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Macro(???????????)

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Ok, ok, ok. Can someone explain macros to me, I use them and all but what exacly are the specifics on them? possibly a tutorial link if it''s so complicated. Any help is greatly appreciated.

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A macro is a symbol whose definition is substituted everywhere it occurs before compilation. The compilation process for a C/C++ source file includes a stage called "preprocessing." It is the preprocessor that handles all those keywords that begin with the hash symbol (#). #include, for example, causes the preprocessor to place the entire included file in place. #define causes an environment symbol to be created (and its value to be substituted everywhere it is referenced in code), which is the basis of macros. Here''s an example:
#define deg2rad(x) (M_PI/180 * x)

Now everywhere in the same file (or files that include this file) where the program sees deg2rad(v) - where v is any value - the preprocessor will substitute (M_PI/180 * v) in its place (note that M_PI is also a "macro" of sorts; it''s a defined symbol). Now if v was a non-numeric value like a string
int y = deg2rad("a");

the compiler will puke where it''s trying to multiply numbers and a string. Unfortunately, that would be the line where the macro was defined, so tracking down the bug would be pretty hard. You see, macros don''t do type-checking, so you can get some very bizarre behavior (which will sometimes compile but crash hard during execution).

If you''re using C++, use inline functions instead (and use const for constant values):
#ifndef M_PI // some platforms define M_PIconst float M_PI = 3.1459;#endifinline float deg2rad(float f) { return (180 / M_PI) * f; }

The problem with this as opposed to macros is that if you pass it an integer, you''ll get a type mismatch. However, since you''re using C++ you might as well make it a template function:
inline template <typename T> T deg2rad(T val) { return (180 / M_PI) * val; }

In conclusion, if you''re a C++ programmer, stick to inlines and const values; they were designer specifically to combat the shortcomings of macros and defines.

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I am C++ guy, but you see all these people demand C instead, so I haveto get used to doing all of this C stuff, hell my whole Win32 book is C and freakin' UNICODE compliance, damn code nazis.

Ok, um, thnx a ton for the explanation, but I got one more thingy thats sorta bothering.

#define __T(x) L##x

Makes a call to __T("Hello!") into a call to L"Hello!" is what this book says. Ummm... does this make sense to you? is this L## thing what I think it is. In which case can I then do like

#define MYCRAZYCRAP##x __T(x)

that way I can do MYCRAZYCRAP"Hello!" which would actually do __T("Hello!") which would actually do L"Hello!"

If thats not the way to define such a thing is there a way?

And can anyone tell me if the Bjorne guys C++ reference would cover all of this, I mean it would right? Cuz I'm thinking of buying that and with it the STL book that amazon recommends. Anyone?

Edited by - Sinner_Zero on November 17, 2001 8:40:39 PM

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The ## preprocessor directive is used to concatenate the tokens before and after it. It's not very useful under most circumstances. Here's an example of how it works:
  #define concat(a, b) a##bint main() { int xy = 10; cout << conat(x,y);}

The preprocessor transforms
cout << concat(x,y);
int
cout << xy;
which prints 10.

-Neophyte

EDIT: Formatting
- Death awaits you all with nasty, big, pointy teeth. -

Edited by - Neophyte on November 17, 2001 8:45:12 PM

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So is there anyway for me to recreate something like

char a[] = L"Hello!";

?

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Try this (I''m not sure if it works, I haven''t had to use Unicode yet).
  wchar_t a[] = L"Hello!";

The "wchar_t" is the standard wide character variable. I''m not sure if the "L" prefix is standard and I''m not sure if they get along together, so...

[Resist Windows XP''s Invasive Production Activation Technology!]

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Bjarne Stroustrup''s book "The C++ Programming Language" cover''s a lot technical, but it doesn''t go much into depth with C technicalities (like how to do asm in C and such), and neither does it go into depth with STL technicalities, i''d recommend for pure C getting Kernighan and Richie''s "The C Programming Language" and P.J. Plauger''s "The C++ Standard Template Library"

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There is little about macros in the Stroustrup (3rd ed).

As for ##, it indeed does token concatenation :

  #define CAT( X, Y ) X##YCAT( 2, 3 ) is 23CAT( A, 5 ) is A5

But, IMHO, more interesting is # which can be very useful to print debugging statements :

  #define STR( X ) #XSTR( x ) is "x"#define DBG( X ) X; // { if( debug_mode ) std::cerr << #X << std::endl; }

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quote:
Original post by abdulla
Bjarne Stroustrup''s book "The C++ Programming Language" cover''s a lot technical, but it doesn''t go much into depth with C technicalities (like how to do asm in C and such)...

Assembly language is not a standard part of C or C++, it is an extension added by the compiler. That''s probably why he doesn''t cover it .

[Resist Windows XP''s Invasive Production Activation Technology!]

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fruny, I didnt get that IMHO part, wtf are you doing there, can you please explain?

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That and the publisher sems to be McGraw here.....hmmm, maybe I''m dumb.

Thnx, I think I''ll really buy that one, especially since you say ALSO and not NO JUST BUY THIS.

That and it''s cheap =)

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Imagine you want to print what piece of code is executed as it is executed. One way to do it is to create a string mirroring the code and printing it out :

  { foo = bar + baz; if (debug) std::cerr << "foo = bar + baz" << std::endl;}

However, you have to take care to keep the string and the code in synch. Well, that is what the # macro is for : it converts source code into a string that the source code can use.

  #define STRING( X ) #X{ foo = bar + baz; if (debug) std::cerr << STRING(foo = bar + baz) << std::endl;}

Which would not buy you much if you couldn't do things with X within the macro itself :

  #define DBG( X ) { X; if( debug ) std::cerr << #X << std::endl; } /* Executes and prints X */{ DBG( foo = bar + baz );}

This is what is used in the assert( X ) macro. It tests the value of the expression X, and if it is false, it prints it out and abort().

Edited by - Fruny on November 17, 2001 12:10:13 AM

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For multiline macros use a forward slash \ not a backslash - and wrap the multiple statements in a do while loop

Check out the C Language faq for details: What''s the best way to write a multi-statement macro?

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late... need... more... caffeine...

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wait, but int he macro whats the first X there for?

{X; ...}

whats that X for???

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quote:
Original post by Sinner_Zero
whats that X for???

To do X.

Remember that X does not necessarily have to be a single variable; it can be a statement (which the preprocessor simply substitutes in place.) For example, in Fruny''s example
foo = bar + baz

is X. The macro therefore ensures that X is executed, and then only if debug is specified is the rest executed.

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IC, X isnt really done since the macro screws it all then.

so the macro hasto actually do X, cool. this stuff is neat.

THNX a ton once again guys, the more I know. heh.

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