl;
}Teacher::~Teacher() { cout << "Teacher destructed is used" << endl; }int Teacher::get_salary() { return this->salary; }void Teacher::display() {cout << "name2:" << get_name() << ",age:" << get_age()\<< "salary:" << salary << endl;
}int main() {People *ptr = new People("people", 20);ptr = new Teacher("child", 10, 1000);delete ptr;return 0;
}

输出:
people class constructed is created
people class constructed is created
Teacher class constructed is created
people destructed is used

上述程序可以观察到，基类中没有使用到虚析构函数，导致delete调用基类析构函数在释放内存时，没有将派生类对象产生的堆内存释放掉，这样就容易造成内存泄漏。那么为什么没有调用派生类的析构函数呢？因为这里函数是非虚函数，通过指针访问非虚函数时，编译器会根据指针的类型来访问析构函数，现在指针的类型是People，那么在没有虚函数的情况下就只能调用People的析构函数。下面将基类的析构函数声明为虚析构函数。

#include <iostream>using namespace std;class People {
public:People(char *name_parameters, int age_parameters);People(const People &people);virtual ~People();virtual void display();char *get_name();int get_age();private:char *name;int age;
};People::People(char *name_parameters, int age_parameters) : name(name_parameters), age(age_parameters) {cout << "people class constructed is created" << endl;
}People::People(const People &people) { cout << "people copy constructed is used" << endl; }People::~People() { cout << "people destructed is used" << endl; }int People::get_age() { return this->age; }char *People::get_name() { return this->name; }void People::display() { cout << "name:" << name << ",age:" << age << endl; }class Teacher : public People {
public:Teacher(char *name, int age, int salary);~Teacher();void display();int get_salary();private:int salary;
};Teacher::Teacher(char *name, int age, int salary) : People(name, age), salary(salary) {cout << "Teacher class constructed is created" << endl;
}Teacher::~Teacher() { cout << "Teacher destructed is used" << endl; }int Teacher::get_salary() { return this->salary; }void Teacher::display() {cout << "name2:" << get_name() << ",age:" << get_age()\<< "salary:" << salary << endl;
}int main() {People *ptr = new People("people", 20);ptr = new Teacher("child", 10, 1000);delete ptr;return 0;
}

输出:
people class constructed is created
people class constructed is created
Teacher class constructed is created
Teacher destructed is used
people destructed is used

通过上述程序的修改，声明基类的虚析构函数之后，delete掉基类的指针，会调用派生类的析构函数和基类的析构函数释放内存。因为虚函数不会在意指针的类型，而是观察指针指向的对象，根据指针指向的对象来调用成员（成员变量和成员函数）。也就是说，指针指向哪个对象，就调用哪个对象的成员。指针ptr指向了派生类的对象，那么在释放内存时会优先调用派生类的析构函数，再根据delete机制调用基类的析构函数，这样就解决了内存泄漏的问题。

构造函数不能是虚函数

主要有两个原因：

1. 派生类不能继承基类的构造函数，因此把基类的构造函数声明为虚函数没有意义，无法实现多态；

2. c++中的构造函数用来在创建对象时进行初始化工作，在执行构造函数的时候，对象尚未创建完成，虚函数表这个时候还不存在， 也没有指向虚函数表的指针，所以此时还无法查询虚函数表。也就不知道调用哪一个构造函数。

虚析构函数的原理分析

1. 由于基类的析构函数为虚函数，所以派生类会在所有属性的前面形成虚表，而虚表内部存储的就是基类的虚函数。
2. 当delete基类的指针时，由于派生类的析构函数与基类的析构函数构成多态，所以得先调动派生类的析构函数；之所以再调动基类的析构函数，是因为delete的机制所引起的,delete 基类指针所指的空间，要调用基类的析构函数。