a cpp program for n students having student name and roll number

[yamada]

so as the title suggests , I wanted to write a program using structures pointers
to enter n students names and roll numbers ,
so I wrote the program below that worked just fine ..,for a first name

#include<iostream>

using namespace std;
struct student
{
    char name[20];
    int roll_no;
};
int main()
{
    int n;
    cout<<"enter number of students that you want to enter their info.\n";
    cin>>n;
    student stu[n],*ptr;
    cout<<"enter data \n";
    for (ptr=stu;ptr<(stu+n);++ptr)
    {cout<<"name:\n";
        cin>>ptr->name;
        cout<<"roll number:\n";
        cin>>ptr->roll_no;}
        cout<<"student data:\n";
        ptr=stu;
        while(ptr<(stu+n))
        { cout<<ptr->name<<"   "<<ptr->roll_no<<endl;
        ptr++;
        }
    }

but I wanted a program to enter the full names for n students using structures' pointers, so I tried using gets(ptr->name) , and tried again using cpp styled strings and getline(cin,ptr->name)
those did not work however for more than one name then the program took no more , the codes below:

#include<iostream>
#include<cstdio>
using namespace std;
struct student
{
    char name[20];
    int roll_no;
};
int main()
{
    int n;
    cout<<"enter number of students that you want to enter their info.\n";
    cin>>n;
    student stu[n],*ptr;
    cout<<"enter data \n";
    for (ptr=stu;ptr<(stu+n);++ptr)
    {cout<<"name:\n";
        gets(ptr->name);
        cout<<"roll number:\n";
        cin>>ptr->roll_no;}
        cout<<"student data:\n";
        ptr=stu;
        while(ptr<(stu+n))
        { cout<<ptr->name<<"   "<<ptr->roll_no<<endl;
        ptr++;
        }
    }

and for getline

#include<iostream>
#include<string>
using namespace std;
struct student
{
    string name;
    int roll_no;
};
int main()
{
    int n;
    cout<<"enter number of students that you want to enter their info.\n";
    cin>>n;
    student stu[n],*ptr;
    cout<<"enter data \n";
    for (ptr=stu;ptr<(stu+n);++ptr)
    {cout<<"name:\n";
        getline(cin,ptr->name);
        cout<<"roll number:\n";
        cin>>ptr->roll_no;}
        cout<<"student data:\n";
        ptr=stu;
        while(ptr<(stu+n))
        { cout<<ptr->name<<"   "<<ptr->roll_no<<endl;
        ptr++;
        }
    }

so where did I go wrong in the two codes below?
and is there any other way of doing this right using structures?
*yes it is mandatory*

[AbstractionAnon]

Your third snippet is closest to being correct. However at line 14 (all snippets) in standard C++, you can not allocate an array like that. In standard C++, the size of an array must be known at compile time. Some compilers do allow this as a non-standard extension.

You also may want to move lines 23-26 outside the for loop if you want to print the roll after all entries have been made.

Line 28: You're missing a close }, but I assume that was just a cut and paste error.