CODEWITHSUNDEEP
c++
Ayaz Uddin
Ayaz Uddin

Reputation: 105

Changing a functions variable through another C++ function

I just started messing around with C++ but I am stuck. Any help would be much appreciated.

I want to change the value of gameState from true to false by passing in another function, however, my thought process seems to be incorrect:

#include <iostream>
#include <vector>
#include <string>
#include <fstream>
using namespace std;

void changeState(bool gameState){
  string answer;
  cout << "End game? Y/N" << endl;
  cin >> answer;

  if(answer == "Y"){
    gameState = false;
  } else if (answer == "N"){
    gameState = true;
  }    
}

void gameLoop(){
  bool gameState = true;
  while(gameState == true){
    cout << "Game is being played" << endl;
    changeState(gameState);
  }    
}

int main(){    
  gameLoop();  
  return 0;
}

Upvotes: 0

Views: 94

Answers (3)

Rama
Rama

Reputation: 3305

Return the gameState varible passed by value, there is no need to pass by reference or pointer in your code, this could lead to hide behavior,

#include <iostream>
#include <vector>
#include <string>
#include <fstream>
using namespace std;

bool changeState(bool gameState){
  string answer;
  cout << "End game? Y/N" << endl;
  cin >> answer;

  if(answer == "Y"){
    gameState = false;
  } else if (answer == "N"){
    gameState = true;
  }  
  return gameState; 
}

void gameLoop(){
  bool gameState = true;
  while(gameState == true){
    cout << "Game is being played" << endl;
    gameState = changeState(gameState);
  }    
}

int main(){    
  gameLoop();  
  return 0;
}

Upvotes: 1

Sami Sallinen
Sami Sallinen

Reputation: 3496

By default, function parameters are passed "by value" to c/c++ functions. This means that the function has its own local copy of the gamestate variable that is initialized to the value given by the caller.

You have 2 possibilities: to change the function to take a reference to the variable or to take a pointer to the variable instead.

A function taking a reference would look like this:

void ChangeState (bool &gameState){
   gameState=false;
}

Using a pointer would look like this:

void ChangeState (bool *gameState){
   *gameState=false;
}

.... ChangeState (&gameState);

Using a reference can be seen as more convenient as the syntax and the use of the local variable is not changed. On the other hand, using a pointer forces the caller to use the & to take the address of the variable, so everybody looking at the function call is made aware that the value may be changed inside the function.

Upvotes: 2

AresCaelum
AresCaelum

Reputation: 1566

In c++ when you pass an argument to a function it generally is passed a copy of that value, that means it is just a copy of what that variable contains, and isn't that variable. In order to create an output parameter a simple solution is to pass by reference or pointer.

I will show the reference version:

void changeState(bool& gameState){
  string answer;
  cout << "End game? Y/N" << endl;
  cin >> answer;

  if(answer == "Y"){
    gameState = false;
  } else if (answer == "N"){
    gameState = true;
  }
}

The code is the exact same the only difference is the & after the bool inside the parameter list.

This tutorial will go into more detail: http://www.cplusplus.com/doc/tutorial/functions/

Also just a fyi, a reference or pointer depending on the OS, could be 4 bytes, where your bool is typically 1 byte, just returning a bool value would take less memory then passing by reference.

Upvotes: 1

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