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

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,
:p


okay anyways here is the code

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

Code:

---

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

---

or

Code:

---

#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:

Code:

---

#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!!
:D

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:

Code:

---

#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:

Code:

---

#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.

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
...

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.

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

Code:

---

#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:

Code:

---

#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:

Code:

---

#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.

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

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

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

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!

:p

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

Code:

---

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

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

---

This is expanded to

Code:

---

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)!!!

indeed :)

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

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

nice template specification :)

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

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

just remember to define NOMINMAX before including windows.h.

I mean do they really belong there?

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

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