C++ struct,union和enum

struct

struct是一组数据元素一个名字，这些数据元素，作为成员，可以有不同类型和不同长度。C++声明struct语法如下：

struct structure_name {
member_type1 member_name1;
member_type2 member_name2;
member_type3 member_name3;
.
.
} object_names;

简单示例如下:

struct product {
  int weight;
  float price;
} ;

product apple;
product banana, melon;

声明struct并定义多个实例:

struct product {
  int weight;
  float price;
} apple, banana, melon;

完整struct例子:

// example about structures
#include <iostream>
#include <string>
#include <sstream>
using namespace std;

struct movies_t {
  string title;
  int year;
} mine, yours;

void printmovie (movies_t movie);

int main ()
{
  string mystr;

  mine.title = "2001 A Space Odyssey";
  mine.year = 1968;

  cout << "Enter title: ";
  getline (cin,yours.title);
  cout << "Enter year: ";
  getline (cin,mystr);
  stringstream(mystr) >> yours.year;

  cout << "My favorite movie is:\n ";
  printmovie (mine);
  cout << "And yours is:\n ";
  printmovie (yours);
  return 0;
}

void printmovie (movies_t movie)
{
  cout << movie.title;
  cout << " (" << movie.year << ")\n";
}

 

struct和指针

struct movies_t {
  string title;
  int year;
};

movies_t amovie;
movies_t * pmovie;
pmovie = &amovie;

完整示例如下:

// pointers to structures
#include <iostream>
#include <string>
#include <sstream>
using namespace std;

struct movies_t {
  string title;
  int year;
};

int main ()
{
  string mystr;

  movies_t amovie;
  movies_t * pmovie;
  pmovie = &amovie;

  cout << "Enter title: ";
  getline (cin, pmovie->title);
  cout << "Enter year: ";
  getline (cin, mystr);
  (stringstream) mystr >> pmovie->year;

  cout << "\nYou have entered:\n";
  cout << pmovie->title;
  cout << " (" << pmovie->year << ")\n";

  return 0;
}

 

内部struct

struct movies_t {
  string title;
  int year;
};

struct friends_t {
  string name;
  string email;
  movies_t favorite_movie;
  } charlie, maria;

friends_t * pfriends = &charlie;

使用如下:

charlie.name
maria.favorite_movie.title
charlie.favorite_movie.year
pfriends->favorite_movie.year

union

union允许以不同的数据类型访问相同的内存，因为他们实际上是在内存中的相同位置,它的声明和struct差不多，但功能完全不同:

union union_name {
  member_type1 member_name1;
  member_type2 member_name2;
  member_type3 member_name3;
  .
  .
} object_names;

union声明的所有元素在内存中占有相同的物理空间,其大小是该声明的最大元素的大小:

union mytypes_t {
  char c;
  int i;
  float f;
  } mytypes;

 

每一个不同的数据类型。由于所有的元素都指的是内存中的同一位置，修改的内容之一，会影响其他元素的内容。

 

匿名union

struct和正常union

struct {
  char title[50];
  char author[50];
  union {
    float dollars;
    int yen;
  } price;
} book;

struct和匿名union

struct {
  char title[50];
  char author[50];
  union {
    float dollars;
    int yen;
  };
} book;

第一种类型的访问方式，如下:

book.price.dollars
book.price.yen

第二种类型的访问方式,如下:

book.dollars
book.yen

 

enum

  定义枚举类型的基本语法：

enum enumeration_name {
  value1,
  value2,
  value3,
  .
  .
} object_names;

例如:

enum Week {SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY};

其中SUNDAY = 0,MONDAY = 1……SATURDAY = 6。也就是说，第１个枚举值代表0，第2个枚举值代表1，这样依次递增1。

也可以在定义时，直接指定某个或某些枚举值的数值,例如:

 enum Week {MONDAY = 1,TUESDAY,WEDNESDAY,THURSDAY,FRIDAY,SATURDAY,SUNDAY};

其中MONDAY等于1，这样，TUESDAY就将等于2，直接到SUNDAY等于7。

 

声明枚举变量:

Week today = TUESDAY;

 

枚举值，我们就称为格举常量，因为它一经定义以后，就不可再改变，以下用法是错误的！

TUESDAY = 10; //错误！我们不能改变一个枚举值的数值。