C++ - How do you pass a variable from one form to another?

Question

I have a Form1.h and a Form2.h

Form1.h is already including Form2.h because i start Form2 by clicking a button in Form1. So how would i pass a variable from Form1.h to Form2.h

Here is an example of Form1.h

#include "Form2.h"

String^ str = "Hello World"; //This variable needs to be passed to Form2.h

//Other windows forms application code here

An example of Form2.h

//#include "Form1.h" this will cause an error

//How would i pass variable str to here?

//Other windows forms application code here

Edit:

This is how i fixed it

This is how i fixed it.

Form1.h

#include "Form1.h"

Form2^ frm = gcnew Form2;
frm->Username = "text here";//This passes the variables.
frm->Password = "other text";

Form2.h

public: String^ Username;
public: String^ Password;

In order to help you, we're going to need to see your code that you are having an issue with. — Aiias, Mar 30 '13 at 02:39
Including Form1.h in Form2.h and accessing directly? Form2 is a variable and you were able to access variable Form2 by means of including Form2.h in Form1.h. It works both ways ;) — Hasan Manzak, Mar 30 '13 at 02:40
@The_aLiEn actually. It just gives me an error because you can't have 2 files including eachother i guess. — FreelanceCoder, Mar 30 '13 at 02:44
Idk what that is but it's not working when i include Form1.h to Form2.h :\ — FreelanceCoder, Mar 30 '13 at 02:47
Make the form's constructor take a reference to the other form (it can be replaced with a specific variable if that's the only one you need to change). — chris, Mar 30 '13 at 02:49
@The_aLiEn Visual Studios 2010, chris i didn't even understand that. — FreelanceCoder, Mar 30 '13 at 02:54
I dont have Visual C++ installed in my VS, I'm using Embarcadero RAD Studio for my visual C++ work.. Anyways, language is language; you should have that worked. Try including form1.h in form2 code area, this is unit2.cpp for my rad std, file name can be different with vs — Hasan Manzak, Mar 30 '13 at 03:00
@The_aLiEn I already tried that and it gives me C4430 errors. — FreelanceCoder, Mar 30 '13 at 03:05
@FreelanceCoder, http://stackoverflow.com/questions/4757565/c-forward-declaration — chris, Mar 30 '13 at 03:06
@FreelanceCoder http://stackoverflow.com/questions/3714808/forward-declarations-for-variables — Hasan Manzak, Mar 30 '13 at 03:07
This is how i fixed it. Form1.h #include "Form1.h" Form2^ frm = gcnew Form2; frm->Username = "text here";//This passes the variables. frm->Password = "other text"; Form2.h public: String^ Username; public: String^ Password; — FreelanceCoder, Mar 30 '13 at 04:01

score 1 · Accepted Answer · answered Mar 30 '13 at 02:55

1

Not exactly sure what your asking, but im assuming that you want to set a variable in Form2 class from inside Form1 class? If so:

class Form1{
  private:
    int data;
  public:
    Form1(){data=4;}
    int getData(){return data;}  //returns Form1 data variable
    void setForm2Data(Form2&);   //sets Form2 data
};

class Form2{
  private:
    int data;
  public:
    void setData(int inData){data = inData;}
};

void Form1::setForm2Data(Form2 &o){
  o->setData(getData());  //argument is a Form2 object, "setData" can set that objects data variable
}

answered Mar 30 '13 at 02:55

mn.

796
6
12

Nope. I already have Form2.h included in Form1.h, I need to pass a variable to Form2 from Form1 without including Form1.h inside Form2.h – FreelanceCoder Mar 30 '13 at 02:59
ah i see. you want something called a "static" function then, check back in 5 mins while i edit my comment – mn. Mar 30 '13 at 03:09
Btw, Form1 and Form2 aren't classes. They are Forms as in Windows Forms Applcations. They also use the .NET Framework. – FreelanceCoder Mar 30 '13 at 03:16
ohh k, you shouldv'e specified that in the question briefly (even with the tag, the question kinda looked like a normal C++ header file question). Unfortunately, can't really help you there then. – mn. Mar 30 '13 at 03:20
@FreelanceCoder You're mentioning .Net now... Is it a Windows Application Visual C# or Windows Application Visual C++ ? – Hasan Manzak Mar 30 '13 at 03:26
@The_aLien Windows Form Application Visual C++ – FreelanceCoder Mar 30 '13 at 03:28
Is anyone still trying to figure this out? – FreelanceCoder Mar 30 '13 at 03:42

Skeptiker · Answer 2 · 2013-03-30T03:49:01.990

0

You get an error because your preprocessor directives cause double inclusion. To avoid this you can use pragma guards

Form1.h:

#ifndef FORM1_H
   #define FORM1_H
   #include "Form2.h"
   extern string str = "Hello World";
#endif

Form2.h:

#include "Form1.h"

edited Mar 30 '13 at 03:49

answered Mar 30 '13 at 03:22

Skeptiker

21
3

score 0 · Answer 3 · answered Mar 30 '13 at 03:49

There are several ways.. one is to use global variables (not necessarily the best way.. depends per case basis)

First, you can solve the multiple inclusion problem by using include guard.

Then you can declare a global variable using extern keyword in the header, and plug in the value in the implementation file.

For example:

//file: mylib.h
#ifndef MYLIB_H
#define MYLIB_H
extern int myGlobalVar;
#endif

//file: mylib.cpp
#include "mylib.h"
int myGlobalVar = 123;

Now you can #include "mylib.h" anywhere in other file as many time as you like and access the same variable

C++ - How do you pass a variable from one form to another?

3 Answers3