向量的结构：添加元素C++

Question

我正在从文件中读取结构，我想将它们添加到结构向量中。 下面是它的外观和工作方式：

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; 
    } 

我应该怎么做，我可以加我的结构为载体，并将其打印正常（本刊只是一种检查真的，如果结构是正确加载成矢量）？

Answer 1

这里是代码的外观在现代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

你的错误是使用指针

std::vector<Student> students; 

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

现在，它会工作。

问题在于您一遍又一遍重复使用了相同的指针。所以你最终得到一个指向同一个对象的指针向量。这将有价值的最后一个学生阅读英寸

这似乎是一个相当普遍的初学特征选择复杂的选择，当更简单的一个是正确的，所以我会有兴趣知道为什么你选择使用指针。

Answer 3

既然你要存储指向学生，但不是在矢量学生。

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 
} 

您已经分配只有一个学生，每次你循环你一次又一次地阅读它。

相反，您应该在每个循环中分配一个新学生并读入新分配的内存。

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 
} 

不要忘记删除每个学生，当你不需要向量了。

Answer 4

您的代码不起作用，因为您每次都有一个Student对象并覆盖其成员。解决方法是每次创建一个新的Student对象并将指针传递给您的向量：

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 
{ 
    // ... 
}