Struct 在全局空间中不适用于一个程序，但适用于另一个程序

问：

我对C++比较陌生。我最近在书中学习了C++的结构，它有以下代码来添加以英尺和英寸为单位的测量值。下面粘贴的代码按预期工作：

#include <iostream>
using namespace std;

struct Distance {
    int feet;
    float inches;
    };

int main(){
    Distance d1, d3; 
    Distance d2 = { 11, 6.25 }; 
    cout << "\nEnter feet: "; cin >> d1.feet;
    cout << "Enter inches: "; cin >> d1.inches;
    d3.inches = d1.inches + d2.inches; 
    d3.feet = 0; 
    if(d3.inches >= 12.0) 
        {
        d3.inches -= 12.0; 
        d3.feet++;
        }
    d3.feet += d1.feet + d2.feet;


    cout << d1.feet << "\'-" << d1.inches << "\" + ";
    cout << d2.feet << "\'-" << d2.inches << "\" = ";
    cout << d3.feet << "\'-" << d3.inches << "\"\n";
    return 0;
}

正如你所看到的，结构变量是在主函数内部定义的，而结构是在主函数外部定义的。

我还决定测试我的知识并制作一个类似的程序，这将增加时间，如下所示：

#include <iostream>
using namespace std;

// This function basically calculates how much is carried over to the next quantity for eg. if seconds      //is 122, it will return 2  which will be carried to minutes.



int carry (int inp) {
    int carry;
    carry = inp / 60;
    return carry;
}

struct time {
int hours;
int minutes;
int seconds;
};
    
int main(){

    char ques = 'y';
    time time1, time2, timetot;

    do {
        cout << "Want to add time (y/n) ? : "; cin >> ques;
        if (ques != 'y') break;
        cout << "Hours   : "  ; cin >> time1.hours  ; cout << endl;
        cout << "Minutes : "  ; cin >> time1.minutes; cout << endl;
        cout << "Seconds : "  ; cin >> time1.seconds; cout << endl;
        cout << "Hours   : "  ; cin >> time2.hours  ; cout << endl;
        cout << "Minutes : "  ; cin >> time2.minutes; cout << endl;
        cout << "Seconds : "  ; cin >> time2.seconds; cout << endl;
        
        if (time1.seconds >=60) { time1.minutes += carry(time1.seconds); time1.seconds %= 60; }
        if (time1.minutes >=60) { time1.hours += carry(time1.minutes);   time1.minutes %= 60; }
        if (time2.seconds >=60) { time2.minutes += carry(time2.seconds); time2.seconds %= 60; }
        if (time2.minutes >=60) { time2.hours += carry(time2.minutes);   time2.minutes %= 60; }

        timetot.hours = time1.hours + time2.hours ;
        timetot.minutes = time1.minutes + time2.minutes ;
        timetot.seconds = time1.seconds + time2.seconds ;

        if (time1.seconds >=60) { time1.minutes += carry(time1.seconds); }
        if (time1.minutes >=60) { time1.hours += carry(time1.minutes); }
        if (time2.seconds >=60) { time2.minutes += carry(time2.seconds); }
        if (time2.minutes >=60) { time2.hours += carry(time2.minutes); }

        cout << "Tot Hours"   << timetot.hours   << endl;
        cout << "Tot minutes" << timetot.minutes << endl;
        cout << "Tot seconds" << timetot.seconds << endl;
    }
    while (ques=='y') ;
    return 0;
};

上面的代码生成错误，即 time1、time2、time3 未定义，但是，我已经明确定义了这些结构变量。我也知道上面的代码没有语法错误，因为当我尝试在 main（） 函数中定义结构时，程序按预期工作。我也在下面粘贴代码：

因为，我的编译器说他如下：

addTime.cpp: In function 'int main()':
addTime.cpp:20:9: error: expected ';' before 'time1'
   20 |     time time1, time2, timetot;
      |         ^~~~~~
      |         ;
addTime.cpp:27:40: error: 'time1' was not declared in this scope; did you mean 'timeb'?
   27 |         cout << "Hours   : "  ; cin >> time1.hours  ; cout << endl;
      |                                        ^~~~~
      |                                        timeb
addTime.cpp:30:40: error: 'time2' was not declared in this scope; did you mean 'timeb'?
   30 |         cout << "Hours   : "  ; cin >> time2.hours  ; cout << endl;
      |                                        ^~~~~
      |                                        timeb
addTime.cpp:39:9: error: 'timetot' was not declared in this scope; did you mean 'time_t'?
   39 |         timetot.hours = time1.hours + time2.hours ;
      |         ^~~~~~~
      |         time_t

我决定将结构定义带入 main（） 函数的范围内，我这样做了，代码可以工作。我粘贴了下面的代码，您也可以将其复制粘贴到编译器中并运行它。这确认了我的代码没有语法错误。那么，为什么书本上写的代码可以工作，而当我做同样的事情时，它不起作用

#include <iostream>
using namespace std;

// This function basically calculates how much is carried over to the next quantitiy for eg. if seconds //in 122, it will return 2  which will be carried to minutes

int carry (int inp) {
    int carry;
    carry = inp / 60;
    return carry;
};
    
int main(){
    struct time {
        int hours;
        int minutes;
        int seconds;
    };

    char ques = 'y';
    time time1, time2, timetot;

    do {

        cout << "Want to add time (y/n) ? : "; cin >> ques;
        if (ques != 'y') break;

        cout << "Hours   : "  ; cin >> time1.hours  ; cout << endl;
        cout << "Minutes : "  ; cin >> time1.minutes; cout << endl;
        cout << "Seconds : "  ; cin >> time1.seconds; cout << endl;
        cout << "Hours   : "  ; cin >> time2.hours  ; cout << endl;
        cout << "Minutes : "  ; cin >> time2.minutes; cout << endl;
        cout << "Seconds : "  ; cin >> time2.seconds; cout << endl;
        
        if (time1.seconds >=60) { time1.minutes += carry(time1.seconds); time1.seconds %= 60; }
        if (time1.minutes >=60) { time1.hours += carry(time1.minutes);   time1.minutes %= 60; }
        if (time2.seconds >=60) { time2.minutes += carry(time2.seconds); time2.seconds %= 60; }
        if (time2.minutes >=60) { time2.hours += carry(time2.minutes);   time2.minutes %= 60; }

        timetot.hours = time1.hours + time2.hours ;
        timetot.minutes = time1.minutes + time2.minutes ;
        timetot.seconds = time1.seconds + time2.seconds ;

        if (time1.seconds >=60) { time1.minutes += carry(time1.seconds); }
        if (time1.minutes >=60) { time1.hours += carry(time1.minutes); }
        if (time2.seconds >=60) { time2.minutes += carry(time2.seconds); }
        if (time2.minutes >=60) { time2.hours += carry(time2.minutes); }

        cout << "Tot Hours"   << timetot.hours   << endl;
        cout << "Tot minutes" << timetot.minutes << endl;
        cout << "Tot seconds" << timetot.seconds << endl;
    }
    while (ques=='y') ;
    return 0;
};

正如你所看到的，我所做的只是在main（）函数中定义结构，程序开始工作。我错过了什么？请帮帮我。