Struct to Class

Question

I'm trying to convert a c program to c++ basically a struct data type to a class but is keeps showing " ‘first’ was not declared in this scope". whereas c program is working just fine.

Original code


#include <stdio.h>
#include <stdlib.h>
struct Node
{
 int data;
 struct Node *next;
}*first=NULL;

void Display(struct Node *p)
{
 while(p!=NULL)
 {
 printf("%d ",p->data);
 p=p->next;
 }
}   


void Insert(struct Node *p,int index,int x)
{
 struct Node *t;
 int i;
 
 if(index < 0 || index > 9)
 return;
 t=(struct Node *)malloc(sizeof(struct Node));
 t->data=x;
 
 if(index == 0)
 {
 t->next=first;
 first=t;
 }
 else
 {
 for(i=0;i<index-1;i++)
 p=p->next;
 t->next=p->next;
 p->next=t;
 
 }
 
 
}
int main()
{
 Insert(first,0,5);
 Insert(first,1,10);
 Insert(first,2,15);
 Display(first);
 return 0;
}

edited Code


#include <iostream>

using namespace std;
class Node{
    public:
    int data;
    Node *next;
};

void Display(struct Node *p){
    while(p!=NULL){
        std::cout << p->data << std::endl;
        p=p->next;
    }
}   


void Insert(struct Node *p,int index,int x){
    Node *t;
    int i;
     
    if(index < 0 || index > 9)
    return;
    t=new Node;
    t->data=x;
 
    if(index == 0){
        t->next=first;
        first=t;
    }
    else
    {
    for(i=0;i<index-1;i++)
    p=p->next;
    t->next=p->next;
    p->next=t;
 
 }
 
 
}
int main()
{
 Node *first=NULL;  // 
 Insert(first,1,10);
 Insert(first,2,15);
 Display(first);
 return 0;
}

error is something related to "Node *first = Null" line something to do with global pointer Insert(first,0,5);

error I'm getting.

`class` and `struct` are two different keywords to declare a class. Only difference is default access. The error you get has nothing to do with swapping the two keywords. Please include the compiler error message as text in the question — 463035818_is_not_an_ai, Jan 17 '23 at 09:07
in the c code you have a global `}*first=NULL;` in the c++ code you moved it to `main`. Voting to close as typo, though this is also relevant https://stackoverflow.com/questions/54585/when-should-you-use-a-class-vs-a-struct-in-c — 463035818_is_not_an_ai, Jan 17 '23 at 09:08
@463035818_is_not_a_number, Not removed, but moved to `main`. To me, that makes it more likely to be a question about scope. — chris, Jan 17 '23 at 09:10
@chris not sure whats the question is really about. OP seems to know already that the issue is a global pointer, but at the same time suggests that the only change is struct to class — 463035818_is_not_an_ai, Jan 17 '23 at 09:13
In case it's not clear, one solution is simply to put `first` back where you found it. `class Node { ... } *first = NULL;` is legal C++, though for many reasons not very good C++. And of course `struct` is perfectly legal in C++ too. — john, Jan 17 '23 at 09:21
Shouldn't conversion uses `std::list` or `std::vector` [Demo](https://godbolt.org/z/GMb7Tx5ca). — Jarod42, Jan 17 '23 at 15:14

Vlad from Moscow · Answer 1 · 2023-01-17T09:27:17.020

In the C program you declared variable first in the file scope.

struct Node
{
 int data;
 struct Node *next;
}*first=NULL;

So it is visible in all functions defined after the structure definition.

In the C++ program you declared the variable first in a block scope of main

int main()
{
 Node *first=NULL; 
 //...

So it is invisible inside the function Insert in this code snippet

if(index == 0){
    t->next=first;
    first=t;
}

In any case the function Insert is invalid because at least there is no check whether p is equal to nullptr in this for loop

for(i=0;i<index-1;i++)
p=p->next;

that can result in undefined behavior.

The first function parameter should have a referenced type as for example

void Insert(struct Node * &p,int index,int x){

In this case you could write for example

if(index == 0){
    t->next=p;
    p=t;
}

and the pointer first defined in main will be changed.

And if you want to rewrite the list in C++ then the provided functions should be declared as member functions of the class.

As extra note, C++ already has `std::list` (and `std::vector`). — Jarod42, Jan 17 '23 at 15:15

Struct to Class

1 Answers1