Thread: Function that returns a class...

So I am working on a software based 3d engine. I have a structure called triangle, and a class called object. The object class contains an array of triangles that make up that object, and position/orientation data.

Anyway, I am writing a function called createCube, which is supposed to return an instance of object (or a pointer to an instance of object). The returned object will contain a bunch of triangles that together resemble a cube (duh). Anyway, Im having some difficulty writing this function. This is what I have, minus the irrelevant parts:

Code:

object createCube(int xpos, ..... )
{
   triangle theTriangles[12];
   //.....
   //code to set triangles to form cube
   //......

   object theCube = new object(xpos, ypos, zpos,0,0,0);
   for (int i=0;i<12;i++)
   {
      theCube.addTriangle(theTriangles[i]);
   }
   return theCube;
}

This does not compile. It says "conversion from 'object*' to nonscalar type 'object' requested" on the line "object theCube= new obj...." So I added a couple astericks (*), changing it to:

Code:

object* createCube(int xpos, ..... )
{
   triangle theTriangles[12];
   //.....
   //code to set triangles to form cube
   //......

   object* theCube = new object(xpos, ypos, zpos,0,0,0);
   for (int i=0;i<12;i++)
   {
      theCube.addTriangle(theTriangles[i]);
   }
   return theCube;
}

But now, it finds a problem with the line "theCube.addTriangle(....)" It says that addTriangle has not been declared (it has) and that their is a request for member of non-aggregate type before '(' token. I tried putting a '&' in front of that line, but it didn't help.

I know the problem has something to do with pointers and reference and dereference operators or something like that. I know its sad that I can figure out 3d math, but not simple classes and pointers. Can anyone help me out? It would be greatly appreciated.

Thanks!

new gives you some memory (or throws an exception) that is suitable to store the object in. It also calls the constructor for the object. new gives back a pointer, and obviously, you can not make an object out of a pointer to an object.

Whether you need a pointer or a regular object is often quite hard to answer. It is one of those that you have to understand from how the object is being used which it is right or wrong (and it may not be CLEAR that one is right and one is wrong, they may be "OK" both of them [or extremely rare "neither"]).

--
Mats

Originally Posted by cjmdjm

So if I don't use "new" does it not call the constructor? Because I was using Daved's method, but I want the constructor to be called, to initialize the position and orientation data.

Thanks.

Sorry if I gave that impression (something to do with "I know what I mean" and other people reading it differently ). No, of course, when you use a local object like that, the constructor is being called too.

--
Mats

Right. Splitting source-code "right" is an art that you learn from experience.

However, for most purposes, this is a good starting point:
Each class is declared in a header-file, usually named to correspond with the class. E.g "myclass.h".

The function definitions for that class (except for real trivial functions, such as access functions [setter/getter functions]) go in a source file of the same name, e.g "myclass.cpp"

The main function is put in a file that corresponds to the application itself, e.g. "myapp.cpp". It includes the header file(s) that it needs to do it's work.

If one class relies on another class, it should include that header file in its header file. So if you have "mybase.h" and "myderived.h", then "mybase.h" would be included by "myderived.h". That way, the programmer that uses "myderived.h" doesn't need to know that "mybase.h" needs to be included as well.

You should have "include-guards" in your header file to prevent the same header file being included multiple times. This is a pattern like this:

Code:

#ifndef SOMEFILE_H
#define SOMEFILE_H
... stuff of the header here ...
#endif

That way, if you have for example:

Code:

#include "myderived.h"
#include "mybase.h"

there won't be an error because mybase.h is included twice.

You should not involve DLL's at this point.

--
Mats

#ifndef SOMEFILE_H
#define SOMEFILE_H
... stuff of the header here ...
#endif

In the above code, why #define SOMEFILE_H, and not #include SOMEFILE_H? Perhaps I don't understand the difference between include and define. And the #endif goes at the very end of the header file? Seems odd.

Also, I have to include header files, eg, #include file.h, but do I have to do the same for .cpp files, eg, #include file.cpp, if I have more than just the main one? And do I need to adjust compilier or linker settings or paths?

And in most of my programs, I define all of a class's functions within the class declaration, eg,

Code:

class object
{
   public:
   void addTriangle(...)
   {
      //code
   }
   //etc
};

as opposed to

Code:

class object
{
   public:
   void addTriangle(...);
   //etc
};
void object::addTriangle(...)
{
   //code
}

Is this bad or not the standard way to do it? Is this a problem if I want to move the class to a header file? ie, do the function declarations have to be in a ".cpp" file but the class itself in a ".h" file?

Sorry to ask so many questions but this is the kind of thing you don't see in tutorials. There are plenty of tutorials on how to use for loops, if statements, classes, etc, but few on how to use header files in your program, or multiple source files, or DLLs, etc.

And yeah, as Daved alluded to, I learned most of what I know about Object Oriented stuff (eg, classes) in Java. I have known C for years, but last year I was required to take a Java class for my major, and finally learned object oriented stuff. But I didn't like Java, and switched to C++ without doing much research on the differences in how classes are handled.

Thanks.