Consider the following simple C/C++ example:
#define FOO
...
#ifdef FOO
bar++;
#endif
OK, now I would like to fit that (and any other similar) conditional into one line for the code readability sake (the code has tens of single-line statements that all need to be conditional each depending on different define). Something that, when used, would look like:
#define FOO
...
MY_IFDEF(FOO,bar++;) //Single-line conditional
The goal is to have a reusable macro that can take an arbitrary identifier and, compile the statement if such identifier has been #define-d previously, and do it all in a single line.
Any ideas?
UPDATE0: the code must compile for both C and C++
You can't use #ifdef while expanding macros, but you can totally precheck it and declare empty statement if condition is not met.
#ifdef FOO
#define MY_IFDEF(x,y) some-processing-you-need
#else
#define MY_IFDEF(x,y) ;
#endif
Also check out new feature of C++: constexpr this can also be usable.
UPDATE4: As pointed out by #Alex Bakanov you can't use #ifdef while expanding macros, so there is no single line solution for your question which works in all cases. Nevertheless, I hope the idea I wrote here may be useful.
if you use #define FOO 1 or #define FOO 0, combination of #define and if constexpr can be used. Note that it gives an error if FOO is not defined. This program gives 1 as result:
#include<iostream>
#define FOO 1
#define MY_IFDEF(x,y) if constexpr (x) y;
int main()
{
int bar =0;
MY_IFDEF(FOO,bar++)
std::cout << bar << "\n";
}
UPDATE: Based on #eerorika's comment to avoid the error if FOO is not defined, the following declaration has to be added:
constexpr bool FOO = false;
UPDATE2: This version works in any circumstances, the only question is that is it worth the effort?
#ifdef FOO
constexpr bool USED_FOO = true;
#else
constexpr bool USED_FOO = false;
#endif
#define MY_IFDEF(x,y) if constexpr (USED_##x) y ;
UPDATE3: C compatible version. Note that in this case in theory it is evaluated runtime not compile time, but the compiler will realize that it is always true/false and generates the code accordingly:
#ifdef FOO
const static bool USED_FOO = true;
#else
const static bool USED_FOO = false;
#endif
#define MY_IFDEF(x,y) if (USED_##x) y;
Related
I know how to mark enums/functions as deprecated by using
__attribute__ ((deprecated)). But how can I mark constant macro
as deprecated?
#define MACRO1 4 //This is deprecated macro
GCC (and possibly others)
__attribute__((deprecated))
For your particular example with just a constant expression, you can use this:
Change
#define X (4)
to
#define X_old (4)
and then add
const int dep __attribute__((deprecated)) = 0;
#define X ((void)dep, X_old)
Addition also works:
#define X (X_old + dep)
For a procedure macro you can do this:
#define P_old do { ... } while(0)
#define P do { (void)dep; P_old; } while(0)
The only function of (void) is to suppress warnings. Thanks Kevin.
#pragma message
Another solution is to put all deprecated macros in a separate header file and use pragma. You could combine this with #ifdef and such:
#pragma message ("This header contains deprecated macros")
All compilers
Unreferenced label
Use an unreferenced label:
#define P_old do { ... } while(0)
#define P do { P_IS_DEPRECATED: P_old; } while(0)
This does not work for constant expression macros and requires you to compile with -Wall to get a warning. Will trigger error if used more than once.
Unused variable:
#define P_old do { ... } while(0)
#define P do { int P_IS_DEPRECATED; P_old; } while(0)
Does not work on constant expressions either. Also requires -Wall but can be used more than once.
Sidenote
Remember to encapsulate constant expression macros in parentheses. The macro #define X 2+3 would make an expression like 2*X to expand to 2*2+3 instead of 2*(2+3).
Ultimately, I want a compile-time-const macro that in itself includes an assertion.
With a real _Static_assert, I can do something like
#define CEXPR_MACRO_WITH_ASSERTION(Assertion) sizeof(struct{char c; _Static_assert(Assertion,""); })?0:42
(meant for stuff like "compile-time-assert" that the macro value computation won't overflow on any target, and I'd like to keep the assertion in the macro so that it's tightly coupled with the value) but compilers like tcc don't have static asserts so I'd need to emulate it.
#define STATIC_ASSERT(Cexpr,Msg) extern STATIC_ASSERT[(Cexpr)?1:-1]
is a common way to do it but with that extern I can't use it in a struct so I could split it in two
#define STATIC_ASSERT(Cexpr,Msg) extern STATIC_ASSERT_(Cexpr,Msg)
#define STATIC_ASSERT_(Cexpr,Msg) char STATIC_ASSERT[sizeof(char [((Cexpr))?1:-1])] /*ignore Msg for simplicity's sake*/
and use the underscore version in the CEXPR_MACRO_WITH_ASSERTION, but in function context, this will give false positives on compilers that support structs with VLAs in them:
#define STATIC_ASSERT(Cexpr,Msg) extern STATIC_ASSERT_(Cexpr,Msg)
#define STATIC_ASSERT_(Cexpr,Msg) char STATIC_ASSERT[sizeof(char [((Cexpr))?1:-1])]
#define CEXPR_MACRO_WITH_ASSERTION(Assert) (sizeof(struct{char c; STATIC_ASSERT_(Assert,""); })?0:42)
int main(void)
{
int x = 0;
CEXPR_MACRO_WITH_ASSERTION(x);
} //compiles on tcc and gcc (clang rejects it because of the vla in a struct)
so I effectively need:
#define STATIC_ASSERT_(Cexpr,Msg) char STATIC_ASSERT[sizeof(char [((Cexpr)&&ENFORCE_ICEXPR(Cexpr))?1:-1])]
Now I realize on tcc in particular, ENFORCE_ICEXPR (enforce integer constant expression) could be simply replaced with __builtin_constant_p but I was curious if I could do it without the platform dependency.
So I thought I could test Cexpr by trying to assign it to an enum constant and I came up with:
#define ENFORCE_Z(X) _Generic(0LL+(X),ullong:(X),llong:(X)) /*could be just `+(X)` cuz I don't care about floats*/
#define ENFORCE_ICEXPR(X) sizeof( void (*)(enum { ENFORCE_ICEXPR = (int)ENFORCE_Z(X) } ) )
but this gets gcc and clang complaining (unsilencably in gcc's case) about the enum not being visible outside of the declaration (which, incidentally, was the intention here) so I resorted to
#define ENFORCE_ICEXPR(X) sizeof(enum { BX_cat(ENFORCE_ICEXPR__,__COUNTER__) = (int)ENFORCE_Z(X) })
relying on the nonstandard magic macro, __COUNTER__.
My question is, is there a better way to write ENFORCE_ICEXPR(X)?
Perl uses a bit-field instead of an array to define a static_assert fallback:
#define STATIC_ASSERT_2(COND, SUFFIX) \
typedef struct { \
unsigned int _static_assertion_failed_##SUFFIX : (COND) ? 1 : -1; \
} _static_assertion_failed_##SUFFIX PERL_UNUSED_DECL
#define STATIC_ASSERT_1(COND, SUFFIX) STATIC_ASSERT_2(COND, SUFFIX)
#define STATIC_ASSERT_DECL(COND) STATIC_ASSERT_1(COND, __LINE__)
No compiler implements variable length bit-fields.
I need to temporarily overwrite a macro and then restore it. Like:
#define FOO X
#save FOO
#define FOO Y
...
#restore FOO
Is it possible in standard C preprocessor? In GCC?
ADDED. About real world example. I use a global macro for error exception. It acts like assert, but for persistent usage, not only for debug versions; so, for example, I usually call functions (with side-effect) inside the macro. It's defined once, but the definition isn't persistent; therefore I don't know it a-priori. For some piece of code I need its own, modified version of the macro, but I want to save general style of code. It's looks ugly when one part of code uses the one macro, other part uses other macro -- both macros have the same purpose, but slightly different implementation.
So, it's good for me to save original macro temporarily, use different version for a part of code, after that restore original macro.
This is possible with #pragma push_macro and #pragma pop_macro. These are not standard C—they're originally an MSVC extension—but clang supports them, and so does GCC.
Example usage:
int main() {
#define SOME_MACRO 1
printf("SOME_MACRO = %d\n", SOME_MACRO);
#pragma push_macro("SOME_MACRO")
#define SOME_MACRO 2
printf("SOME_MACRO = %d\n", SOME_MACRO);
#pragma pop_macro("SOME_MACRO")
printf("SOME_MACRO = %d\n", SOME_MACRO);
return 0;
}
prints:
SOME_MACRO = 1
SOME_MACRO = 2
SOME_MACRO = 1
You can also #undef a macro inside a push_macro / pop_macro pair, and the pop_macro call will redefine it.
As already said, it is not really possible. Depending on the situation, this might be a workaround:
#include "generalmacrodefs.h" // put them in here or include them indirectly
#undef macro1
#define macro1 "specialized temporary value"
#undef macro1
#include "generalmacrodefs.h" // restores
This requires that generalmacrodefs.h uses a pattern like this at least for the definitions you might temporarily overwrite:
#ifndef macro1
#define macro1 "original value"
#endif
The closest you can come in C is the #undef directive, which simply undefines the macro, allowing it to be replaced:
#define FOO X
...
#undef FOO
#define FOO Y
...
#undef FOO
#define FOO X
The problem is that you cannot know the 'old' value of FOO once you redefine it - so your values must be hard-coded in one place.
You cannot create a macro to save the values for you either, as it isn't possible to have a macro that creates other preprocessor directives in standard C.
I know that this will not work, but hopefully you can see what I'm trying to do
#if ASSIGN_ALLOWED
#define MAYBE_SKIP_REST_OF_LINE
#else
#define MAYBE_SKIP_REST_OF_LINE ; //
#endif
char str[80] MAYBE_SKIP_REST_OF_LINE = "Hello\n";
long array[3] MAYBE_SKIP_REST_OF_LINE = { 7,8,9 };
int x MAYBE_SKIP_REST_OF_LINE = 3;
//...many many more similar lines...
Is there a way to do this such that it works?
Sure:
#ifdef ASSIGN_ALLOWED
#define OPTIONAL_INITIALISER(x) = x
#else
#define OPTIONAL_INITIALISER(x)
#endif
char str[80] OPTIONAL_INTIALISER("Hello\n");
#define ARRAY_INIT { 7,8,9 }
long array[3] OPTIONAL_INITIALISER(ARRAY_INIT);
#undef ARRAY_INIT
int x OPTIONAL_INITIALISER(3);
Any initialisers containing commas, like for array in the example, will need to be expanded from their own macro, like ARRAY_INIT in the above. If your compiler supports C99 varargs macros, then you can instead do it in a cleaner way:
#ifdef ASSIGN_ALLOWED
#define OPTIONAL_INITIALISER(...) = __VA_ARGS__
#else
#define OPTIONAL_INITIALISER(...)
#endif
char str[80] OPTIONAL_INTIALISER("Hello\n");
long array[3] OPTIONAL_INITIALISER({ 7,8,9 });
int x OPTIONAL_INITIALISER(3);
As comments are filtered out in the preprocessor run, i dont think so
It would depend on how the preprocessor worked with comments and macros. If it strips comments after macro expansion then your smooth sailing, but otherwise it might not work simply due to the preprocessor implementation.
You could try this? (it would be messy though).
#define MAYBE_SKIP(code) code
#define MAYBE_SKIP(code) /* code */
The preprocessor strips out commented sections. Try running
gcc -E source.c
This will run the preprocessor on your code but will not actually compile it, allowing you to see what happens after macro expansion. You should notice that all the comments are gone from any expanded macros.
I'm tidying up some older code that uses 'magic numbers' all over the place to set hardware registers, and I would like to use constants instead of these numbers to make the code somewhat more expressive (in fact they will map to the names/values used to document the registers).
However, I'm concerned that with the volume of changes I might break the magic numbers. Here is a simplified example (the register set is more complex):
const short mode0 = 0;
const short mode1 = 1;
const short mode2 = 2;
const short state0 = 0;
const short state1 = 4;
const short state2 = 8;
so instead of :
set_register(5);
we have:
set_register(state1|mode1);
What I'm looking for is a build time version of:
ASSERT(5==(state1|mode1));
Update
#Christian, thanks for the quick response, I'm interested on a C / non-boost environment answer too because this is driver/kernel code.
NEW ANSWER :
In my original answer (below), I had to have two different macros to support assertions in a function scope and at the global scope. I wondered if it was possible to come up with a single solution that would work in both scopes.
I was able to find a solution that worked for Visual Studio and Comeau compilers using extern character arrays. But I was able to find a more complex solution that works for GCC. But GCC's solution doesn't work for Visual Studio. :( But adding a '#ifdef __ GNUC __', it's easy to choose the right set of macros for a given compiler.
Solution:
#ifdef __GNUC__
#define STATIC_ASSERT_HELPER(expr, msg) \
(!!sizeof \ (struct { unsigned int STATIC_ASSERTION__##msg: (expr) ? 1 : -1; }))
#define STATIC_ASSERT(expr, msg) \
extern int (*assert_function__(void)) [STATIC_ASSERT_HELPER(expr, msg)]
#else
#define STATIC_ASSERT(expr, msg) \
extern char STATIC_ASSERTION__##msg[1]; \
extern char STATIC_ASSERTION__##msg[(expr)?1:2]
#endif /* #ifdef __GNUC__ */
Here are the error messages reported for STATIC_ASSERT(1==1, test_message); at line 22 of test.c:
GCC:
line 22: error: negative width in bit-field `STATIC_ASSERTION__test_message'
Visual Studio:
test.c(22) : error C2369: 'STATIC_ASSERTION__test_message' : redefinition; different subscripts
test.c(22) : see declaration of 'STATIC_ASSERTION__test_message'
Comeau:
line 22: error: declaration is incompatible with
"char STATIC_ASSERTION__test_message[1]" (declared at line 22)
ORIGINAL ANSWER :
I do something very similar to what Checkers does. But I include a message that'll show up in many compilers:
#define STATIC_ASSERT(expr, msg) \
{ \
char STATIC_ASSERTION__##msg[(expr)?1:-1]; \
(void)STATIC_ASSERTION__##msg[0]; \
}
And for doing something at the global scope (outside a function) use this:
#define GLOBAL_STATIC_ASSERT(expr, msg) \
extern char STATIC_ASSERTION__##msg[1]; \
extern char STATIC_ASSERTION__##msg[(expr)?1:2]
There is an article by
Ralf Holly that examines different options for static asserts in C.
He presents three different approaches:
switch case values must be unique
arrays must not have negative dimensions
division by zero for constant expressions
His conclusion for the best implementation is this:
#define assert_static(e) \
do { \
enum { assert_static__ = 1/(e) }; \
} while (0)
Checkout boost's static assert
You can roll your own static assert if you don't have access to a third-party library static assert function (like boost):
#define STATIC_ASSERT(x) \
do { \
const static char dummy[(x)?1:-1] = {0};\
} while(0)
The downside is, of course, that error message is not going to be very helpful, but at least, it will give you the line number.
#define static_assert(expr) \
int __static_assert(int static_assert_failed[(expr)?1:-1])
It can be used anywhere, any times.
I think it is the easiest solution.
Before usage, test it with your compiler carefully.
Any of the techniques listed here should work and when C++0x becomes available you will be able to use the built-in static_assert keyword.
If you have Boost then using BOOST_STATIC_ASSERT is the way to go. If you're using C or don't want to get Boost
here's my c_assert.h file that defines (and explains the workings of) a few macros to handle static assertions.
It's a bit more convoluted that it should be because in ANSI C code you need 2 different macros - one that can work in the area where you have declarations and one that can work in the area where normal statements go. There is a also a bit of work that goes into making the macro work at global scope or in block scope and a bunch of gunk to ensure that there are no name collisions.
STATIC_ASSERT() can be used in the variable declaration block or global scope.
STATIC_ASSERT_EX() can be among regular statements.
For C++ code (or C99 code that allow declarations mixed with statements) STATIC_ASSERT() will work anywhere.
/*
Define macros to allow compile-time assertions.
If the expression is false, an error something like
test.c(9) : error XXXXX: negative subscript
will be issued (the exact error and its format is dependent
on the compiler).
The techique used for C is to declare an extern (which can be used in
file or block scope) array with a size of 1 if the expr is TRUE and
a size of -1 if the expr is false (which will result in a compiler error).
A counter or line number is appended to the name to help make it unique.
Note that this is not a foolproof technique, but compilers are
supposed to accept multiple identical extern declarations anyway.
This technique doesn't work in all cases for C++ because extern declarations
are not permitted inside classes. To get a CPP_ASSERT(), there is an
implementation of something similar to Boost's BOOST_STATIC_ASSERT(). Boost's
approach uses template specialization; when expr evaluates to 1, a typedef
for the type
::interslice::StaticAssert_test< sizeof( ::interslice::StaticAssert_failed<true>) >
which boils down to
::interslice::StaticAssert_test< 1>
which boils down to
struct StaticAssert_test
is declared. If expr is 0, the compiler will be unable to find a specialization for
::interslice::StaticAssert_failed<false>.
STATIC_ASSERT() or C_ASSERT should work in either C or C++ code (and they do the same thing)
CPP_ASSERT is defined only for C++ code.
Since declarations can only occur at file scope or at the start of a block in
standard C, the C_ASSERT() or STATIC_ASSERT() macros will only work there. For situations
where you want to perform compile-time asserts elsewhere, use C_ASSERT_EX() or
STATIC_ASSERT_X() which wrap an enum declaration inside it's own block.
*/
#ifndef C_ASSERT_H_3803b949_b422_4377_8713_ce606f29d546
#define C_ASSERT_H_3803b949_b422_4377_8713_ce606f29d546
/* first some utility macros to paste a line number or counter to the end of an identifier
* this will let us have some chance of generating names that are unique
* there may be problems if a static assert ends up on the same line number in different headers
* to avoid that problem in C++ use namespaces
*/
#if !defined( PASTE)
#define PASTE2( x, y) x##y
#define PASTE( x, y) PASTE2( x, y)
#endif /* PASTE */
#if !defined( PASTE_LINE)
#define PASTE_LINE( x) PASTE( x, __LINE__)
#endif /* PASTE_LINE */
#if!defined( PASTE_COUNTER)
#if (_MSC_VER >= 1300) /* __COUNTER__ introduced in VS 7 (VS.NET 2002) */
#define PASTE_COUNTER( x) PASTE( x, __COUNTER__) /* __COUNTER__ is a an _MSC_VER >= 1300 non-Ansi extension */
#else
#define PASTE_COUNTER( x) PASTE( x, __LINE__) /* since there's no __COUNTER__ use __LINE__ as a more or less reasonable substitute */
#endif
#endif /* PASTE_COUNTER */
#if __cplusplus
extern "C++" { // required in case we're included inside an extern "C" block
namespace interslice {
template<bool b> struct StaticAssert_failed;
template<> struct StaticAssert_failed<true> { enum {val = 1 }; };
template<int x> struct StaticAssert_test { };
}
}
#define CPP_ASSERT( expr) typedef ::interslice::StaticAssert_test< sizeof( ::interslice::StaticAssert_failed< (bool) (expr) >) > PASTE_COUNTER( IntersliceStaticAssertType_)
#define STATIC_ASSERT( expr) CPP_ASSERT( expr)
#define STATIC_ASSERT_EX( expr) CPP_ASSERT( expr)
#else
#define C_ASSERT_STORAGE_CLASS extern /* change to typedef might be needed for some compilers? */
#define C_ASSERT_GUID 4964f7ac50fa4661a1377e4c17509495 /* used to make sure our extern name doesn't collide with something else */
#define STATIC_ASSERT( expr) C_ASSERT_STORAGE_CLASS char PASTE( PASTE( c_assert_, C_ASSERT_GUID), [(expr) ? 1 : -1])
#define STATIC_ASSERT_EX(expr) do { enum { c_assert__ = 1/((expr) ? 1 : 0) }; } while (0)
#endif /* __cplusplus */
#if !defined( C_ASSERT) /* C_ASSERT() might be defined by winnt.h */
#define C_ASSERT( expr) STATIC_ASSERT( expr)
#endif /* !defined( C_ASSERT) */
#define C_ASSERT_EX( expr) STATIC_ASSERT_EX( expr)
#ifdef TEST_IMPLEMENTATION
C_ASSERT( 1 < 2);
C_ASSERT( 1 < 2);
int main( )
{
C_ASSERT( 1 < 2);
C_ASSERT( 1 < 2);
int x;
x = 1 + 4;
C_ASSERT_EX( 1 < 2);
C_ASSERT_EX( 1 < 2);
return( 0);
}
#endif /* TEST_IMPLEMENTATION */
#endif /* C_ASSERT_H_3803b949_b422_4377_8713_ce606f29d546 */
Try:
#define STATIC_ASSERT(x, error) \
do { \
static const char error[(x)?1:-1];\
} while(0)
Then you can write:
STATIC_ASSERT(a == b, a_not_equal_to_b);
Which may give you a better error message (depending on your compiler).
The common, portable option is
#if 5 != (state1|mode1)
# error "aaugh!"
#endif
but it doesn't work in this case, because they're C constants and not #defines.
You can see the Linux kernel's BUILD_BUG_ON macro for something that handles your case:
#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
When condition is true, this becomes ((void)sizeof(char[-1])), which is illegal and should fail at compile time, and otherwise it becomes ((void)sizeof(char[1])), which is just fine.
Ensure you compile with a sufficiently recent compiler (e.g. gcc -std=c11).
Then your statement is simply:
_Static_assert(state1|mode1 == 5, "Unexpected change of bitflags");
#define MODE0 0
#define MODE1 1
#define MODE2 2
#define STATE0 0
#define STATE1 4
#define STATE2 8
set_register(STATE1|STATE1); //set_register(5);
#if (!(5==(STATE1|STATE1))) //MY_ASSERT(5==(state1|mode1)); note the !
#error "error blah blah"
#endif
This is not as elegant as a one line MY_ASSERT(expr) solution. You could use sed, awk, or m4 macro processor before compiling your C code to generate the DEBUG code expansion of MY_ASSERT(expr) to multiple lines or NODEBUG code which removes them for production.