Returning pointers from functions problem.

Hi,

I have been wrestling with this program all day and am looking for some help.

I have never separated a program up before into separate files (main.cpp, base.h and base.cpp).

What I'm trying to do is call a function from main() and have that function return a pointer to a new object.

Once the pointer has been returned, I'm trying to push it into the std::list.

The problem I've been having is this compile error:

illegal call of non-static member function

I have wrote a simple version of my program which reproduces the same problem:

main.cpp :

Code:

#include <iostream>
#include <list>
#include <string>
#include "base.h"
using namespace std;

int main()
{
	typedef list<Base *>BaseList;

	do{
		char choice;
		cout << "a) Create then add a base class pointer into the list" << endl;
		cin >> choice;
		cin.ignore();

		switch(choice)
		{
		case 'a':
			{
				//Create an object and return a pointer to that object
				//(I will be making many of these objects)
				Base::ReturnPtrToANewBaseClassObject();

				//push the returned pointer onto the std::list
				BaseList.push_back( ptr );

				cout << "Object now pushed onto list" << endl;
				break;
			}
		default:
			{
				cout << "You made an invalid choice" << endl;
				break;
			}
		}
	}
	while(true);

	system("pause");
}

base.h :

Code:

#ifndef BASE_H
#define BASE_H

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

class Base
{
public:
	//constructor
	Base(string mystring);

	//Member Functions:
	Base * ReturnPtrToANewBaseClassObject();//Create an object and return a pointer to it

private:

protected:

	//Data Members common to all objects (abstract base class):
	string _mystring;

};
#endif

base.cpp :

Code:

#pragma once

#include <iostream>
#include <string>
#include "base.h"


//Base class contructor

Base::Base(string mystring)
{
	_mystring = mystring;
}

//Base class function
Base * ReturnPtrToANewBaseClassObject()
{
	//Build an object, and then return a pointer to that object
	//Wthin main() the pointer will be be pushed into the std:list

	string mystring;
	cout << "Value? " << endl;
	getline (cin, mystring);

	//Get a pointer to the new object
	Base *ptr = new Base(mystring);

	//return the pointer to main()
	return (ptr);
}

Here is what Visual Studio reports when I try to build:

------ Build started: Project: myproject, Configuration: Debug Win32 ------
Compiling...
main.cpp
c:\main.cpp(22) : error C2352: 'Base::ReturnPtrToANewBaseClassObject' : illegal call of non-static member function
c:\base.h(15) : see declaration of 'Base::ReturnPtrToANewBaseClassObject'
c:\main.cpp(25) : error C2143: syntax error : missing ';' before '.'
c:\main.cpp(25) : error C2143: syntax error : missing ';' before '.'
Build log was saved at "file://c:\Debug\BuildLog.htm"
myproject - 3 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

I have a very similar program working when I use a single main.cpp file, but I want to stop using single files, and this problem has really been holding me back.

Hope you guys and gals can help me with this.

Thanks very much!!!

Boy, let me crack my knuckles and get to it:
#1

Code:

typedef list<Base *>BaseList;

...

BaseList.push_back( ptr );

Baselist is a typedef(alias) for an actual object of type list<Base*>. You need to create an instance of that type and use that instance not the typedef when performing the push_back.

#2

Code:

Base::ReturnPtrToANewBaseClassObject();

Base is a class, not an instance of said class. To call a member function you either need to create an instance of the object first and then call the member function for a particular instance, or you need to make the member function static which means it can be called as you are doing without an actual instance.

#3 Since you're storing pointers in your list container, you need to iterate through the list and call the destructor for each object manually before the program ends or you'll be leaking memory. Either that, or switch from using a simple pointer to type Base and instead use some form of smart pointer such that this will be taken care of automatically.

Also,

Code:

BaseList.push_back( ptr );

"ptr" isn't declared in "main". You probably wanted to save the result of the previous line, for example (after fixing the above mentioned errors)

Code:

Base* ptr = Base::ReturnPtrToANewBaseClassObject();

Hey hk_mp5kpdw thanks so much!

I've followed your instructions as best I can, and it is indeed compiling further now.

here is my code right now:



main.cpp :

Code:

#include <iostream>
#include <list>
#include <string>
#include "base.h"
using namespace std;

int main()
{
	list<Base *>BaseList;

	do{
		char choice;
		cout << "a) Create then add a base class pointer into the list" << endl;
		cin >> choice;
		cin.ignore();

		switch(choice)
		{
		case 'a':
			{
				//Create an object and return a pointer to that object
				//(I will be making many of these objects)
				Base::ReturnPtrToANewBaseClassObject();

				//push the returned pointer onto the std::list
				BaseList.push_back( ptr );

				cout << "Object now pushed onto list" << endl;
				break;
			}
		default:
			{
				cout << "You made an invalid choice" << endl;
				break;
			}
		}
	}
	while(true);

	system("pause");
}

base.h :

Code:

#ifndef BASE_H
#define BASE_H

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

class Base
{
public:
	//constructor
	Base(string mystring);

	//Member Functions:
	static Base * ReturnPtrToANewBaseClassObject();//Create an object and return a pointer to it

private:

protected:

	//Data Members common to all objects (abstract base class):
	string _mystring;

};
#endif

base.cpp :

Code:

#pragma once

#include <iostream>
#include <string>
#include "base.h"


//Base class contructor

Base::Base(string mystring)
{
	_mystring = mystring;
}

//Base class function
Base * ReturnPtrToANewBaseClassObject()
{
	//Build an object, and then return a pointer to that object
	//Wthin main() the pointer will be be pushed into the std:list

	string mystring;
	cout << "Value? " << endl;
	getline (cin, mystring);

	//Get a pointer to the new object
	Base *ptr = new Base(mystring);

	//return the pointer to main()
	return (ptr);
}

Here is what Visual Studio reports when I try to build:

------ Build started: Project: myproject, Configuration: Debug Win32 ------
Compiling...
main.cpp
c:\main.cpp(26) : error C2065: 'ptr' : undeclared identifier
Build log was saved at "file://c:\Debug\BuildLog.htm"
myproject - 1 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

I don't see why the pointer wouldn't be available (undeclared identifier). I expect the function would be returning the pointer to main(), and so using push_back I expected to work.

Originally Posted by nadroj

Also,

Code:

BaseList.push_back( ptr );

"ptr" isn't declared in "main". You probably wanted to save the result of the previous line, for example (after fixing the above mentioned errors)

Code:

Base* ptr = Base::ReturnPtrToANewBaseClassObject();

As I said, you call the function, the function returns the value, and you ignore the value. You probably wanted to save it.

Originally Posted by hk_mp5kpdw

#3 Since you're storing pointers in your list container, you need to iterate through the list and call the destructor for each object manually before the program ends or you'll be leaking memory. Either that, or switch from using a simple pointer to type Base and instead use some form of smart pointer such that this will be taken care of automatically.

I meant you'd need to manually delete the pointers stored in the list not call the destructor's as calling delete on the pointer would call the destructor for the object pointed to by the pointer.

As for the ptr issue you really don't need a pointer variable at all, you can just directly use the return value from the static function call in the push_back function:

Code:

BaseList.push_back( Base::ReturnPtrToANewBaseClassObject() );

cout << "Object now pushed onto list" << endl;
break;

Everyone thanks so much for your help.

I have tried everything and am getting a problem with some kind of unresovled external.

#1 Remove the using namespace std; bit in the header and explicitly qualify any necessary objects that need it with std:: in said header.

#2 Remove the #ifndef/#define/#endif bits from base.cpp.

#3 You have a multi-source project (two source files). Are you compiling/linking them both together as part of a project? Your project should have both source files added to it. Your prior build output does not indicate this so maybe you should go ahead and do that.

I got it working in the end.

Thanks to everyone who helped out.

this was more about the separation of the files more than anything else.

It is a nice template for me to use now in the future.

Here is a good rule to remember:
When returning a value, let's say you have a box. Inside that box is a value.
When you write code to return "the box", the compiler actually instead opens the box and returns its contents! So the caller of the function doesn't get the box. The box exists only (for the sake of simplicity, this isn't always the case) in the local function that is called.
Hence, if the caller wants to save this value, it must create a box and store the value inside it.