Getting an identifer is undefined issue C++

Question

I want to make it so my program will create an object based on user input. When I run the program I get the error "identifier "plant" is undefined" on the first call of plant which is "while(plant.getDays() > 0){".

#include "Carrot.h"
#include "Potato.h"
#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main(){
    int gameState = 1;
    cout << "WELCOME TO THE CROP GAME" << endl;
    while(gameState == 1){
        cout << endl << "PICK A CROP TO GROW" << endl;
        cout << "1:CARROT - SHORT" << endl;
        cout << "2:POTATO - LONG" << endl;
        int userGameChoice;
        
        cin >> userGameChoice;
        while(userGameChoice < 1 || userGameChoice > 2){
            if(userGameChoice == 1){
                Carrot plant;
                cout << "YOU HAVE CHOSEN TO GROW CARROTS" << endl;
            }
            else if(userGameChoice == 2){
                Potato plant;
                cout << "YOU HAVE CHOSEN TO GROW POTATOES" << endl;
            }
            else{
                cout << "INVALID CHOICE" << endl; 
                cin >> userGameChoice;
            }
            
        } 
        
        while(plant.getDays() > 0){
            int userChoice;
            //STATS
            plant.DisplayStats();
            //MENU
            plant.Menu();

Both `plant` objects you're using in this code have scope-lifetime of their surrounding `{ }`. They don't exist by the time you get to `plant.getDays()`. In short, there is no `plant` by then. — WhozCraig, Apr 30 '22 at 07:00
Maybe relevant https://stackoverflow.com/questions/15188894/why-doesnt-polymorphism-work-without-pointers-references — n. m. could be an AI, Apr 30 '22 at 07:02

Dominik Rafacz · Answer 1 · 2022-04-30T07:16:35.043

You declare variables plant in nested code blocks, so they cannot be seen in the outside blocks, they are visible only in the specific branches of if-else statement. When the compiler gets to your while loop, there is no visible variable.

If you want it to work, you need to declare variable before you use them in a visible scope, like:


int main(){
    int gameState = 1;
    cout << "WELCOME TO THE CROP GAME" << endl;
    while(gameState == 1){
        cout << endl << "PICK A CROP TO GROW" << endl;
        cout << "1:CARROT - SHORT" << endl;
        cout << "2:POTATO - LONG" << endl;
        int userGameChoice;

        Plant *plant;
        
        cin >> userGameChoice;
        while(userGameChoice < 1 || userGameChoice > 2){
            if(userGameChoice == 1){
                plant = new Carrot()
                cout << "YOU HAVE CHOSEN TO GROW CARROTS" << endl;
            }
            else if(userGameChoice == 2){
                plant = new Potato()
                cout << "YOU HAVE CHOSEN TO GROW POTATOES" << endl;
            }
            else{
                cout << "INVALID CHOICE" << endl; 
                cin >> userGameChoice;
            }
            
        } 
        
        while(plant->getDays() > 0){
            int userChoice;
            //STATS
            plant->DisplayStats();
            //MENU
            plant->Menu();
        }
        delete plant;
    }
}

However, if you want it to work, Carrot and Potato need to be descendants of some Plant class. Here, plant is a pointer to object of class Plant and the object is created dynamically, using new keyword inside of the if-else branches. Since it's created dynamically, it also needs to be deleted after all. You probably need to understand more about polymorphism in C++, the source linked in your comments might be useful for you.

if you really want it to 'work' the syntax for the member access would be using `plant->` rather than `plant.`. The above code, which won't compile without that fix. And `delete *plant;` is wrong too; it should be `delete plant;`. — WhozCraig, Apr 30 '22 at 07:13
Ahh, yes, you're right, I've written it too quickly. I'll edit it. Those are the C++ syntax details that I always forget and just remember about them after the first compilation. — Dominik Rafacz, Apr 30 '22 at 07:16

Getting an identifer is undefined issue C++

1 Answers1