设计国家类Country，包含国名（char name[40]）、首都（char capital[40]）和人口数量（int Population）。根据给定的main函数设计必要的成员函数。

以下是根据给定的main函数设计的Country类：

#include<iostream>
#include<cstring>

using namespace std;

class Country {
private:
    char name[40];
    char capital[40];
    int population;
public:
    Country(const char* cou = "Russia", const char* cap = "Moscow", int pop = 143000000);
    const char* GetName() const;
    const char* GetCapital() const;
    int GetPopulation() const;
    void Set(const char* cou = "Russia", const char* cap = "Moscow", int pop = 143000000);
    void Show();
};

Country::Country(const char* cou, const char* cap, int pop) {
    strcpy(name, cou);
    strcpy(capital, cap);
    population = pop;
}

const char* Country::GetName() const {
    return name;
}

const char* Country::GetCapital() const {
    return capital;
}

int Country::GetPopulation() const {
    return population;
}

void Country::Set(const char* cou, const char* cap, int pop) {
    strcpy(name, cou);
    strcpy(capital, cap);
    population = pop;
}

void Country::Show() {
    cout << name << "-" << capital << "-" << population << endl;
}

int main() {
    Country c1;
    cout << "Show object c1:";
    cout << c1.GetName() << "-" << c1.GetCapital() << "-" << c1.GetPopulation() << endl;

    char name[40], capital[40];
    int population;
    cin >> name >> capital >> population;

    Country c2(name, capital, population);
    cout << "Show object c2:";
    cout << c2.GetName() << "-" << c2.GetCapital() << "-" << c2.GetPopulation() << endl;

    cin >> name >> capital >> population;

    cout << "Reset and Show object c1:";
    c1.Set(name, capital, population);
    c1.Show();

    cout << "Reset and Show object c2:";
    c2.Set();
    c2.Show();

    return 0;
}

在该类中，我们将国名、首都和人口数量分别用char数组、char数组和int类型来表示，并提供了构造函数、获取成员变量的函数、设置成员变量的函数和展示成员变量的函数。其中，构造函数和设置成员变量的函数都提供了默认参数，使得在创建对象时可以不提供参数。