在 C++ 中创建一个类的实例向量
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create a vector of instances of a class in c++
提问by mazlor
i created a class its name is Student as follows:
我创建了一个名为 Student 的类，如下所示：
class Student
{
private:
unsigned int id; // the id of the student
public:
unsigned int get_id(){return id;};
void set_id(unsigned int value) {id = value;};
Student(unsigned int init_val) {id = init_val;}; // constructor
~Student() {}; // destructor
};
then after i wanted to have a container ( say a vector ) its elements are instances of class Student, but i found myself not able to understand this situation , here is my issue:
然后在我想要一个容器（比如一个向量）之后，它的元素是类 Student 的实例，但我发现自己无法理解这种情况，这是我的问题：
first i run this code:
首先我运行这个代码：
#include<iostream>
#include<vector>
using namespace std;
const unsigned int N = 5;
Student ver_list[2] = {7, 9};
int main()
{
cout<< "Hello, This is a code to learn classes"<< endl;
cout<< ver_list[1].get_id() << endl;
return 0;
}
everything is fine and the output is :
一切都很好，输出是：
Hello, This is a code to learn classes
9
now when i try these options:
现在当我尝试这些选项时：
option #1:
选项1：
#include<iostream>
#include<vector>
using namespace std;
const unsigned int N = 5;
vector <Student> ver[N]; // Create vector with N elements
for(unsigned int i = 0; i < N; ++i )
ver[i].set_id(i);
int main()
{
cout<< "Hello, This is a code to learn classes"<< endl;
cout<< ver[1].get_id() << endl;
return 0;
}
i got this output "error" :
我得到了这个输出“错误”：
test.cpp:26:3: error: expected unqualified-id before 'for'
for(unsigned int i = 0; i < N; ++i )
^
test.cpp:26:27: error: 'i' does not name a type
for(unsigned int i = 0; i < N; ++i )
^
test.cpp:26:34: error: expected unqualified-id before '++' token
for(unsigned int i = 0; i < N; ++i )
^
test.cpp: In function 'int main()':
test.cpp:43:15: error: 'class std::vector<Student>' has no member named 'get_id'
cout<< ver[1].get_id() << endl;
^
option #2:
选项#2：
#include<iostream>
#include<vector>
using namespace std;
const unsigned int N = 5;
Student ver[N]; // Create one dimensional array with N elements
for(unsigned int i = 0; i < N; ++i )
ver[i].set_id(i);
int main()
{
cout<< "Hello, This is a code to learn classes"<< endl;
cout<< ver[1].get_id() << endl;
return 0;
}
the output "error" was :
输出“错误”是：
test.cpp:30:14: error: no matching function for call to 'Student::Student()'
Student ver[5];
^
test.cpp:30:14: note: candidates are:
test.cpp:14:2: note: Student::Student(unsigned int)
Student(unsigned int init_val) {id = init_val;}; // constructor
^
test.cpp:14:2: note: candidate expects 1 argument, 0 provided
test.cpp:7:7: note: Student::Student(const Student&)
class Student
^
test.cpp:7:7: note: candidate expects 1 argument, 0 provided
test.cpp:31:1: error: expected unqualified-id before 'for'
for(unsigned int i = 0; i < N; ++i )
^
test.cpp:31:25: error: 'i' does not name a type
for(unsigned int i = 0; i < N; ++i )
^
test.cpp:31:32: error: expected unqualified-id before '++' token
for(unsigned int i = 0; i < N; ++i )
^
In the first try everything was looking ok , but when i tried the two next options , i received errors , i wish that i can understand what wrong i am doing.
在第一次尝试中，一切看起来都很好，但是当我尝试下两个选项时，我收到了错误，我希望我能理解我做错了什么。
Thanks.
谢谢。
回答by Martin York
This:
这个：
vector <Student> ver[N];
Creates an array of Nelements. Each element is vector<Student>. This is not you want. You were probably trying to create a vector of Nelements. The syntax for this is:
创建N元素数组。每个元素都是vector<Student>。这不是你想要的。您可能正在尝试创建N元素向量。其语法是：
vector <Student> ver(N);
But you can't use this because your class does not have a default constructor. So your next alternative is to initializae all the objects with the same element.
但是您不能使用它，因为您的类没有默认构造函数。因此，您的下一个选择是使用相同的元素初始化所有对象。
vector <Student> ver(N, Student(0));
You also tried to create an array of students like this:
您还尝试创建一组像这样的学生：
Student ver[N];
This will not work. Because it tries to initialize every element in the array with the default constructor. But your class does not have a default constructor. So this will not work. But this is why your original code did work:
这是行不通的。因为它尝试使用默认构造函数初始化数组中的每个元素。但是您的类没有默认构造函数。所以这行不通。但这就是您的原始代码确实有效的原因：
Student ver_list[2] = {7, 9}; // Here you are using the constructor for your object.
// It uses the normal constructor you provided not the default one.
The other issues is that you can not run code outside a function(method).
So this will not work:
另一个问题是您不能在函数（方法）之外运行代码。
所以这行不通：
for(unsigned int i = 0; i < N; ++i )
ver[i].set_id(i);
In C++11 you can initialize a vector the same way as an array:
在 C++11 中，您可以像数组一样初始化向量：
vector<Student> ver = { 0, 1, 2, 3, 4, 5};
If you don't have C++11 or initialization is more complex. Then you need to write a wrapper.
如果您没有 C++11 或初始化更复杂。然后你需要写一个包装器。
class VecWrapper
{
public:
std::vector<Student> ver;
VecWrapper()
{
ver.reserve(N);
for(unsigned int i = 0; i < N; ++i )
ver.push_back(Student(i));
}
};
Now You can place this in global scope and it will auto init.
现在你可以把它放在全局范围内，它会自动初始化。
VecWrapper myData; // myData.vec initializaed before main entered.
int main()
{}
Full solution:
完整解决方案：
Option 2:
选项 2：
#include<iostream>
#include<vector>
using namespace std;
const unsigned int N = 5;
// The following is not correct
// This creates an arrya of `N` elements each element is `vector <Student>`
//
// vector <Student> ver[N]; // Create vector with N elements
//
// The following lines are not allowed.
// All code has to be inside a function.
//
// for(unsigned int i = 0; i < N; ++i )
// ver[i].set_id(i);
// What you want is:
// I use the following because it is unclear if you have C++11 or not.
class VecWrapper
{
public:
std::vector<Student> vec;
VecWrapper()
{
vec.reserve(N);
for(unsigned int i = 0; i < N; ++i )
vec.push_back(Student(i));
}
};
VecWrapper myData; // myData.vec
int main()
{
cout<< "Hello, This is a code to learn classes"<< endl;
cout<< myData.vec[1].get_id() << endl;
return 0;
}
回答by Zach Stark
The main problem is you are trying to execute a for loop at global scope. It is acceptable to define and initialize variables outside of a function, but using a for loop or assignment operator is not. Put the for loop into main() (and I would recommend you also put N and the vector/student array into main() and everything should work.
Additionally, the compiler is complaining because when you declare Student array[5];or vector<Student> ver[N];it is looking for a default constructor for Student called Student(), which just sets default values for a class. You need to provide this inside the Student class; set the id to some value that can never be an actual student ID, something like -1.
主要问题是您试图在全局范围内执行 for 循环。在函数之外定义和初始化变量是可以接受的，但使用 for 循环或赋值运算符则不然。将 for 循环放入 main() （我建议您也将 N 和 vector/student 数组放入 main() 中，一切都应该正常工作。
此外，编译器正在抱怨，因为当您声明Student array[5];或vector<Student> ver[N];正在寻找默认构造函数时对于名为 Student() 的 Student，它只是为类设置默认值。您需要在 Student 类中提供它；将 id 设置为某个永远不可能是实际学生 ID 的值，例如 -1。
回答by Alexey Teplyakov
Option #1:
选项1：
You should replace vector <Student> ver[N]with vector<Student> ver(N)
你应该vector <Student> ver[N]用vector<Student> ver(N)
The std::vector is a class, that represents the vector by himself, you shouldn't create an array of vectors, you should just pass N(vector size) to it's constructor. Check this link
std::vector 是一个类，它自己代表向量，你不应该创建一个向量数组，你应该只将 N(vector size) 传递给它的构造函数。检查此链接
Option #2:
选项#2：
Student ver[N];
is incorrect, since the Default Constructor Student() is invoked N times, but you haven't implement it.
So you have to use array initilizer Student ver[5] = {1, 2, 3, 4, 5}or implement the default constructor explicitly.
不正确，因为默认构造函数 Student() 被调用 N 次，但您还没有实现它。因此，您必须使用数组初始化程序Student ver[5] = {1, 2, 3, 4, 5}或显式实现默认构造函数。
And of course - the "for" loop has to be used inside function body.
当然 - 必须在函数体内使用“for”循环。
回答by Thomas Benard
This is actually not linked at all with vectors. You just need to move your "for" statement into your main
这实际上与向量根本没有联系。你只需要将你的“for”语句移到你的主要
回答by Van N Tran
#include<iostream>
using namespace std;
class Student
{
private:
int id;
public:
// Mutator
void setId(int i){
id = i;
}
// Accessor
int getId()const{
return id;}
};
int main()
{
const unsigned int N = 5;
Student ver[N]; // Define instances as an array
// of the Student class
int idStudent[N] = { 11, 32, 37, 4, 50}; // Create
// one dimensional array with N elements of the
// student ID
for(unsigned int i = 0; i < N; i++ ){ // Assign
//student ID for each object of the class
ver[i].setId(idStudent[i]);}
cout<< "Hello, This is a code to learn classes : "
<< endl << endl; // Display the student ID
for(unsigned int i = 0; i < N; i++ ){
cout<< "Student ID #" << i+1 << " of "
<< N << " : " << ver[i].getId() << endl;}
return 0;
}
回答by Van N Tran
The solution to the question asked is to create an array of instances of the Student class as Student ver[N]. Next, using the mutator function, setID(int i) to assign the elements from a given student ID in array format ( int idStudent[N] = { 11, 32, 37, 4, 50};) to each perspective instance of the Student class: ver[i].setId(idStudent[i]) and for loop to finish the job.
所问问题的解决方案是将 Student 类的实例数组创建为 Student ver[N]。接下来，使用 mutator 函数 setID(int i) 以数组格式 ( int idStudent[N] = { 11, 32, 37, 4, 50};) 将给定学生 ID 中的元素分配给每个透视图实例学生类： ver[i].setId(idStudent[i]) 和 for 循环来完成工作。
Last, using the accessor function, ver[i].getID() and the for loop to display the info.
最后，使用访问器函数 ver[i].getID() 和 for 循环来显示信息。
