In C and C++, when using a macro like so:
#if ( 1 == __MY_MACRO__ )
// Some code
#endif
The compiler will not catch if MY_MACRO is not defined and will consider it 0. This could cause a lot of hidden bugs when the design of the code is intended such that the macro must be defined (non-zero).
Is there away to get to compiler to report this, even if the compiler natively doesn't look for such thing?
Use #if defined(__MY_MACRO__) to test if the macro value is defined.
#ifndef __MY_MACRO__
#error "MY_MACRO NOT DEFINED"
#endif
You can use #ifdef or ifndef to check if a macro is defined of not.
Example :
#ifndef MY_MACRO
# error "MY_MACRO is not defined"
#endif
More informations can be found here : https://gcc.gnu.org/onlinedocs/cpp/Ifdef.html
I have to resort to
#if !defined( __MY_MACRO__ )
MACRO_NOT_DEFINED; //to cause compiler error
#endif
#if ( 1 == __MY_MACRO__ )
//code
#endif
Which looks rather ugly.
Someone I know came up with a clever 1 liner
#if ( (1/defined(_MY_MACRO__) && 1 == _MY_MACRO__ ) )
//code
#endif
If _MY_MACRO__ is not defined, it will cause divide by zero error.
If the macro is used as compile switch try this way.
in a configuration file:
#define FEATURE_1_ENABLED() (1)
#define FEATURE_2_ENABLED() (0)
in place you checking the value:
#if FEATURE_1_ENABLED()
// whatever
#endif
in contrast to your example this does show error when macro is not visible by mistake (at least in my ide)
Related
Since preprocessor don't report an error when checking value of preprocessor's symbol that isn't actually defined (usually due to the lack of #include "some_header.h"), I use this cumbersome three line construction with "defined":
#if !defined(SOME_SYMBOL)
#error "some symbol isn't defined"
#endif
#if SOME_SYMBOL == 1
// Here is my conditionally compiled code
#endif
And the way with "#ifndef" is the same.
Is there a more elegant way to perform this check?
In your construction you could use an else block to skip the check for defined:
#if SOME_SYMBOL == 1
// Here is my conditionally compiled code
#else
// error
#endif
But in principle the comments are right. #if !defined and the shorthand #ifndef are the two available versions.
Currently, you are checking if SOME_SYMBOL is equals to 1. Do you execute different code based on that value ?
If not, you could simply use:
#ifdef SOME_SYMBOL
// Here is my conditionally compiled code
#else
// error
#endif
And now it's a short step to the typical c++ include guards. Copying from that wikipedia link, here is a grandparent.h file:
#ifndef GRANDPARENT_H
#define GRANDPARENT_H
struct foo {
int member;
};
#endif /* GRANDPARENT_H */
Now, even if you end up including this header twice, it will only be executed once.
Can I achieve something similar to following code:
#define MODULE base
#if defined (MODULE ## _dll) <-- this should do `#ifdef base_dll`
...
#else
...
#endif
second line is obviously wrong. Can I do this somehow?
Thanks
I don't think it is possible to check the definition of token-pasted macro like that (at least I don't know the way) but you can do this:
#define JOIN_INTERNAL(a,b) a ## b
#define JOIN(a,b) JOIN_INTERNAL(a,b)
// switch 1/0
#define base_dll 1
#define MODULE base
#if JOIN(MODULE,_dll)
// the base_dll is 1
#else
// the base_dll is 0 or not defined (in MSVC at least)
#endif
Perhaps if you describe what do you actually want to achieve there might be another way to do that.
In my C++ code I have to execute certain code under two conditions: because of a preprocessor macro OR a boolean variable check. For example:
bool done=false;
#ifdef _DEBUG
executeDebugCode();
done=true;
#endif
if (inputParam && !done)
executeDebugCode();
Is there a way to write the above code in a more elegant way, without repeating the executeDebugCode() function call two times?
EDIT:
the executeDebugCode() function should be executed once, and if one of the two condition is met. For example a function that should be executed in DEBUG mode only, that could be set by preprocessor macro or command line parameter.
Assuming that you want to execute this code only once, if at least one of these conditions is true:
if ( inputParam
#ifdef DEBUG
|| true
#endif
)
{
executeDebugCode();
}
The form I see most for this, and which tends to work well, is do make the exact check performed depend on _DEBUG, so you'd get:
#ifdef _DEBUG
#define SHOULD_EXECUTE_DEBUG_CODE() 1
#else
#define SHOULD_EXECUTE_DEBUG_CODE() inputParam
#endif
if (SHOULD_EXECUTE_DEBUG_CODE())
executeDebugCode();
Note that if inputParam is a local variable (as Sambuca points out in the comments), this macro SHOULD_EXECUTE_DEBUG_CODE cannot be used in other functions, and for maintainability, it may be worth adding #undef SHOULD_EXECUTE_DEBUG_CODE at the end of the function to prevent accidental misuse.
How about something like this :
bool debugEnabled = inputParam;
#ifdef _DEBUG
debugEnabled = true;
#endif
if (debugEnabled)
executeDebugCode()
ie. use one flag to control the code behavior, but allow that flag to be set in different ways.
My approach would be something like this
#ifdef _DEBUG
#define SHOULD_EXECUTE 1
#else
#define SHOULD_EXECUTE 0
#endif
if (SHOULD_EXECUTE || inputParam) {
executeDebugCode();
}
This way your if-statement shows right away that you're checking on a preprocessor define and another (boolean) condition.
This will not generate any overhead in runtime if DEBUG is not enabled.
#ifdef _DEBUG
#define MY_DEBUG true
#else
#define MY_DEBUG false
#endif
if ( inputParam || MY_DEBUG )
executeDebugCode();
I have encountered the #define pre-processor directive before while learning C, and then also encountered it in some code I read. But apart from using it to definite substitutions for constants and to define macros, I've not really understook the special case where it is used without a "body" or token-string.
Take for example this line:
#define OCSTR(X)
Just like that! What could be the use of this or better, when is this use of #define necessary?
This is used in two cases. The first and most frequent involves
conditional compilation:
#ifndef XYZ
#define XYZ
// ...
#endif
You've surely used this yourself for include guards, but it can also be
used for things like system dependencies:
#ifdef WIN32
// Windows specific code here...
#endif
(In this case, WIN32 is more likely defined on the command line, but it
could also be defined in a "config.hpp" file.) This would normally
only involve object-like macros (without an argument list or
parentheses).
The second would be a result of conditional compilation. Something
like:
#ifdef DEBUG
#define TEST(X) text(X)
#else
#define TEST(X)
#endif
That allows writing things like:
TEST(X);
which will call the function if DEBUG is defined, and do nothing if it
isn't.
Such macro usually appears in pair and inside conditional #ifdef as:
#ifdef _DEBUG
#define OCSTR(X)
#else
#define OCSTR(X) SOME_TOKENS_HERE
#endif
Another example,
#ifdef __cplusplus
#define NAMESPACE_BEGIN(X) namespace X {
#define NAMESPACE_END }
#else
#define NAMESPACE_BEGIN(X)
#define NAMESPACE_END
#endif
One odd case that I recently dug up to answer a question turned out to be simply commentary in nature. The code in question looked like:
void CLASS functionName(){
//
//
//
}
I discovered it was just an empty #define, which the author had chosen to document that the function accessed global variables in the project:
C++ syntax: void CLASS functionName()?
So not really that different from if it said /* CLASS */, except not allowing typos like /* CLAAS */...some other small benefits perhaps (?)
I agree with every answer, but I'd like to point out a small trivial thing.
Being a C purist I've grown up with the assertion that EACH AND EVERY #define should be an expression, so, even if it's common practice using:
#define WHATEVER
and test it with
#ifdef WHATEVER
I think it's always better writing:
#define WHATEVER (1)
also #debug macros shall be expressions:
#define DEBUG (xxx) (whatever you want for debugging, value)
In this way, you are completely safe from misuse of #macros and prevents nasty problems (especially in a 10 million line C project)
This can be used when you may want to silent some function. For example in debug mode you want to print some debug statements and in production code you want to omit them:
#ifdef DEBUG
#define PRINT(X) printf("%s", X)
#else
#define PRINT(X) // <----- silently removed
#endif
Usage:
void foo ()
{
PRINT("foo() starts\n");
...
}
#define macros are simply replaced, literally, by their replacement text during preprocessing. If there is no replacement text, then ... they're replaced by nothing! So this source code:
#define FOO(x)
print(FOO(hello world));
will be preprocessed into just this:
print();
This can be useful to get rid of things you don't want, like, say, assert(). It's mainly useful in conditional situations, where under some conditions there's a non-empty body, though.
As you can see in the above responses, it can be useful when debugging your code.
#ifdef DEBUG
#define debug(msg) fputs(__FILE__ ":" (__LINE__) " - " msg, stderr)
#else
#define debug(msg)
#endif
So, when you are debugging, the function will print the line number and file name so you know if there is an error. And if you are not debugging, it will just produce no output
There are many uses for such a thing.
For example, one is for the macro to have different behavior in different builds. For example, if you want debug messages, you could have something like this:
#ifdef _DEBUG
#define DEBUG_LOG(X, ...) however_you_want_to_print_it
#else
#define DEBUG_LOG(X, ...) // nothing
#endif
Another use could be to customize your header file based on your system. This is from my mesa-implemented OpenGL header in linux:
#if !defined(OPENSTEP) && (defined(__WIN32__) && !defined(__CYGWIN__))
# if defined(__MINGW32__) && defined(GL_NO_STDCALL) || defined(UNDER_CE) /* The generated DLLs by MingW with STDCALL are not compatible with the ones done by Microsoft's compilers */
# define GLAPIENTRY
# else
# define GLAPIENTRY __stdcall
# endif
#elif defined(__CYGWIN__) && defined(USE_OPENGL32) /* use native windows opengl32 */
# define GLAPIENTRY __stdcall
#elif defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__) >= 303
# define GLAPIENTRY
#endif /* WIN32 && !CYGWIN */
#ifndef GLAPIENTRY
#define GLAPIENTRY
#endif
And used in header declarations like:
GLAPI void GLAPIENTRY glClearIndex( GLfloat c );
GLAPI void GLAPIENTRY glClearColor( GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha );
GLAPI void GLAPIENTRY glClear( GLbitfield mask );
...
(I removed the part for GLAPI)
So you get the picture, a macro that is used in some cases and not used in other cases could be defined to something on those cases and nothing to those other cases.
Other cases could be as follows:
If the macro doesn't take parameters, it could be just to declare some case. A famous example is to guard header files. Another example would be something like this
#define USING_SOME_LIB
and later could be used like this:
#ifdef USING_SOME_LIB
...
#else
...
#endif
Could be that the macro was used at some stage to do something (for example log), but then on release the owner decided the log is not useful anymore and simply removed the contents of the macro so it becomes empty. This is not recommended though, use the method I mentioned in the very beginning of the answer.
Finally, it could be there just for more explanation, for example you can say
#define DONT_CALL_IF_LIB_NOT_INITIALIZED
and you write functions like:
void init(void);
void do_something(int x) DONT_CALL_IF_LIB_NOT_INITIALIZED;
Although this last case is a bit absurd, but it would make sense in such a case:
#define IN
#define OUT
void function(IN char *a, OUT char *b);
Sorry I know this is basic, but perhaps it doesn't exist or I'm not googling the right words.
Is there and an if not (is that ifndef?) an AND and an OR so I could do something like:
if not DEBUG and MACOS
I think something like #if !defined(DEBUG) && defined(MACOS) should do it.
#ifndef and #if do different things so it depends on what you want. #ifndef is true when there is no defined preprocessor symbol that matches the name following. #if is true when the following preprocessor expression evaluates to non-zero.
You can use the standard && and || operators.
#if !defined(DEBUG) && defined(MACOS)
#error "Ouch!"
#endif
tests, if those macros/values are defined (even set to 0 means defined). Leave out the "defined()" and test again a value, depending on your macros, like
#if DEBUG==0 && MACOS==1
#error "Spam!"
#endif
#if !DEBUG && MACROS
or
#if !DEBUG & !MACROS
depending on what you are looking for. defined() can also help
#if !defined(DEBUG) && defined(MACROS)
#if !(defined(DEBUG) && defined(MACOS))
or
#if !defined(DEBUG) && !defined(MACOS)
depending on what you're trying to evaluate.
#if, #else and #endif are general.
Use #define to declare and #undef to undeclare.
Use #ifdef to check if is declared and #ifndef to check, if is not declared.
Example:
#ifndef LABEL
#define LABEL some_value // declares LABEL as some_value
#else
#undef LABEL // undeclare previously declared LABEL...
#define LABEL new_value // to declare a new_value
#endif
Check out the Boost preprocessing library. It can accomplish a large number of tasks using the preprocessor.