C++ struct,union和enum

struct

struct是一組資料元素一個名字，這些資料元素，作為成員，可以有不同類型和不同長度。C++聲明struct文法如下：

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

簡單樣本如下:

struct product {
int weight;
float price;
} ;

product apple;
product banana, melon;

聲明struct並定義多個執行個體:

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和指標

struct movies_t {
string title;
int year;
};

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

完整樣本如下:

// 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;
}

 內部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允許以不同的資料類型訪問相同的記憶體，因為他們實際上是在記憶體中的相同位置,它的聲明和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聲明的所有元素在記憶體中佔有相同的物理空間,其大小是該聲明的最大元素的大小:

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

 

每一個不同的資料類型。由於所有的元素都指的是記憶體中的同一位置，修改的內容之一，會影響其他元素的內容。

 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;

第一種類型的訪問方式，如下:

book.price.dollars
book.price.yen

第二種類型的訪問方式,如下:

book.dollars
book.yen

 

enum

  定義枚舉類型的基本文法：

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。也就是說，第１個枚舉值代表0，第2個枚舉值代表1，這樣依次遞增1。

也可以在定義時，直接指定某個或某些枚舉值的數值,例如:

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

其中MONDAY等於1，這樣，TUESDAY就將等於2，直接到SUNDAY等於7。

 

聲明枚舉變數:

Week today = TUESDAY;

 

枚舉值，我們就稱為格舉常量，因為它一經定義以後，就不可再改變，以下用法是錯誤的！

TUESDAY = 10; //錯誤！我們不能改變一個枚舉值的數值。