回顧struct,union的申明,初始化,io處理

/*  * File:   main.cpp * Author: Vicky.H * Email:  eclipser@163.com */#include <iostream>#include <typeinfo>#include <string.h>void f(void) {    struct A {        short age;        int money;        char name[12];    }; // 12 + 2 + 4 = 18  但實際上是20,因為 結構體的長度以 4 * n    std::cout << sizeof(struct A) << "\t" << typeid(struct A).name() << std::endl;    union B {        short age;        int money;        char name[12];    }; // 公用體的長度以 4 * n        std::cout << sizeof(union B) << "\t" << typeid(union B).name() << std::endl;}/* *  */int main(void) {        f();    f();    f();        struct A {};    std::cout << typeid(struct A).name() << std::endl;        std::cout << "---------------------------" << std::endl;        union B {        short age;        int money;        char name[12];    };    B b;    b.age = 2;    std::cout << b.age << std::endl;    b.money = 999;    std::cout << b.age << "\t" << b.money << std::endl;    strcpy(b.name,"Jack");    std::cout << b.age << "\t" << b.money << "\t" << b.name << std::endl;            return 0;}

執行:

20      Z1fvE1A
12      Z1fvE1B
20      Z1fvE1A
12      Z1fvE1B
20      Z1fvE1A
12      Z1fvE1B
Z4mainE1A
---------------------------
2
999     999
24906   1801675082      Jack

 

理解編譯期與運行期:
這裡主要討論的是struct 與 union 申明位置.這個執行個體中,同事聲明了2個struct A 以及 2個union B,分別在main()函數之外的函數體內申明以及main()函數之內申明.

需要注意的是,即便是在函數體中申明的結構體,也是在編譯期間就定義好的,也就是說,是靜態產生的,並非程式執行到申明結構體的代碼才動態產生的!這和全域變數,局部變數,全域函數,局部函數,以及全域#include,局部#include,全域using name namespace xxx.局部using namespace xxx是一樣的,都是靜態編譯期間就完工定義,並非動態時定義!不同之處就是限制了使用的範圍而已.本質上沒有任何區別,不會導致更多的效能消耗,這裡我們使用typeid來證明了並非程式執行到申明結構體的行才動態申明結構體,因為,他們typeid是相同的!!!

 

 

共用體,IO處理:

/*  * File:   main.cpp * Author: Vicky.H * Email:  eclipser@163.com */#include <iostream>#include <fstream>   #include <sstream>   #include <cstring> struct Animal {    int kind;        union {        char human[10];        char bird[20];        char fish[30];    };        char* getName(void) {        if (kind == 1) {            return human;        } else if (kind == 2) {            return bird;        } else {            return fish;        }    }        // 考慮IO處理    friend std::ostream& operator<<(std::ostream& os, const Animal& ref);      friend std::istream& operator>>(std::istream& is, Animal& ref);  };std::ostream& operator<<(std::ostream& os, const Animal& ref) {      os << ref.kind << '\n' /**需要分行符號*/;//    if (ref.kind == 1) {//        os << ref.human;//    } else if (ref.kind == 2) {//        os << ref.bird;//    } else {//        os << ref.fish;//    }    os << ref.fish;    return os;  }    std::istream& operator>>(std::istream& is, Animal& ref) {      is >> ref.kind >> ref.fish;      return is;  }  /* *  */int main(void) {    Animal human = {1,"Jack"};    std::cout << human.getName() << std::endl;        std::ofstream ofs("./tmp.txt", std::ios::trunc | std::ios::binary);      ofs << human; // 將對象輸入到檔案中       ofs.flush();      ofs.close();        std::ifstream ifs("./tmp.txt", std::ios::in | std::ios::binary);      Animal a1;      ifs >> a1; //  讀取檔案內容,將其轉換為對象       ifs.close();      std::cout << a1 << std::endl;          std::cout << "---------------------------" << std::endl;        Animal bird = {2,"Sparrow"};    std::cout << bird.getName() << std::endl;        std::ofstream ofs2("./tmp2.txt", std::ios::trunc | std::ios::binary);      ofs2 << bird; // 將對象輸入到檔案中       ofs2.flush();      ofs2.close();        std::ifstream ifs2("./tmp2.txt", std::ios::in | std::ios::binary);      Animal a2;      ifs2 >> a2; //  讀取檔案內容,將其轉換為對象       ifs2.close();      std::cout << a2 << std::endl;          std::cout << "---------------------------" << std::endl;        Animal fish = {3, "Dolphin"};    std::cout << fish.getName() << std::endl;        std::ofstream ofs3("./tmp3.txt", std::ios::trunc | std::ios::binary);      ofs3 << fish; // 將對象輸入到檔案中       ofs3.flush();      ofs3.close();        std::ifstream ifs3("./tmp3.txt", std::ios::in | std::ios::binary);      Animal a3;      ifs3 >> a3; //  讀取檔案內容,將其轉換為對象       ifs3.close();      std::cout << a3 << std::endl;                  return 0;}

 

運行:
Jack
1
Jack
---------------------------
Sparrow
2
Sparrow
---------------------------
Dolphin
3
Dolphin