i know what this means
#define M(B) (1U << ((sizeof(x) * CHAR_BIT) - B)) // CHAR_BIT=bits/byte
but i dont understand well this one
#undef M
after this what happens? M is cleared or deleted or?
views:
98answers:
3
+9
A:
After the #undef, it's as if the #define M... line never existed.
int a = M(123); // error, M is undefined
#define M(B) (1U << ((sizeof(x) * CHAR_BIT) - B))
int b = M(123); // no error, M is defined
#undef M
int c = M(123); // error, M is undefined
Dean Harding
2010-07-08 07:12:57
This is a good explanation of what #undef does. But I would say that the most common usage of #undef is in conjunction with simple definitions like "#define DEBUG" and not macro definitions as shown in the question/example. And remember that these are preprocessor definitions and not processed by the compiler.
rotti2
2010-07-08 08:04:36
@rotti2: yes, that's a good point. Though I also use `#undef` to undefine those annoying `min` and `max` macros that are in windows.h :-)
Dean Harding
2010-07-08 08:13:44
@rotti2: Another use for undef is to replace the definition of a macro. That is, you do `#ifdef A` followed by `#undef A`, `#define A [...]`, then `#endif`.I think if you skip the `#undef A` line the compiler will issue a warning if you `#define A` and A is already defined.
utnapistim
2010-07-08 08:31:54
+2
A:
Here is the MSDN article about it: http://msdn.microsoft.com/en-us/library/ts4w8783(VS.80).aspx
My understanding is that it removes the definition of M so that it may be used to define something else.
E.G.
#define M(X) 2*(X)
int a = M(2);
ASSERT(a == 4);
#undefine M
#define M(X) 3*(X)
int b = M(2);
ASSERT(b == 6);
It seems like a confusing thing to use but may come up in practice if you need to work with someone else's macros.
TJB
2010-07-08 07:13:39
A:
#define and #undef are preprocessor directives.
E.g. #define M(X) 2*(X)
M(1);
#undef M
M(2)
Because are preprocessor directives before compilation the preprocessor will simply replace after the #define M(X) 2*(X) in that source file.
M(1) with 2 * 1
if preprocessor finds #undef M it will not replace anymore
M(2) with 2 * 2 because M is destroyed when #undef M is found.
#undef is ussualy used if want to give another definition for an existing macro
Manu
2010-07-08 07:43:26