#define,#ifdef,#else,#endif questions! book isn't very clear how they work..

[struntz]

Hello everyone,
I'm not new to programming in C++, i just bought a new book, a "software eingeering" one. and he's covering some things i must have skipped along the ways of reading bad online tutorials,



okay anyways here is the code

#define MT
    #ifdef MT
    #define NFILE 40
    #else
    #define NFILE 20
#end if

alright so called explanation which i don't get really:
This code is similiar to what you find in the header files; if hte symbol MT is defined, the limit for the number of files is 40; if we remove its definition form the source file, the lmit will be 20.

now what does it mean "if the symbol MT is defined" ?
can u give me an example when number one will be true? the number of files == 40, and one of the examples when the number of files == 20? or make up your own example that is eaiseer to understand
also i don't understand

#ifdef CPLUSPLUS
//whatever is needed when the program is wirtten in C++
#endif

or

#define __cplusplus
   #ifdef __cplusplus
//whatever is needed when the program is written in C++
#end if

can u give me an code example that makes those statements run.

I understand how a simple one works like this:

#include <iostream>
#include <cmath>
#define Pi 3.1415926
using namespace std;


int main()
{
	double x,y = 1;
	double result;

	cout <<"Welcome to the vb-world.com!" << endl;
	x = y + 1;
	result = pow(Pi,x);
	cout <<"In that world, pi square is: " << result << endl;
	cout <<"Have a nice day!" << endl;
return 0;
}

thansk for your time!!

[Zaei]

Using #define <something> without a value (#define MT, for example), you define a symbol. Then, you can use #ifdef, #else, and #endif just like you would a regular boolean statement. So:

#define MT
#ifdef MT
#define FILES 40
#else
#define FILES 20
#endif

is just like a boolean statement. In this case, FILES gets 40, and #define FILES 20 nevers gets executed. If you remove the #define MT:

#ifdef MT
#define FILES 40
#else
#define FILES 20
#endif

MT is not defined, so #define FILES 40 doesnt get executed, and #define FILES 20 does.

Get it? =).

Z.

[kedaman]

Try to avoid macros, they are not type safe. Use constants and typedefs instead of defines:


#define Pi 3.1415926

can be replaced with

const double Pi = 3.1415926;

#define MT
#ifdef MT
#define NFILE 40
#else
#define NFILE 20
#end if

can be replaced with

struct useMT{const int NFILE=40;};
struct noMT{const int NFILE=20;};

typedef MT useMT;
...
MT::NFILE
...

[Zaei]

You can usually make Macros type safe. Anything with a decimal defaults to a double, but if you want a float, append an "f" to the end of the value. If no decimal, the default is an int, etc.

Z.

[CornedBee]

Keda is right about the constants thing, but I have to disagree with the other thing:

#ifdef _cplusplus
extern "C" {
#endif

  // all things go here

#ifdef _cplusplus
}
#endif

What about this?

struntz: everything that starts with # is a precompiler directive. Those will be processed before the real compiler does it's work.
#define PI 3.1415926
tells the precompiler to find every occurence of PI in the file and replace it with 3.1415926. In this case, the C++ method:
const double PI = 3.1415926;
is better because it's a double, and nothing else.
#define MT
defines a symbol. It just tells the precompiler that this is defined. For example, _cplusplus is automatically defined by most compilers when it's compiling C++ code, while it's not defined when compiling C code. MT is defined by VC++ when the application is multithreaded.
You can later test for these symbols using the defined keyword and the directives for conditioned compiling:

#if defined(MT) // if this is not defined REMOVE the code after this
// code
#else // else remove the code after this
// code
#endif // until here

You can also use !defined (not defined). You can concatenate them using || and &&. You can use #ifdef instead of #if defined and #ifndef instead of #if !defined.
You will often find this:

#ifndef _SOMENAME_
#define _SOMENAME_
// code
#endif

This is used in header files: if a unique symbol is not defined, define it and include all the stuff in the header. If the header is included a second time, the symbol will already be defined and the code therefore not included.

[kedaman]

Well that's not the thing i was mentioning, I was pointing out how you can defy macros for alternative values.

[CornedBee]

I fully agree that you should replace defines by real C++ constants and macros by inline functions, but symbols should still be used, even in C++.

#define MT
#ifdef MT
#define NFILE 40
#else
#define NFILE 20
#end if

should therefore be replaced by

#define MT
#ifdef MT
const unsigned int NFILE = 40;
#else
const unsigned int NFILE = 20;
#end if

[kedaman]

the double header avoiding #include and #pragma's are ok

[struntz]

Thanks guys, that hepled me understand it alot better, and i rember the book saying In C++, we use inline functions istead of macros. This wis why i will not discuss macros in further detail, even though a few years ago you had to know how to write macros to pass as a C programmmer. Macros are great fun, but they are a source of errors that are diffuclt to debug.

Your examples were very helpful,

thanks again!

[CornedBee]

Well, I once had a terrible problem with the ni/max macros:

#define min(a, b) (((a) < (b)) ? (a) : (b))

int SomeRecursive(int i)
{
   return min(SomeRecursive(i+1), 1);
}

This is expanded to

int SomeRecursive(int i)
{
   return (((SomeRecursive(i+1)) < (1)) ? (SomeRecursive(i+1)) : (1));
}

And suddenly I had two function calls... (My function added contents to a tree control, some items appeared twice)

Therefore: AVIOD MACROS WHENEVER POSSIBLE (= always)!!!

[kedaman]

indeed

template <class T>
inline T min(T a,T b){return a<b?a:b;};

[CornedBee]

template <> inline char* min(char* a, char* b)
{
return (strcmp(a, b) < 0) ? a : b;
}

[kedaman]

nice template specification

[CornedBee]

standard one - learned it together with the templates themselves

Oh yeah, windows.h contains the min/max macros - tell parksie to replace them by the inline templates

[kedaman]

If he haven't done that allready, I'll put them in, just wondering what they are doing in windows.h?

[CornedBee]

just remember to define NOMINMAX before including windows.h.

[kedaman]

I mean do they really belong there?

[CornedBee]

Well, they are useful (sometimes). And they don't hurt. But one doesn't want them in C++.

[kedaman]

I mean windows.h what else do you put in there just because it's "useful" And yep, they're not advocating OOP really