向量的結構：添加元素C++

Question

我正在從文件中讀取結構，我想將它們添加到結構向量中。 下面是它的外觀和工作方式：

typedef struct 
{ 
    int ID; 
    string name; 
    string surname; 
    int points; 
} 
Student; 

int main() 
{ 
    ifstream theFile("test.txt"); 
    std::vector<Student*> students; 

    Student* s = new Student(); 

    while(theFile >> s->ID >> s->name >> s->surname >> s->points) 
    { 
     studenti.push_back(s); // here I would like to add this struct s from a file 
    } 

// here I want to print each struct's values on the screen, but the output is always ONLY last struct N times, and not all of them, each only once 


    std::vector<Student*>::const_iterator it; 
    for(it = students.begin(); it != students.end(); it+=1) 
    { 
     std::cout << (*it)->ID <<" " << (*it)->name << " " << (*it)->surname <<" " << (*it)->points <<endl; 
    } 

我應該怎麼做，我可以加我的結構爲載體，並將其打印正常（本刊只是一種檢查真的，如果結構是正確加載成矢量）？

Answer 1

這裏是代碼的外觀在現代C++：

#include <string> 
#include <istream> 
#include <vector> 

struct Student 
{ 
    int ID; 
    std::string name; 
    std::string surname; 
    int points; 

    Student(int i, std::string n, std::string s, int p) 
    : ID(i), name(std::move(n)), surname(std::move(s)), points(p) {} 
}; 

std::vector<Student> read_students(std::istream & is) 
{ 
    std::vector<Student> result; 

    std::string name, surname; 
    int id, points; 

    while (is >> id >> name >> surname >> points) 
    { 
     result.emplace_back(id, name, surname, points); 
    } 

    return result; 
} 

用法：

#include <fstream> 
#include <iostream> 

int main() 
{ 
    std::ifstream infile("test.txt"); 
    auto students = read_students(infile); 

    // ... 
} 

Answer 2

你的錯誤是使用指針

std::vector<Student> students; 

Student s; 
while(theFile >> s.ID >> s.name >> s.surname >> s.points) 
{ 
    students.push_back(s); 
} 

現在，它會工作。

問題在於您一遍又一遍重複使用了相同的指針。所以你最終得到一個指向同一個對象的指針向量。這將有價值的最後一個學生閱讀英寸

這似乎是一個相當普遍的初學特徵選擇複雜的選擇，當更簡單的一個是正確的，所以我會有興趣知道爲什麼你選擇使用指針。

Answer 3

既然你要存儲指向學生，但不是在矢量學生。

Student* s = new Student(); 

while(theFile >> s->ID >> s->name >> s->surname >> s->points) 
{ 
    students.push_back(s); // here I would like to add this struct s from a file 
} 

您已經分配只有一個學生，每次你循環你一次又一次地閱讀它。

相反，您應該在每個循環中分配一個新學生並讀入新分配的內存。

Student* s; 
int tmpId, tmpPoints; 
string tmpname, tmpsur; 

while(theFile >> tmpId >> tmpname >> tmpsur >> tmpPoints) 
{ 
    s = new Student(); 

    s->ID = tmpId ; 
    s->name = tmpname; 
    s->sur = tmpsur ; 
    s->points= tmpPoints; 

    studenti.push_back(s); // here You push a pointer to the newly allocated student 
} 
else 
{ 
    // There is error reading data 
} 

不要忘記刪除每個學生，當你不需要向量了。

Answer 4

您的代碼不起作用，因爲您每次都有一個Student對象並覆蓋其成員。解決方法是每次創建一個新的Student對象並將指針傳遞給您的向量：

std::vector<Student*> students; 
int tmpId, tmpPoints; 
string tmpname, tmpsur; 

while(theFile >> tmpId >> tmpname >> tmpsur >> tmpPoints) 
{ 
    Student* s = new Student(); 
    s->ID = tmpId ; 
    s->name = tmpname; 
    s->sur = tmpsur ; 
    s->points= tmpPoints; 

    students.push_back(s); // push a pointer to new student object 
} 
else 
{ 
    // ... 
}