设计并实现一个ObjectCounter类，要求： （1）对系统中现存的本类实例计数； （2）每个对象都有唯一的整型ID，按照创建的次序建立； （3）获取当前系统中本类对象的个数； （4）获取每个对象的ID。编写测试程序

下面是一个可能的ObjectCounter类的实现：

class ObjectCounter {
public:
    ObjectCounter() {
        id_ = count_++;
    }

    virtual ~ObjectCounter() {
        --count_;
    }

    static size_t getCount() {
        return count_;
    }

    int getID() const {
        return id_;
    }

private:
    static size_t count_;
    size_t id_;
};

size_t ObjectCounter::count_ = 0;

这个类有一个静态成员变量 `count_`，用于记录当前系统中本类对象的个数。每次创建一个新对象时，它的构造函数会将 `count_` 增加，同时为该对象分配一个唯一的ID。当对象被销毁时，`count_` 会减少。

使用该类的示例测试程序：

#include <iostream>
#include <vector>

class MyClass : public ObjectCounter {
public:
    MyClass() {
        std::cout << "MyClass #" << getID() << " created" << std::endl;
    }

    ~MyClass() {
        std::cout << "MyClass #" << getID() << " destroyed" << std::endl;
    }
};

int main() {
    std::vector<MyClass*> objs;

    for (int i = 0; i < 5; ++i) {
        objs.push_back(new MyClass());
    }

    std::cout << "Number of MyClass objects: " << ObjectCounter::getCount() << std::endl;

    for (auto obj : objs) {
        std::cout << "Object #" << obj->getID() << std::endl;
        delete obj;
    }

    std::cout << "Number of MyClass objects: " << ObjectCounter::getCount() << std::endl;

    return 0;
}

输出结果为：

MyClass #0 created
MyClass #1 created
MyClass #2 created
MyClass #3 created
MyClass #4 created
Number of MyClass objects: 5
Object #0
MyClass #0 destroyed
Object #1
MyClass #1 destroyed
Object #2
MyClass #2 destroyed
Object #3
MyClass #3 destroyed
Object #4
MyClass #4 destroyed
Number of MyClass objects: 0