独習C++ 2章

2.1 構造体、共用体、列挙体

2.1.1 構造体

#include <iostream>

struct product
{
   int id;
   int price;
   int stock;
};

int main()
{
   product pen;
   pen.id = 0;
   pen.price = 100;
   pen.stock = 200;
   product * ptr = &pen;

   std::cout << "商品ID: " << ptr->id << std::endl;
   std::cout << "単価: " << ptr->price << std::endl;
   std::cout << "在庫数: " << ptr->stock << std::endl;
}

#include <iostream>

struct product
{
   int id;
   int price;
   int stock;
};

int main()
{
   product pen = {0,100,200};
   product * ptr = &pen;

   std::cout << "商品ID: " << ptr->id << std::endl;
   std::cout << "単価: " << ptr->price << std::endl;
   std::cout << "在庫数: " << ptr->stock << std::endl;
}

We have to write initial values in the same order as they are written in the struct definition.

#include <iostream>

struct product
{
   int id;
   int price;
   int stock;
};

void show_product(struct product* ptr)
{
   std::cout << "商品ID: " << ptr->id << std::endl;
   std::cout << "単価: " << ptr->price << std::endl;
   std::cout << "在庫数: " << ptr->stock << std::endl;
}

int main()
{
   product pen = {0, 100, 200};
   product *ptr = &pen;
   show_product(ptr);
}

2.1.2 共用体

member variable are in the same address.
So, if we change any of them, other variables are changed.

#include <iostream>

union U
{
   int a;
   int b;
   int c;
};

int main()
{
   U u = {42};
   std::cout << "u.aのアドレスは " << &u.a << std::endl;
   std::cout << "u.bのアドレスは " << &u.b << std::endl;
   std::cout << "u.cのアドレスは " << &u.c << std::endl;
}

2.1.3 enum

#include <iostream>

enum class category {
   Value1,
   Value2,
   Value3 = 100,
   Value4
};

int main()
{
   category cat = category::Value4;
   std::cout << static_cast<int>(cat) << std::endl;
}

列挙体変数が何バイト使うのかを決めるには、underlying-typeを指定する

enum class category : char {
   Value1,
   Value2,
   Value3 = 100,
   Value4
};

char type is 1 byte. So, in this case, the value of each enum is 0 <= and < 127.