NeHe Productions: OpenGL Article #05

Article 05




Handy Compilation Of C/C++ Methods


By: Paul Frazee (The Rainmaker)



Introduction

C++ is a very expansive language with many cool features that most people
don't know- that is, until now! This collection of articles is designed to
teach you, the adept C/C++ programmer, the tips and tricks of C and how
you too can exploit them in your daily programming. Some of the methods
will range from the commonly known yet under-used macros, to the
incredible expandability of inheritance, to the utter power of memory
functions. Much of it will surprise you with how easy it is to implement,
so read on!


About The Author

I am Paul Frazee, amateur programmer and computer game die-hard. I am
fifteen years old and wrote most of these articles in, yes, the basement
of my parent's house. I have been involved any many small projects, and
hope to release a commercial game some day. I use a very laid-back writing
style, to make things informative but fun to read too! I hope you enjoy
it!

You can email me at paulfrazee@cox.net with your questions, comments, and
corrections. Don't hesitate to contact me; I enjoy the feedback!


Update (07/21/03):

Looking back at this article, I realize there were a lot of silly mistakes
and some down-right misinformation, and all with my name on it. I
apologize for not fixing them earlier, but now I have set all of that
right. I am tempted to delete the above paragraph (*sigh*), but I will
leave it there just for old-timesł sake. Note also that my email has
changed. Code on!

Macros:

Macros have to be the most under-used feature for new programmers. They
are very useful, and a good programmer should take note of them. Why?
Well, for instance, you can streamline a function that will be called
often and with congruent parameters. The windows API call MessageBox is a
good example:


#define MSGBOX(body,title) MessageBox(NULL,body,title,MB_OK);

Now all you have to do is call MSGBOX("","") instead of
MessageBox(NULL,"","",MB_OK)! Nice, huh?

Another common use is to define a standard function prototype that can be
reproduced in an easy and clean way. For example, let's say you want to
make a standard naming convention for a library you are making:


#define FUNC(name) void FUNC_name(void)

Now you can easily declare and define a function that has FUNC_ at the
front of the name, and is of type void. Observe:


FUNC(myfunc); // Declaration
...
FUNC(myfunc) // Definition
{
printf("MyFunc!!\n");
}
...
void main()
{
FUNC_myfunc(); // Calling
}

The output of that program would be "MyFunc!!" Cool, huh? Now that you
have seen the wonders of macros, let's get a more in-depth look at them.

What Are Macros?

Macros are, simply, groupings of code with a name attached. Like
functions, they can have parameters passed to them, but it is important to
note that they are not the same as functions. Functions are code compiled
into a static memory location whose definition will appear in the actual
binary output only once. Macros are, during early compilation stages,
replace with the code they are defined as. Macros are pretty much always
in all caps (by convention rather than necessity). You declare and define
them all at once, and it is safe to define them in headers. Here is the
format:


#define ()

You do not have to surround the contents with brackets ({}), although
there are certain exceptions. If you ever wish to continue a macro past
its declaration line, you append that line with a back-slash (\):


#define MyMacro(parameter) line 1 \
line 2 \
... \
line N

Another interesting feature is that the parameters are not type based,
either. You can pass an integer, a string, or even flat out code to
parameter. As long as you keep in mind what a macro truly is, you be able
to use it effectively.

In case you are having trouble grasping this whole concept, I will provide
you with an example. Observe:


#define MyMacro(text) printf(text);

As you can see, MyMacro is a macro that wraps the function printf. So...


void main()
{
MyMacro("Hello World!\n");
}

is the exact same as saying


void main()
{
printf("Hello World!\n");
}

Got it? Good. And remember, macros are not magical ways around casts. In
the previous example, the data passed to MyMacro would have to be a string
(char array).

When Should Macros Be Used?

By now you may be asking yourself why one would ever really want to use a
macro, especially when a function can do exactly the same thing. Well what
you should understand about the macro is that it is designed simply for
code cleanliness, a tool for the programmer. In the end product, it is no
different from just typing out the body of the macro. So when is best? I
recommend using it in the following situations:
1. When you have a common calculation or small code snippet that isnłt
worthy of a whole function. I especially recommend this for situations
where you want to save on speed, since a function call just adds
unnecessary operations (but that is serious operation-crunching). I find
macros to be especially good for calculations, since the data cast is
fairly inconsequential, where in a function you would have to either
change the cast or write a version for each type.
2. When you have common code that must be inserted to the calling
position, such as common variable declaration.
3. When you are enforcing a syntax standard, such as in a library.
4. One word
obfuscation.
Use well.

This Pointer:

The "this" pointer is so important that I canłt imagine you donłt already
know of it. When programming data structures
usually classes - it may
become necessary to refer to the instance of class within itself. The
answer to this problem is the "this" pointer. It is a pointer of the type
of the class it is being used in, to the instance using it. Behold:


class CMyClass
{

int int_variable;
void MyFunction( int int_variable );

};

void CMyClass::MyFunction(int int_variable)
{
this->int_variable = int_variable;
}

In this example, the programmer with horrible naming conventions had an
ambiguous variable situation. The rules of C/C++ state that a parameter or
locally defined variable overtakes a class member or global variable,
which made access to CMyClass::int_variable impossible. To specify which
variable he was talking about, the programmer used the this pointer.
Another common use is to pass itself into a function, like in the
following example:


void AddToInteger(CMyClass *MyClassInstance, int value)
{
MyClassInstance->int_variable += value; // This Function Adds To
// The Class' Variable
}
...
void CMyClass::MyFunction(int int_variable)
{
AddToInteger(this,int_variable); // We Pass This For The Class parameter
}

Aha, very useful indeed! Note that the this pointer works just as well for
structs.

Dynamic Memory:

As you create more complex programs, you will find that arrays of a fixed
size are not cutting it anymore. Eventually you are going to need to
create arrays of variable size, allocate and delete objects, and more, and
I can tell you how. But first, letłs have a little technical stuff. This
may be over your head
if it is, simply skip it. I would, however,
recommend reading it; it is quite interesting.

First of all, you must understand that the program, when compiled,
translates into memory positions in the executable (all of my discussion
pertains to the PE executable file format common to Windows, though can
apply in other situations). When the program is loaded into the memory,
those memory positions remain intact, but are simply relative to the
position the program was loaded into. So if a line of code was in position
4 in the executable file, and the file was loaded into position 1000 in
the memory, that line of code would be at position 1004 in the memory.

Now, how are variables put into the memory? Well, that depends on the
variablełs declaration situation. If a variable is global, it is actually
put into the executable the same way code is, meaning that a 4 byte
integer at position 20 would take up the positions 20 through 23 in the
exe memory. That is why global variables are generally frowned upon
they
make the executable take up more memory. Now, function-local, fixed-size
variables (the ones you are used to) are different. They are allocated to
what is called the "stack" at the beginning of a function call. The
problem with those is that, as I previously mentioned, they are
fixed-sized! The alternative is a dynamically allocated variable, which is
managed through pointers.

Dynamically allocated variables are pointers that, instead of pointing to
fixed-size variables in the memory, point to allocated memory in the
"heap". Because these variables are not allocated until instructed to do
so, and because the memory is managed by a flexible pointer, the
programmer has the power to decide exactly how much memory the variable
will need and when to allocate it. What this means for you is that you can
create arrays of a size determined at runtime. I find the most common
application to be with strings. Time to find out how this can be done!

Make Me Some Memory

There are actually a couple ways to allocate memory in C++. The most
common is new and delete, the C++ commands. New allocates the specified
amount of memory and returns a pointer to it, and delete deallocates it.
You should never use one without the other for stability reasons. Allocate
with new but forget to free up with delete? You have what is called a
memory leak
memory is being taken and not given back. Try to deallocate
with delete but never allocated with a new? You program is going to crash
through an assertion, because you just tried to deallocate "bogus" or null
memory. The functionsł usages are as follows:


new ( ); // Returns An Instance Of An Object
delete ; // Deletes An Instance Of An Object
new []; // Returns An Array
delete [] ; // Deletes An Array

The first two usages have to do with classes, and I will not delve into
that any further. However, the latter two I will go into. You use those to
create dynamic arrays
they are very simple. Here is an example:


int *intarray=NULL; // Create A Null Pointer Of Type Int
intarray = new int[50]; // Allocate An Array Of 50 Slots
...
if(intarray) // Always Check To Make Sure It Exists To Avoid Crashes

delete [] intarray; // Delete Intarray
intarray = NULL; // Delete Does Not Set To Null, So It Is Best We Do

I recommend adhering to the example above pretty closely for allocation
and deallocation, as failure to do so can very well lead to bugs.

The other way to allocate and deallocate memory is through the three
functions left over from the C days: malloc, realloc, and free. How are
they different? Well mostly in usage. New simplifies the allocation by
calculating how much memory will be required and casting it for you (it
multiplies the size of the type times the number of elements in the
array). Malloc requires you to do all of that. So why use malloc and free?
Personal preference, really. Here is the usage:


malloc( ); // Allocate specified bytes
free( ); // Deallocate variable memory

Malloc, free, and realloc all make use of the void pointer (a cast-less
pointer). As such, you must cast the data, like so:


int* pData = (int *)malloc( sizeof( int ) * 100 ); // Allocates an integer array of 100 elements

if( pData )
free( pData ); // Deallocate pData
pData = NULL; // Free does not set to null, so we should

Malloc may look very intimidating, but it really isnłt as complex as it
looks. In the end, however, I recommend using new/delete, for a few
reasons. First of all, when you are instantiating data objects with
constructors or destroying those with destructors (not covered in this
article), the constructor or destructor will be called. The other reason
is that since you are probably writing in C++, then you will want to stick
to the C++ commands.

Ah, one little function left
realloc. There will be times when you will
want to resize your dynamic arrays. When this is the case, I would
recommend using STLłs vector or something along those lines, but I am
betting that you donłt know STL yet. So, we better reallocate that array!
While you could just make a new array of the desired size, copy the old
arrayłs data, and delete the old array, it is much easier to just use
realloc:


realloc( , );

Realloc returns the newly allocated data as a void pointer, and also takes
the variable as a void pointer, requiring some casting on your part:


pData = (int *)realloc( (void *)pData, sizeof( int ) * 1000 );

That isnłt so hard! And now you too can make dynamic arrays.

One final subject I should like to touch upon. You may be asking yourself
whether it is safe to mix new/delete and malloc/realloc/free. The truth is
that yes, it is completely safe. Data is data, no matter which function
was used to attain it. However, it is considered to be good coding not to
mix them. Now that you understand dynamic arrays, go grab yourself a book
on STL and forget most of this! :D

Logic Without The If( ):

Standard programming languages today are built on logic. And while if
statements are all good, sometimes you just need a quick expression check
that doesn't require all those brackets... and so the ? was born. ? is an
expression evaluator, like the if statement but designed to work as a
one-liner. Here is the syntax:


? :

That may take a little explaining. is the logic in question
(e.g. a==0 or handle!=NULL). is the code that will be
executed if the expression evaluates to true. is the code
that will be executed from the operator if the expression evaluates to
false. If you are still stumped, consider this:


int a = 6;
int value = (a==6) ? 3 : 6;
printf("a=%d value=%d",value);

The output would be "a=6 value=3", because the expression (a==6) was true.
You can also run functions instead of just returning a variable, like in
this example:


int a = rand( )%2; // Get A Random Number (0 or 1)
a==1 ? printf("Random Returned 1!\n") : printf("Random Returned 0!\n");

How does that work? Well remember how macros would simply insert its code
where it was used? This is kind of like a conditional macro that has its
contents inserted at runtime. So if the condition evaluates to true, it is
just like the true code was there, and the same for the false code when
false. I find this particularly useful for when you are passing parameters
or doing conditional math.

Getting That Log, Even In A Crash:

When you start to make a complex system, especially a game, you need to
have a logging system so that you can find out exactly what made your game
crash. Unfortunately, those that have made them are certain to have
experienced the log-destroying crash. An undefeatable problem? I think
not.

What is happening when you lose your log? Well that has to do with the way
file input/output works in C/C++. When you use file writing functions, the
data is not immediately written, but rather qued for writing in the
memory. The actual file writing will not occur until either the file is
closed or until the file is flushed. When the program crashes, the data
that was qued was never written
it was still in the memory. So as a good
rule of thumb, when you write to a log, flush it immediately thereafter.
If you are using the stdio FILE operations, you do that with fflush:


fflush( ); // Flushes output

Thatłs all it takes! No more lost logs! Yay!

Whelp, that concludes the article. Hope you learned something!

Paul Frazee (The Rainmaker)



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