dynamic initalisations of c++ objects based on user input - c++

Question

Hey sorry for the previous Question
OK..My project is to create and run a database for a college using c++
I have to use USN which is a Unique Student Number to access the database :
So i wrote the following program :
#include<iostream>
# include<conio.h>
#include<iomanip>
#include<string.h>
#include<stdlib.h>
int checkinarray(char[],char*);
using namespace std;
class student
{
private :
int sem;
float cgpa;
char password[11];
char passwordtrial[11];
void readdata();
void checkpassword();
void createpassword();
public :
char name[50];
int roll;
void printdata();
char USN[11];
static int number;
void opendatabase();
void firsttime();
public:
student(char n[50]="NONE")
{
number++;
roll=number;
cout<<endl<<"New record under the name of "<<n<<" has been created !"<<endl;
cout<<"Roll number set for new student as : "<<roll<<endl;
cout<<"Use this Roll number for further usage"<<endl<<endl;
};
};
void student::opendatabase()
{
int ch;
cout<<"Enter your name:"<<endl;
cin>>name;
cout<<"Enter your password"<<endl;
cin>>passwordtrial;
if(!strcmp(passwordtrial,password))
{
cout<<"Do you want to read or write";
cin>>ch;
switch(ch)
{
case 0 :
readdata();
break;
case 1 :
printdata();
break;
}
}
else
cout<<"Try Again";
};
void student::readdata()
{
cout <<endl<<"Enter the name of the student : ";
cin >> name;
cout <<endl<<"Enter the semester of the student : ";
cin >> sem;
cout <<endl<<" Enter the cgpa of the student : ";
cin >> cgpa;
};
void student :: printdata()
{
cout << "The name is : " << name << endl;
cout << "The sem is : " << sem << endl;
cout << "The roll is : " << roll << endl;
cout << "The cgpa is : " << cgpa << endl;
}
void student::firsttime()
{
cout<<"Enter your name :";
cin>>name;
cout<<"Hey "<<name<<" Welcome to DBMS "<<endl;
createpassword();
};
void student::createpassword()
{
cout<<"Please enter your 6 character password.. : ";
cin>>password;
cout<<"Please Input your Data here.... :" ;
readdata();
};
int student::number=0;
int main()
{
enum status {existing,newacc,exit};
enum functi{read,update};
char entry1[40];
char entry[10];
char usn[15];
char a[1000][15];
char n[40];
int i,j=0,k;
int s=2;
cout<<endl<<"WELCOME TO COLLEGE NAME"<<endl<<"Press enter to access Database...";
getch();
cout<<endl<<"Welcome to the Main Page of our Database : "<<endl;
while(1)
{//User option
cout<<endl<<"Do you want to access an old entry : "<<endl<<"OR"<<"Create a NEW entry : ";
cin>>entry1;
if(!strcmp(entry1,"old"))
s=existing;
else if(!strcmp(entry1,"new"))
s=newacc;
else
s=exit;
switch(s)
{
case existing:
{
i=1;
break;
}
case newacc:
{ i=1;
cout<<endl<<"Enter your usn : "<<endl;
cin>>usn;
strcpy(a[j],usn);
j++;
strcpy(n,usn);
cout<<n;
student usn(n);
usn.firsttime(); //Start here!! use i to go to next loop or stay in this loop,,change name entry to usn
break;
}
default :{cout<<"Error Input";i=0;break;}
}
if(i)
continue;
cout<<endl<<"What do u want to do??"<<endl<<"Read Entries "<<endl<<"Update entries";
cin>>entry;
if(!strcmp(entry,"read"))
s=read;
else if(!strcmp(entry,"update"))
s=update;
else
s=exit;
cout<<endl<<"Enter your usn : "<<endl;
cin>>usn;
if(checkinarray(a[15],usn))
{
switch(s)
{
case read:{
usn.printdata();
break;
}
case update:{
usn.firsttime();
break;
}
default :
cout<<"Are you sure you want to exit?"<<endl<<"Press 0 to exit"<<endl<<"to back to menu press 1";
cin>>k;
break;
}
if(!k)
break;
}
else cout<<"Invalid Roll number try again!";
}
}
int checkinarray(char a[][15],char b[])
{
int len;
//Finding the length of the string :
len=(sizeof(a)/sizeof(a[0]));
//Checking Conditions for roll number:
for(int k=0;k<len;k++)
{
if(strcmp(a[k],b))
return 1;//stringcompare!!
}
return 0;
}
okay so when i run this i get the following error :
request for member 'printdata' in 'usn', which is of non-class type 'char [15]'
AND
request for member 'firsttime' in 'usn', which is of non-class type 'char [15]'
So please help me overcome this error by suggesting different ways to create and call objects based on user input

Answer 1

OP's problem can be reduced to the following example:
#include<iostream>
#include<string.h>
using namespace std;
class student
{
public:
student(char n[50])
{
}
};
int main()
{
char usn[15];
char n[40];
{
cin >> usn;
strcpy(n, usn);
student usn(n);
}
usn.printdata();
}
This is what is meant by a Minimal, Complete, and Verifiable example. Not everything, but everything needed to reproduce the problem. The beauty of the MCVE is it has reduced the problem to the point where all of it can fit on the screen and probably within the brain, making it easy to analyze and test.
Things to note:
There are two variables named usn
char usn[15];
is an automatic variable within main. It is visible only within main and will expire at the end of main
student usn(n);
is an automatic variable within an anonymous block within main. It is visible only within this block and will expire at the end of the block.
Annotating this block to better explain what is happening, we get
{
cin >> usn; // uses char usn[15];
strcpy(n, usn);
student usn(n); // declares a new variable named usn that replaces char usn[15];for the remainder of this block
} // student usn(n); ends right here and is destroyed.
usn.printdata(); //uses char usn[15]; which has no printdata method.
So how do we fix this?
student usn(n); must have a wider scope.
one of these two variables must change names because once student usn(n); has wider scope it will collide with char usn[15];
Lets give that a quick try.
int main()
{
char usn[15];
char n[40];
student stud(n);
{
cin >> usn;
strcpy(n, usn);
}
stud.printdata();
}
Isn't possible because there is no data in n with which we can make stud. At least not for this minimal example.
int main()
{
char usn[15];
char n[40];
student * stud;
{
cin >> usn;
strcpy(n, usn);
stud = new student(n);
}
stud->printdata();
delete stud;
}
Solves that by dynamically allocating stud so that it is no longer scoped by the braces. Unfortunately that does pick up some extra memory management woes. stud must be deleted. Added a delete, but what if printdata throws an exception and delete stud; is skipped?
int main()
{
char usn[15];
char n[40];
std::unique_ptr<student> stud;
{
cin >> usn;
strcpy(n, usn);
stud.reset(new student(n));
}
stud->printdata();
}
std::unique_ptr takes care of that. But... What about that whole database thing? Why not store stud in a list?
int main()
{
char usn[15];
char n[40];
std::vector<student> studs;
{
cin >> usn;
strcpy(n, usn);
studs.emplace_back(n); // or studs.push_back(student(n));
}
for (student & stud: studs)
{
stud.printdata();
}
}
std::vector solves both problems at once.