Related
I came across this code that involved variadic Macros and I wanted to know what that meant
#define DECLARE_LEGACY_TYPES(...) //This all of the macro - I am not holding out on anything
Now There is this class as this
Header file: .h
namespace LG_Wrapper
{
template <LG_Thread Thread>
class EffectApplication : public ktApplication
{
public:
static EffectApplication<Thread>& GetInstance();
protected:
.....
.....
static boost::recursive_mutex mResource;
}
}
DECLARE_LEGACY_TYPES(EffectApplication); <---- What does this do ?
I wanted to know what effect the macro has ?
Update:
I have received numerous downvotes on this as this question gives of the impression that something is missing that I did not post the entire content of the macro. There is nothing more to the macro. I wish there was. This question is related to this which was closed. The macro literally just ends after (...)
#define DECLARE_LEGACY_TYPES(...)
but there isnt. That is one of the reason why I am here as I am not sure how to deal with this situation. Does this macro have not effect then ?
More Info:
This is what I have in another file
I am using the following defined in my project setting
LG_WRAPPER_EXPORTS
LG_THREAD_NAME=GAME
Following is the code
namespace LG_Wrapper
{
enum LG_Thread
{
GAME,
OTHER
};
/*
If the library itself is including this file
*/
#ifdef LG_WRAPPER_EXPORTS
#ifndef LG_THREAD_NAME
#error You must define LG_THREAD_NAME!
#endif
//Legacy types should not be used internally
#define DECLARE_LEGACY_TYPES(...)
#else // LG_WRAPPER_EXPORTS
//Legacy typenames are provided for convenience to the client
#define DECLARE_LEGACY_TYPES(ClassType) \
typedef LG_Wrapper::##ClassType##<LG_Wrapper::GAME> ClassType; \
#endif // LG_WRAPPER_EXPORTS
}
This is actually pretty common, but it depends on other code that wasn't mentioned in the other code you looked at:
#if USING_OLD_COMPILER //when using an older compiler, use this to declare legacy types
#define DECLARE_LEGACY_TYPES(...) STUFF(__VA_ARGS__)
#else //new compiler doesn't have to do anything special
#define DECLARE_LEGACY_TYPES(...)
#endif
//in older compilers we had to declare legacy types for this
//newer compilers don't need this step, so this does nothing at all in them.
DECLARE_LEGACY_TYPES(EffectApplication);
I don't actually know this macro, so I don't know it's actual purpose. But it's common to see macros without definitions for similar tricks as this.
Closed. This question needs to be more focused. It is not currently accepting answers.
Want to improve this question? Update the question so it focuses on one problem only by editing this post.
Closed 4 years ago.
Improve this question
The C preprocessor is justifiably feared and shunned by the C++ community. In-lined functions, consts and templates are usually a safer and superior alternative to a #define.
The following macro:
#define SUCCEEDED(hr) ((HRESULT)(hr) >= 0)
is in no way superior to the type safe:
inline bool succeeded(int hr) { return hr >= 0; }
But macros do have their place, please list the uses you find for macros that you can't do without the preprocessor.
Please put each use-cases in a seperate answer so it can be voted up and if you know of how to achieve one of the answers without the preprosessor point out how in that answer's comments.
As wrappers for debug functions, to automatically pass things like __FILE__, __LINE__, etc:
#ifdef ( DEBUG )
#define M_DebugLog( msg ) std::cout << __FILE__ << ":" << __LINE__ << ": " << msg
#else
#define M_DebugLog( msg )
#endif
Since C++20 the magic type std::source_location can however be used instead of __LINE__ and __FILE__ to implement an analogue as a normal function (template).
Methods must always be complete, compilable code; macros may be code fragments. Thus you can define a foreach macro:
#define foreach(list, index) for(index = 0; index < list.size(); index++)
And use it as thus:
foreach(cookies, i)
printf("Cookie: %s", cookies[i]);
Since C++11, this is superseded by the range-based for loop.
Header file guards necessitate macros.
Are there any other areas that necessitate macros? Not many (if any).
Are there any other situations that benefit from macros? YES!!!
One place I use macros is with very repetitive code. For example, when wrapping C++ code to be used with other interfaces (.NET, COM, Python, etc...), I need to catch different types of exceptions. Here's how I do that:
#define HANDLE_EXCEPTIONS \
catch (::mylib::exception& e) { \
throw gcnew MyDotNetLib::Exception(e); \
} \
catch (::std::exception& e) { \
throw gcnew MyDotNetLib::Exception(e, __LINE__, __FILE__); \
} \
catch (...) { \
throw gcnew MyDotNetLib::UnknownException(__LINE__, __FILE__); \
}
I have to put these catches in every wrapper function. Rather than type out the full catch blocks each time, I just type:
void Foo()
{
try {
::mylib::Foo()
}
HANDLE_EXCEPTIONS
}
This also makes maintenance easier. If I ever have to add a new exception type, there's only one place I need to add it.
There are other useful examples too: many of which include the __FILE__ and __LINE__ preprocessor macros.
Anyway, macros are very useful when used correctly. Macros are not evil -- their misuse is evil.
Mostly:
Include guards
Conditional compilation
Reporting (predefined macros like __LINE__ and __FILE__)
(rarely) Duplicating repetitive code patterns.
In your competitor's code.
Inside conditional compilation, to overcome issues of differences between compilers:
#ifdef WE_ARE_ON_WIN32
#define close(parm1) _close (parm1)
#define rmdir(parm1) _rmdir (parm1)
#define mkdir(parm1, parm2) _mkdir (parm1)
#define access(parm1, parm2) _access(parm1, parm2)
#define create(parm1, parm2) _creat (parm1, parm2)
#define unlink(parm1) _unlink(parm1)
#endif
When you want to make a string out of an expression, the best example for this is assert (#x turns the value of x to a string).
#define ASSERT_THROW(condition) \
if (!(condition)) \
throw std::exception(#condition " is false");
String constants are sometimes better defined as macros since you can do more with string literals than with a const char *.
e.g. String literals can be easily concatenated.
#define BASE_HKEY "Software\\Microsoft\\Internet Explorer\\"
// Now we can concat with other literals
RegOpenKey(HKEY_CURRENT_USER, BASE_HKEY "Settings", &settings);
RegOpenKey(HKEY_CURRENT_USER, BASE_HKEY "TypedURLs", &URLs);
If a const char * were used then some sort of string class would have to be used to perform the concatenation at runtime:
const char* BaseHkey = "Software\\Microsoft\\Internet Explorer\\";
RegOpenKey(HKEY_CURRENT_USER, (string(BaseHkey) + "Settings").c_str(), &settings);
RegOpenKey(HKEY_CURRENT_USER, (string(BaseHkey) + "TypedURLs").c_str(), &URLs);
Since C++20 it is however possible to implement a string-like class type that can be used as a non-type template parameter type of a user-defined string literal operator which allows such concatenation operations at compile-time without macros.
When you want to change the program flow (return, break and continue) code in a function behaves differently than code that is actually inlined in the function.
#define ASSERT_RETURN(condition, ret_val) \
if (!(condition)) { \
assert(false && #condition); \
return ret_val; }
// should really be in a do { } while(false) but that's another discussion.
The obvious include guards
#ifndef MYHEADER_H
#define MYHEADER_H
...
#endif
Let's say we'll ignore obvious things like header guards.
Sometimes, you want to generate code that needs to be copy/pasted by the precompiler:
#define RAISE_ERROR_STL(p_strMessage) \
do \
{ \
try \
{ \
std::tstringstream strBuffer ; \
strBuffer << p_strMessage ; \
strMessage = strBuffer.str() ; \
raiseSomeAlert(__FILE__, __FUNCSIG__, __LINE__, strBuffer.str().c_str()) \
} \
catch(...){} \
{ \
} \
} \
while(false)
which enables you to code this:
RAISE_ERROR_STL("Hello... The following values " << i << " and " << j << " are wrong") ;
And can generate messages like:
Error Raised:
====================================
File : MyFile.cpp, line 225
Function : MyFunction(int, double)
Message : "Hello... The following values 23 and 12 are wrong"
Note that mixing templates with macros can lead to even better results (i.e. automatically generating the values side-by-side with their variable names)
Other times, you need the __FILE__ and/or the __LINE__ of some code, to generate debug info, for example. The following is a classic for Visual C++:
#define WRNG_PRIVATE_STR2(z) #z
#define WRNG_PRIVATE_STR1(x) WRNG_PRIVATE_STR2(x)
#define WRNG __FILE__ "("WRNG_PRIVATE_STR1(__LINE__)") : ------------ : "
As with the following code:
#pragma message(WRNG "Hello World")
it generates messages like:
C:\my_project\my_cpp_file.cpp (225) : ------------ Hello World
Other times, you need to generate code using the # and ## concatenation operators, like generating getters and setters for a property (this is for quite a limited cases, through).
Other times, you will generate code than won't compile if used through a function, like:
#define MY_TRY try{
#define MY_CATCH } catch(...) {
#define MY_END_TRY }
Which can be used as
MY_TRY
doSomethingDangerous() ;
MY_CATCH
tryToRecoverEvenWithoutMeaningfullInfo() ;
damnThoseMacros() ;
MY_END_TRY
(still, I only saw this kind of code rightly used once)
Last, but not least, the famous boost::foreach !!!
#include <string>
#include <iostream>
#include <boost/foreach.hpp>
int main()
{
std::string hello( "Hello, world!" );
BOOST_FOREACH( char ch, hello )
{
std::cout << ch;
}
return 0;
}
(Note: code copy/pasted from the boost homepage)
Which is (IMHO) way better than std::for_each.
So, macros are always useful because they are outside the normal compiler rules. But I find that most the time I see one, they are effectively remains of C code never translated into proper C++.
Unit test frameworks for C++ like UnitTest++ pretty much revolve around preprocessor macros. A few lines of unit test code expand into a hierarchy of classes that wouldn't be fun at all to type manually. Without something like UnitTest++ and it's preprocessor magic, I don't know how you'd efficiently write unit tests for C++.
You can't perform short-circuiting of function call arguments using a regular function call. For example:
#define andm(a, b) (a) && (b)
bool andf(bool a, bool b) { return a && b; }
andm(x, y) // short circuits the operator so if x is false, y would not be evaluated
andf(x, y) // y will always be evaluated
To fear the C preprocessor is like to fear the incandescent bulbs just because we get fluorescent bulbs. Yes, the former can be {electricity | programmer time} inefficient. Yes, you can get (literally) burned by them. But they can get the job done if you properly handle it.
When you program embedded systems, C uses to be the only option apart form assembler. After programming on desktop with C++ and then switching to smaller, embedded targets, you learn to stop worrying about “inelegancies” of so many bare C features (macros included) and just trying to figure out the best and safe usage you can get from those features.
Alexander Stepanov says:
When we program in C++ we should not be ashamed of its C heritage, but make
full use of it. The only problems with C++, and even the only problems with C, arise
when they themselves are not consistent with their own logic.
Some very advanced and useful stuff can still be built using preprocessor (macros), which you would never be able to do using the c++ "language constructs" including templates.
Examples:
Making something both a C identifier and a string
Easy way to use variables of enum types as string in C
Boost Preprocessor Metaprogramming
We use the __FILE__ and __LINE__ macros for diagnostic purposes in information rich exception throwing, catching and logging, together with automated log file scanners in our QA infrastructure.
For instance, a throwing macro OUR_OWN_THROW might be used with exception type and constructor parameters for that exception, including a textual description. Like this:
OUR_OWN_THROW(InvalidOperationException, (L"Uninitialized foo!"));
This macro will of course throw the InvalidOperationException exception with the description as constructor parameter, but it'll also write a message to a log file consisting of the file name and line number where the throw occured and its textual description. The thrown exception will get an id, which also gets logged. If the exception is ever caught somewhere else in the code, it will be marked as such and the log file will then indicate that that specific exception has been handled and that it's therefore not likely the cause of any crash that might be logged later on. Unhandled exceptions can be easily picked up by our automated QA infrastructure.
Code repetition.
Have a look to boost preprocessor library, it's a kind of meta-meta-programming. In topic->motivation you can find a good example.
One common use is for detecting the compile environment, for cross-platform development you can write one set of code for linux, say, and another for windows when no cross platform library already exists for your purposes.
So, in a rough example a cross-platform mutex can have
void lock()
{
#ifdef WIN32
EnterCriticalSection(...)
#endif
#ifdef POSIX
pthread_mutex_lock(...)
#endif
}
For functions, they are useful when you want to explicitly ignore type safety. Such as the many examples above and below for doing ASSERT. Of course, like a lot of C/C++ features you can shoot yourself in the foot, but the language gives you the tools and lets you decide what to do.
I occasionally use macros so I can define information in one place, but use it in different ways in different parts of the code. It's only slightly evil :)
For example, in "field_list.h":
/*
* List of fields, names and values.
*/
FIELD(EXAMPLE1, "first example", 10)
FIELD(EXAMPLE2, "second example", 96)
FIELD(ANOTHER, "more stuff", 32)
...
#undef FIELD
Then for a public enum it can be defined to just use the name:
#define FIELD(name, desc, value) FIELD_ ## name,
typedef field_ {
#include "field_list.h"
FIELD_MAX
} field_en;
And in a private init function, all the fields can be used to populate a table with the data:
#define FIELD(name, desc, value) \
table[FIELD_ ## name].desc = desc; \
table[FIELD_ ## name].value = value;
#include "field_list.h"
Something like
void debugAssert(bool val, const char* file, int lineNumber);
#define assert(x) debugAssert(x,__FILE__,__LINE__);
So that you can just for example have
assert(n == true);
and get the source file name and line number of the problem printed out to your log if n is false.
If you use a normal function call such as
void assert(bool val);
instead of the macro, all you can get is your assert function's line number printed to the log, which would be less useful.
#define ARRAY_SIZE(arr) (sizeof arr / sizeof arr[0])
Unlike the 'preferred' template solution discussed in a current thread, you can use it as a constant expression:
char src[23];
int dest[ARRAY_SIZE(src)];
You can use #defines to help with debugging and unit test scenarios. For example, create special logging variants of the memory functions and create a special memlog_preinclude.h:
#define malloc memlog_malloc
#define calloc memlog calloc
#define free memlog_free
Compile you code using:
gcc -Imemlog_preinclude.h ...
An link in your memlog.o to the final image. You now control malloc, etc, perhaps for logging purposes, or to simulate allocation failures for unit tests.
When you are making a decision at compile time over Compiler/OS/Hardware specific behavior.
It allows you to make your interface to Comppiler/OS/Hardware specific features.
#if defined(MY_OS1) && defined(MY_HARDWARE1)
#define MY_ACTION(a,b,c) doSothing_OS1HW1(a,b,c);}
#elif define(MY_OS1) && defined(MY_HARDWARE2)
#define MY_ACTION(a,b,c) doSomthing_OS1HW2(a,b,c);}
#elif define(MY_SUPER_OS)
/* On this hardware it is a null operation */
#define MY_ACTION(a,b,c)
#else
#error "PLEASE DEFINE MY_ACTION() for this Compiler/OS/HArdware configuration"
#endif
Compilers can refuse your request to inline.
Macros will always have their place.
Something I find useful is #define DEBUG for debug tracing -- you can leave it 1 while debugging a problem (or even leave it on during the whole development cycle) then turn it off when it is time to ship.
You can #define constants on the compiler command line using the -D or /D option. This is often useful when cross-compiling the same software for multiple platforms because you can have your makefiles control what constants are defined for each platform.
In my last job, I was working on a virus scanner. To make thing easier for me to debug, I had lots of logging stuck all over the place, but in a high demand app like that, the expense of a function call is just too expensive. So, I came up with this little Macro, that still allowed me to enable the debug logging on a release version at a customers site, without the cost of a function call would check the debug flag and just return without logging anything, or if enabled, would do the logging... The macro was defined as follows:
#define dbgmsg(_FORMAT, ...) if((debugmsg_flag & 0x00000001) || (debugmsg_flag & 0x80000000)) { log_dbgmsg(_FORMAT, __VA_ARGS__); }
Because of the VA_ARGS in the log functions, this was a good case for a macro like this.
Before that, I used a macro in a high security application that needed to tell the user that they didn't have the correct access, and it would tell them what flag they needed.
The Macro(s) defined as:
#define SECURITY_CHECK(lRequiredSecRoles) if(!DoSecurityCheck(lRequiredSecRoles, #lRequiredSecRoles, true)) return
#define SECURITY_CHECK_QUIET(lRequiredSecRoles) (DoSecurityCheck(lRequiredSecRoles, #lRequiredSecRoles, false))
Then, we could just sprinkle the checks all over the UI, and it would tell you which roles were allowed to perform the action you tried to do, if you didn't already have that role. The reason for two of them was to return a value in some places, and return from a void function in others...
SECURITY_CHECK(ROLE_BUSINESS_INFORMATION_STEWARD | ROLE_WORKER_ADMINISTRATOR);
LRESULT CAddPerson1::OnWizardNext()
{
if(m_Role.GetItemData(m_Role.GetCurSel()) == parent->ROLE_EMPLOYEE) {
SECURITY_CHECK(ROLE_WORKER_ADMINISTRATOR | ROLE_BUSINESS_INFORMATION_STEWARD ) -1;
} else if(m_Role.GetItemData(m_Role.GetCurSel()) == parent->ROLE_CONTINGENT) {
SECURITY_CHECK(ROLE_CONTINGENT_WORKER_ADMINISTRATOR | ROLE_BUSINESS_INFORMATION_STEWARD | ROLE_WORKER_ADMINISTRATOR) -1;
}
...
Anyways, that's how I've used them, and I'm not sure how this could have been helped with templates... Other than that, I try to avoid them, unless REALLY necessary.
I use macros to easily define Exceptions:
DEF_EXCEPTION(RessourceNotFound, "Ressource not found")
where DEF_EXCEPTION is
#define DEF_EXCEPTION(A, B) class A : public exception\
{\
public:\
virtual const char* what() const throw()\
{\
return B;\
};\
}\
If you have a list of fields that get used for a bunch of things, e.g. defining a structure, serializing that structure to/from some binary format, doing database inserts, etc, then you can (recursively!) use the preprocessor to avoid ever repeating your field list.
This is admittedly hideous. But maybe sometimes better than updating a long list of fields in multiple places? I've used this technique exactly once, and it was quite helpful that one time.
Of course the same general idea is used extensively in languages with proper reflection -- just instrospect the class and operate on each field in turn. Doing it in the C preprocessor is fragile, illegible, and not always portable. So I mention it with some trepidation. Nonetheless, here it is...
(EDIT: I see now that this is similar to what #Andrew Johnson said on 9/18; however the idea of recursively including the same file takes the idea a bit further.)
// file foo.h, defines class Foo and various members on it without ever repeating the
// list of fields.
#if defined( FIELD_LIST )
// here's the actual list of fields in the class. If FIELD_LIST is defined, we're at
// the 3rd level of inclusion and somebody wants to actually use the field list. In order
// to do so, they will have defined the macros STRING and INT before including us.
STRING( fooString )
INT( barInt )
#else // defined( FIELD_LIST )
#if !defined(FOO_H)
#define FOO_H
#define DEFINE_STRUCT
// recursively include this same file to define class Foo
#include "foo.h"
#undef DEFINE_STRUCT
#define DEFINE_CLEAR
// recursively include this same file to define method Foo::clear
#include "foo.h"
#undef DEFINE_CLEAR
// etc ... many more interesting examples like serialization
#else // defined(FOO_H)
// from here on, we know that FOO_H was defined, in other words we're at the second level of
// recursive inclusion, and the file is being used to make some particular
// use of the field list, for example defining the class or a single method of it
#if defined( DEFINE_STRUCT )
#define STRING(a) std::string a;
#define INT(a) long a;
class Foo
{
public:
#define FIELD_LIST
// recursively include the same file (for the third time!) to get fields
// This is going to translate into:
// std::string fooString;
// int barInt;
#include "foo.h"
#endif
void clear();
};
#undef STRING
#undef INT
#endif // defined(DEFINE_STRUCT)
#if defined( DEFINE_ZERO )
#define STRING(a) a = "";
#define INT(a) a = 0;
#define FIELD_LIST
void Foo::clear()
{
// recursively include the same file (for the third time!) to get fields.
// This is going to translate into:
// fooString="";
// barInt=0;
#include "foo.h"
#undef STRING
#undef int
}
#endif // defined( DEFINE_ZERO )
// etc...
#endif // end else clause for defined( FOO_H )
#endif // end else clause for defined( FIELD_LIST )
I've used the preprocesser to calculate fixed-point numbers from floating point values used in embedded systems that cannot use floating point in the compiled code. It's handy to have all of your math in Real World Units and not have to think about them in fixed-point.
Example:
// TICKS_PER_UNIT is defined in floating point to allow the conversions to compute during compile-time.
#define TICKS_PER_UNIT 1024.0
// NOTE: The TICKS_PER_x_MS will produce constants in the preprocessor. The (long) cast will
// guarantee there are no floating point values in the embedded code and will produce a warning
// if the constant is larger than the data type being stored to.
// Adding 0.5 sec to the calculation forces rounding instead of truncation.
#define TICKS_PER_1_MS( ms ) (long)( ( ( ms * TICKS_PER_UNIT ) / 1000 ) + 0.5 )
Yet another foreach macros. T: type, c: container, i: iterator
#define foreach(T, c, i) for(T::iterator i=(c).begin(); i!=(c).end(); ++i)
#define foreach_const(T, c, i) for(T::const_iterator i=(c).begin(); i!=(c).end(); ++i)
Usage (concept showing, not real):
void MultiplyEveryElementInList(std::list<int>& ints, int mul)
{
foreach(std::list<int>, ints, i)
(*i) *= mul;
}
int GetSumOfList(const std::list<int>& ints)
{
int ret = 0;
foreach_const(std::list<int>, ints, i)
ret += *i;
return ret;
}
Better implementations available: Google "BOOST_FOREACH"
Good articles available: Conditional Love: FOREACH Redux (Eric Niebler) http://www.artima.com/cppsource/foreach.html
Maybe the greates usage of macros is in platform-independent development.
Think about cases of type inconsistency - with macros, you can simply use different header files -- like:
--WIN_TYPES.H
typedef ...some struct
--POSIX_TYPES.h
typedef ...some another struct
--program.h
#ifdef WIN32
#define TYPES_H "WINTYPES.H"
#else
#define TYPES_H "POSIX_TYPES.H"
#endif
#include TYPES_H
Much readable than implementing it in other ways, to my opinion.
I have, in a .c file, the following (this is a much smaller array to exemplify):
static const char* __someNames[] =
{
"Fox",
"Wulf",
"Cat"
};
Then later I am defining a macro like this:
#define EXAMPLE(N) XXX
where I would like XXX to be replaced by __someNames[N] without the "..i.e I would like
#define EXAMPLE(1)
to expand to:
Wulf
How can I go about this?
EDIT
Some cool ideas. This is more out of intellectual curiosity that I want to do this. I like the idea of the #include "file.h# twice with a redefined macro..what if I can also use the boost preprocessor (I added c++ and boost tags to question) - would I be able to get around having the extra "file.h"? The one with a #define per string is also cool though slightly less idea given how many #defines I would have...
I think the solution to your X problem (where you have asked about the Y solution) is to turn the thing around and use macros to put the string in, and then use the same macro redefined to do whatever else it is you want to do.
For example:
#define EXAMPLE(x) #x,
static const char* __someNames[] =
{
#include "file.h"
};
#undef EXAMPLE
#define EXAMPLE(x) x,
enum animals
{
#include "file.h"
};
where file.h contains:
EXAMPLE(Fox)
EXAMPLE(Wulf)
EXAMPLE(Cat)
There is no way to do this with the standard C or C++ pre-processor.
The pre-processor scans your code for identifiers, and if an identifier matches a macro, it expands the macro. The expanded macro, in this case, would actually be the array name followed by the input number in brackets.
It is a limitation of the way the pre-processor works!
A work-around is like this...
#define EXAMPLE_1 Wolf
#define EXAMPLE_2 Dog
#define EXAMPLE_3 Cat
#define EXAMPLE_X(n) EXAMPLE_##n
// Then say
EXAMPLE_X(1); // will put Wolf;
int EXAMPLE_X(2) = 0;// expands to int Dog = 0;
Unfortunately that only works if you pass in constants like 1 or 2 etc... Variables would not work.
OR ...
static const char* __someNames[] =
{
#define __someNames_1 Fox
"Fox",
#define __someNames_2 Wulf
"Wulf",
#define __someNames_3 Cat
"Cat"
};
#define EXAMPLE_X(n) __someNames_##n
That will work the way you expect and keep the same scheme, but it will put a lot of defines for big arrays!.
#define EXAMPLE(_x) __someNames[(_x)]
For some reason I need to temporarily disable some macros in a header file and the #undef MACRONAME will make the code compile but it will undef the existing macro.
Is there a way of just disabling it?
I should mention that you do not really know the values of the macros and that I'm looking for a cross compiler solution (should work at least in GCC and MSVC).
In MSVC you could use push_macro pragma, GCC supports it for compatibility with Microsoft Windows compilers.
#pragma push_macro("MACRONAME")
#undef MACRONAME
// some actions
#pragma pop_macro("MACRONAME")
Using just the facilities defined by Standard C (C89, C99 or C11), the only 'disable' mechanism is #undef.
The problem is there is no 're-enable' mechanism.
As others have pointed out, if the header file containing the macro definitions is structured so that it does not contain any typedef or enum declarations (these cannot be repeated; function and variable declarations can be repeated), then you could #undef the macro, do what you need without the macro in effect, and then re-include the header, possibly after undefining its protection against reinclusion.
If the macros are not defined in a header, of course, you are stuck until you refactor the code so that they are in a header.
One other trick is available - if the macros are function-like macros and not object-like macros.
#define nonsense(a, b) b /\= a
int (nonsense)(int a, int b)
{
return (a > b) ? a : b;
}
The function nonsense() is defined fine, despite the macro immediately before it. This is because a macro invocation - for a function-like macro - must be immediately followed by an open parenthesis (give or take white space, possibly including comments). In the function definition line, the token after 'nonsense' is a close parenthesis, so it is not an invocation of the nonsense macro.
Had the macro been an argument-less object-like macro, the trick would not work:
#define nonsense min
int (nonsense)(int a, int b)
{
// Think about it - what is the function really called?
return (a > b) ? a : b;
}
This code defines a bogus function that's called min and is nonsensical. And there's no protection from the macro.
This is one of the reasons why the standard is careful to define which namespaces are reserved for 'The Implementation'. The Implementation is allowed to define macros for any purpose it desires or needs, of any type (function-like or object-like) it desires or needs, provided those names are reserved to the implementation. If you as a consumer of the services of The Implementation try to use or define a name reserved to the implementation, you must be aware that your code will probably break sooner or later, and that it will be your fault, not the fault of The Implementation.
Macros make my knees go weak, but wouldn't the most universal solution be to restructure your code so that you wouldn't need to reenable the macro again in the same source file? Wouldn't it be possible to extract some code into a separate function and a separate source file where you can undef the offending macro.
The macros come from some header file, so you should have access to their values. You can then do something like
#include <foo.h> // declares macro FOO
// Do things with FOO
#undef FOO
// do things without FOO
#include <foo.h> // reenable FOO
Your header should then be designed along these lines
#ifndef FOO
#define FOO do_something(x,y)
#endif
EDIT:
You may think that it's that easy:
#ifdef macro
#define DISABLED_macro macro
#undef macro
#endif
// do what you want with macro
#ifdef DISABLED_macro
#define macro DISABLED_macro
#endif
But it's not (like the following example demonstrates)!
#include <iostream>
#include <limits>
#include <windows.h>
#ifdef max
#define DISABLED_max max
#undef max
#endif
int main()
{
std::cout << std::numeric_limits<unsigned long>::max() << std::endl;
#ifdef DISABLED_max
#define max DISABLED_max
#endif
std::cout << max(15,3) << std::endl; // error C3861: "max": identifier not found
return 0;
}
Using #undef on the macro and re-including the original header is also not likely to work, because of the header guards.
So what's left is using the push_macro/pop_macro #pragma directives.
#pragma push_macro("MACRO")
#undef MACRO
// do what you want
#pragma pop_macro("MACRO")
There are specific rules for function-like macroses invokation in C/C++ language.
The function-like macroses have to be invoked in the following way:
Macros-name
Left parethesis
One token for each argument separated by commas
Each token in this list can be separared from another by whitespaces (i.e. actual whitespaces and commas)
With one trick you "disable preprocessor mechanism" with breaking rules for function-like macro invokation, but be still within a rules of function calling mechanism...
#include <iostream>
using namespace std;
inline const char* WHAT(){return "Hello from function";}
#define WHAT() "Hello from macro"
int main()
{
cout << (*WHAT)() << "\n"; // use function
cout << (WHAT)() << "\n"; // use function
cout << WHAT () << "\n"; // use macro
return 0;
}
Closed. This question needs to be more focused. It is not currently accepting answers.
Want to improve this question? Update the question so it focuses on one problem only by editing this post.
Closed 4 years ago.
Improve this question
The C preprocessor is justifiably feared and shunned by the C++ community. In-lined functions, consts and templates are usually a safer and superior alternative to a #define.
The following macro:
#define SUCCEEDED(hr) ((HRESULT)(hr) >= 0)
is in no way superior to the type safe:
inline bool succeeded(int hr) { return hr >= 0; }
But macros do have their place, please list the uses you find for macros that you can't do without the preprocessor.
Please put each use-cases in a seperate answer so it can be voted up and if you know of how to achieve one of the answers without the preprosessor point out how in that answer's comments.
As wrappers for debug functions, to automatically pass things like __FILE__, __LINE__, etc:
#ifdef ( DEBUG )
#define M_DebugLog( msg ) std::cout << __FILE__ << ":" << __LINE__ << ": " << msg
#else
#define M_DebugLog( msg )
#endif
Since C++20 the magic type std::source_location can however be used instead of __LINE__ and __FILE__ to implement an analogue as a normal function (template).
Methods must always be complete, compilable code; macros may be code fragments. Thus you can define a foreach macro:
#define foreach(list, index) for(index = 0; index < list.size(); index++)
And use it as thus:
foreach(cookies, i)
printf("Cookie: %s", cookies[i]);
Since C++11, this is superseded by the range-based for loop.
Header file guards necessitate macros.
Are there any other areas that necessitate macros? Not many (if any).
Are there any other situations that benefit from macros? YES!!!
One place I use macros is with very repetitive code. For example, when wrapping C++ code to be used with other interfaces (.NET, COM, Python, etc...), I need to catch different types of exceptions. Here's how I do that:
#define HANDLE_EXCEPTIONS \
catch (::mylib::exception& e) { \
throw gcnew MyDotNetLib::Exception(e); \
} \
catch (::std::exception& e) { \
throw gcnew MyDotNetLib::Exception(e, __LINE__, __FILE__); \
} \
catch (...) { \
throw gcnew MyDotNetLib::UnknownException(__LINE__, __FILE__); \
}
I have to put these catches in every wrapper function. Rather than type out the full catch blocks each time, I just type:
void Foo()
{
try {
::mylib::Foo()
}
HANDLE_EXCEPTIONS
}
This also makes maintenance easier. If I ever have to add a new exception type, there's only one place I need to add it.
There are other useful examples too: many of which include the __FILE__ and __LINE__ preprocessor macros.
Anyway, macros are very useful when used correctly. Macros are not evil -- their misuse is evil.
Mostly:
Include guards
Conditional compilation
Reporting (predefined macros like __LINE__ and __FILE__)
(rarely) Duplicating repetitive code patterns.
In your competitor's code.
Inside conditional compilation, to overcome issues of differences between compilers:
#ifdef WE_ARE_ON_WIN32
#define close(parm1) _close (parm1)
#define rmdir(parm1) _rmdir (parm1)
#define mkdir(parm1, parm2) _mkdir (parm1)
#define access(parm1, parm2) _access(parm1, parm2)
#define create(parm1, parm2) _creat (parm1, parm2)
#define unlink(parm1) _unlink(parm1)
#endif
When you want to make a string out of an expression, the best example for this is assert (#x turns the value of x to a string).
#define ASSERT_THROW(condition) \
if (!(condition)) \
throw std::exception(#condition " is false");
String constants are sometimes better defined as macros since you can do more with string literals than with a const char *.
e.g. String literals can be easily concatenated.
#define BASE_HKEY "Software\\Microsoft\\Internet Explorer\\"
// Now we can concat with other literals
RegOpenKey(HKEY_CURRENT_USER, BASE_HKEY "Settings", &settings);
RegOpenKey(HKEY_CURRENT_USER, BASE_HKEY "TypedURLs", &URLs);
If a const char * were used then some sort of string class would have to be used to perform the concatenation at runtime:
const char* BaseHkey = "Software\\Microsoft\\Internet Explorer\\";
RegOpenKey(HKEY_CURRENT_USER, (string(BaseHkey) + "Settings").c_str(), &settings);
RegOpenKey(HKEY_CURRENT_USER, (string(BaseHkey) + "TypedURLs").c_str(), &URLs);
Since C++20 it is however possible to implement a string-like class type that can be used as a non-type template parameter type of a user-defined string literal operator which allows such concatenation operations at compile-time without macros.
When you want to change the program flow (return, break and continue) code in a function behaves differently than code that is actually inlined in the function.
#define ASSERT_RETURN(condition, ret_val) \
if (!(condition)) { \
assert(false && #condition); \
return ret_val; }
// should really be in a do { } while(false) but that's another discussion.
The obvious include guards
#ifndef MYHEADER_H
#define MYHEADER_H
...
#endif
Let's say we'll ignore obvious things like header guards.
Sometimes, you want to generate code that needs to be copy/pasted by the precompiler:
#define RAISE_ERROR_STL(p_strMessage) \
do \
{ \
try \
{ \
std::tstringstream strBuffer ; \
strBuffer << p_strMessage ; \
strMessage = strBuffer.str() ; \
raiseSomeAlert(__FILE__, __FUNCSIG__, __LINE__, strBuffer.str().c_str()) \
} \
catch(...){} \
{ \
} \
} \
while(false)
which enables you to code this:
RAISE_ERROR_STL("Hello... The following values " << i << " and " << j << " are wrong") ;
And can generate messages like:
Error Raised:
====================================
File : MyFile.cpp, line 225
Function : MyFunction(int, double)
Message : "Hello... The following values 23 and 12 are wrong"
Note that mixing templates with macros can lead to even better results (i.e. automatically generating the values side-by-side with their variable names)
Other times, you need the __FILE__ and/or the __LINE__ of some code, to generate debug info, for example. The following is a classic for Visual C++:
#define WRNG_PRIVATE_STR2(z) #z
#define WRNG_PRIVATE_STR1(x) WRNG_PRIVATE_STR2(x)
#define WRNG __FILE__ "("WRNG_PRIVATE_STR1(__LINE__)") : ------------ : "
As with the following code:
#pragma message(WRNG "Hello World")
it generates messages like:
C:\my_project\my_cpp_file.cpp (225) : ------------ Hello World
Other times, you need to generate code using the # and ## concatenation operators, like generating getters and setters for a property (this is for quite a limited cases, through).
Other times, you will generate code than won't compile if used through a function, like:
#define MY_TRY try{
#define MY_CATCH } catch(...) {
#define MY_END_TRY }
Which can be used as
MY_TRY
doSomethingDangerous() ;
MY_CATCH
tryToRecoverEvenWithoutMeaningfullInfo() ;
damnThoseMacros() ;
MY_END_TRY
(still, I only saw this kind of code rightly used once)
Last, but not least, the famous boost::foreach !!!
#include <string>
#include <iostream>
#include <boost/foreach.hpp>
int main()
{
std::string hello( "Hello, world!" );
BOOST_FOREACH( char ch, hello )
{
std::cout << ch;
}
return 0;
}
(Note: code copy/pasted from the boost homepage)
Which is (IMHO) way better than std::for_each.
So, macros are always useful because they are outside the normal compiler rules. But I find that most the time I see one, they are effectively remains of C code never translated into proper C++.
Unit test frameworks for C++ like UnitTest++ pretty much revolve around preprocessor macros. A few lines of unit test code expand into a hierarchy of classes that wouldn't be fun at all to type manually. Without something like UnitTest++ and it's preprocessor magic, I don't know how you'd efficiently write unit tests for C++.
You can't perform short-circuiting of function call arguments using a regular function call. For example:
#define andm(a, b) (a) && (b)
bool andf(bool a, bool b) { return a && b; }
andm(x, y) // short circuits the operator so if x is false, y would not be evaluated
andf(x, y) // y will always be evaluated
To fear the C preprocessor is like to fear the incandescent bulbs just because we get fluorescent bulbs. Yes, the former can be {electricity | programmer time} inefficient. Yes, you can get (literally) burned by them. But they can get the job done if you properly handle it.
When you program embedded systems, C uses to be the only option apart form assembler. After programming on desktop with C++ and then switching to smaller, embedded targets, you learn to stop worrying about “inelegancies” of so many bare C features (macros included) and just trying to figure out the best and safe usage you can get from those features.
Alexander Stepanov says:
When we program in C++ we should not be ashamed of its C heritage, but make
full use of it. The only problems with C++, and even the only problems with C, arise
when they themselves are not consistent with their own logic.
Some very advanced and useful stuff can still be built using preprocessor (macros), which you would never be able to do using the c++ "language constructs" including templates.
Examples:
Making something both a C identifier and a string
Easy way to use variables of enum types as string in C
Boost Preprocessor Metaprogramming
We use the __FILE__ and __LINE__ macros for diagnostic purposes in information rich exception throwing, catching and logging, together with automated log file scanners in our QA infrastructure.
For instance, a throwing macro OUR_OWN_THROW might be used with exception type and constructor parameters for that exception, including a textual description. Like this:
OUR_OWN_THROW(InvalidOperationException, (L"Uninitialized foo!"));
This macro will of course throw the InvalidOperationException exception with the description as constructor parameter, but it'll also write a message to a log file consisting of the file name and line number where the throw occured and its textual description. The thrown exception will get an id, which also gets logged. If the exception is ever caught somewhere else in the code, it will be marked as such and the log file will then indicate that that specific exception has been handled and that it's therefore not likely the cause of any crash that might be logged later on. Unhandled exceptions can be easily picked up by our automated QA infrastructure.
Code repetition.
Have a look to boost preprocessor library, it's a kind of meta-meta-programming. In topic->motivation you can find a good example.
One common use is for detecting the compile environment, for cross-platform development you can write one set of code for linux, say, and another for windows when no cross platform library already exists for your purposes.
So, in a rough example a cross-platform mutex can have
void lock()
{
#ifdef WIN32
EnterCriticalSection(...)
#endif
#ifdef POSIX
pthread_mutex_lock(...)
#endif
}
For functions, they are useful when you want to explicitly ignore type safety. Such as the many examples above and below for doing ASSERT. Of course, like a lot of C/C++ features you can shoot yourself in the foot, but the language gives you the tools and lets you decide what to do.
I occasionally use macros so I can define information in one place, but use it in different ways in different parts of the code. It's only slightly evil :)
For example, in "field_list.h":
/*
* List of fields, names and values.
*/
FIELD(EXAMPLE1, "first example", 10)
FIELD(EXAMPLE2, "second example", 96)
FIELD(ANOTHER, "more stuff", 32)
...
#undef FIELD
Then for a public enum it can be defined to just use the name:
#define FIELD(name, desc, value) FIELD_ ## name,
typedef field_ {
#include "field_list.h"
FIELD_MAX
} field_en;
And in a private init function, all the fields can be used to populate a table with the data:
#define FIELD(name, desc, value) \
table[FIELD_ ## name].desc = desc; \
table[FIELD_ ## name].value = value;
#include "field_list.h"
Something like
void debugAssert(bool val, const char* file, int lineNumber);
#define assert(x) debugAssert(x,__FILE__,__LINE__);
So that you can just for example have
assert(n == true);
and get the source file name and line number of the problem printed out to your log if n is false.
If you use a normal function call such as
void assert(bool val);
instead of the macro, all you can get is your assert function's line number printed to the log, which would be less useful.
#define ARRAY_SIZE(arr) (sizeof arr / sizeof arr[0])
Unlike the 'preferred' template solution discussed in a current thread, you can use it as a constant expression:
char src[23];
int dest[ARRAY_SIZE(src)];
You can use #defines to help with debugging and unit test scenarios. For example, create special logging variants of the memory functions and create a special memlog_preinclude.h:
#define malloc memlog_malloc
#define calloc memlog calloc
#define free memlog_free
Compile you code using:
gcc -Imemlog_preinclude.h ...
An link in your memlog.o to the final image. You now control malloc, etc, perhaps for logging purposes, or to simulate allocation failures for unit tests.
When you are making a decision at compile time over Compiler/OS/Hardware specific behavior.
It allows you to make your interface to Comppiler/OS/Hardware specific features.
#if defined(MY_OS1) && defined(MY_HARDWARE1)
#define MY_ACTION(a,b,c) doSothing_OS1HW1(a,b,c);}
#elif define(MY_OS1) && defined(MY_HARDWARE2)
#define MY_ACTION(a,b,c) doSomthing_OS1HW2(a,b,c);}
#elif define(MY_SUPER_OS)
/* On this hardware it is a null operation */
#define MY_ACTION(a,b,c)
#else
#error "PLEASE DEFINE MY_ACTION() for this Compiler/OS/HArdware configuration"
#endif
Compilers can refuse your request to inline.
Macros will always have their place.
Something I find useful is #define DEBUG for debug tracing -- you can leave it 1 while debugging a problem (or even leave it on during the whole development cycle) then turn it off when it is time to ship.
You can #define constants on the compiler command line using the -D or /D option. This is often useful when cross-compiling the same software for multiple platforms because you can have your makefiles control what constants are defined for each platform.
In my last job, I was working on a virus scanner. To make thing easier for me to debug, I had lots of logging stuck all over the place, but in a high demand app like that, the expense of a function call is just too expensive. So, I came up with this little Macro, that still allowed me to enable the debug logging on a release version at a customers site, without the cost of a function call would check the debug flag and just return without logging anything, or if enabled, would do the logging... The macro was defined as follows:
#define dbgmsg(_FORMAT, ...) if((debugmsg_flag & 0x00000001) || (debugmsg_flag & 0x80000000)) { log_dbgmsg(_FORMAT, __VA_ARGS__); }
Because of the VA_ARGS in the log functions, this was a good case for a macro like this.
Before that, I used a macro in a high security application that needed to tell the user that they didn't have the correct access, and it would tell them what flag they needed.
The Macro(s) defined as:
#define SECURITY_CHECK(lRequiredSecRoles) if(!DoSecurityCheck(lRequiredSecRoles, #lRequiredSecRoles, true)) return
#define SECURITY_CHECK_QUIET(lRequiredSecRoles) (DoSecurityCheck(lRequiredSecRoles, #lRequiredSecRoles, false))
Then, we could just sprinkle the checks all over the UI, and it would tell you which roles were allowed to perform the action you tried to do, if you didn't already have that role. The reason for two of them was to return a value in some places, and return from a void function in others...
SECURITY_CHECK(ROLE_BUSINESS_INFORMATION_STEWARD | ROLE_WORKER_ADMINISTRATOR);
LRESULT CAddPerson1::OnWizardNext()
{
if(m_Role.GetItemData(m_Role.GetCurSel()) == parent->ROLE_EMPLOYEE) {
SECURITY_CHECK(ROLE_WORKER_ADMINISTRATOR | ROLE_BUSINESS_INFORMATION_STEWARD ) -1;
} else if(m_Role.GetItemData(m_Role.GetCurSel()) == parent->ROLE_CONTINGENT) {
SECURITY_CHECK(ROLE_CONTINGENT_WORKER_ADMINISTRATOR | ROLE_BUSINESS_INFORMATION_STEWARD | ROLE_WORKER_ADMINISTRATOR) -1;
}
...
Anyways, that's how I've used them, and I'm not sure how this could have been helped with templates... Other than that, I try to avoid them, unless REALLY necessary.
I use macros to easily define Exceptions:
DEF_EXCEPTION(RessourceNotFound, "Ressource not found")
where DEF_EXCEPTION is
#define DEF_EXCEPTION(A, B) class A : public exception\
{\
public:\
virtual const char* what() const throw()\
{\
return B;\
};\
}\
If you have a list of fields that get used for a bunch of things, e.g. defining a structure, serializing that structure to/from some binary format, doing database inserts, etc, then you can (recursively!) use the preprocessor to avoid ever repeating your field list.
This is admittedly hideous. But maybe sometimes better than updating a long list of fields in multiple places? I've used this technique exactly once, and it was quite helpful that one time.
Of course the same general idea is used extensively in languages with proper reflection -- just instrospect the class and operate on each field in turn. Doing it in the C preprocessor is fragile, illegible, and not always portable. So I mention it with some trepidation. Nonetheless, here it is...
(EDIT: I see now that this is similar to what #Andrew Johnson said on 9/18; however the idea of recursively including the same file takes the idea a bit further.)
// file foo.h, defines class Foo and various members on it without ever repeating the
// list of fields.
#if defined( FIELD_LIST )
// here's the actual list of fields in the class. If FIELD_LIST is defined, we're at
// the 3rd level of inclusion and somebody wants to actually use the field list. In order
// to do so, they will have defined the macros STRING and INT before including us.
STRING( fooString )
INT( barInt )
#else // defined( FIELD_LIST )
#if !defined(FOO_H)
#define FOO_H
#define DEFINE_STRUCT
// recursively include this same file to define class Foo
#include "foo.h"
#undef DEFINE_STRUCT
#define DEFINE_CLEAR
// recursively include this same file to define method Foo::clear
#include "foo.h"
#undef DEFINE_CLEAR
// etc ... many more interesting examples like serialization
#else // defined(FOO_H)
// from here on, we know that FOO_H was defined, in other words we're at the second level of
// recursive inclusion, and the file is being used to make some particular
// use of the field list, for example defining the class or a single method of it
#if defined( DEFINE_STRUCT )
#define STRING(a) std::string a;
#define INT(a) long a;
class Foo
{
public:
#define FIELD_LIST
// recursively include the same file (for the third time!) to get fields
// This is going to translate into:
// std::string fooString;
// int barInt;
#include "foo.h"
#endif
void clear();
};
#undef STRING
#undef INT
#endif // defined(DEFINE_STRUCT)
#if defined( DEFINE_ZERO )
#define STRING(a) a = "";
#define INT(a) a = 0;
#define FIELD_LIST
void Foo::clear()
{
// recursively include the same file (for the third time!) to get fields.
// This is going to translate into:
// fooString="";
// barInt=0;
#include "foo.h"
#undef STRING
#undef int
}
#endif // defined( DEFINE_ZERO )
// etc...
#endif // end else clause for defined( FOO_H )
#endif // end else clause for defined( FIELD_LIST )
I've used the preprocesser to calculate fixed-point numbers from floating point values used in embedded systems that cannot use floating point in the compiled code. It's handy to have all of your math in Real World Units and not have to think about them in fixed-point.
Example:
// TICKS_PER_UNIT is defined in floating point to allow the conversions to compute during compile-time.
#define TICKS_PER_UNIT 1024.0
// NOTE: The TICKS_PER_x_MS will produce constants in the preprocessor. The (long) cast will
// guarantee there are no floating point values in the embedded code and will produce a warning
// if the constant is larger than the data type being stored to.
// Adding 0.5 sec to the calculation forces rounding instead of truncation.
#define TICKS_PER_1_MS( ms ) (long)( ( ( ms * TICKS_PER_UNIT ) / 1000 ) + 0.5 )
Yet another foreach macros. T: type, c: container, i: iterator
#define foreach(T, c, i) for(T::iterator i=(c).begin(); i!=(c).end(); ++i)
#define foreach_const(T, c, i) for(T::const_iterator i=(c).begin(); i!=(c).end(); ++i)
Usage (concept showing, not real):
void MultiplyEveryElementInList(std::list<int>& ints, int mul)
{
foreach(std::list<int>, ints, i)
(*i) *= mul;
}
int GetSumOfList(const std::list<int>& ints)
{
int ret = 0;
foreach_const(std::list<int>, ints, i)
ret += *i;
return ret;
}
Better implementations available: Google "BOOST_FOREACH"
Good articles available: Conditional Love: FOREACH Redux (Eric Niebler) http://www.artima.com/cppsource/foreach.html
Maybe the greates usage of macros is in platform-independent development.
Think about cases of type inconsistency - with macros, you can simply use different header files -- like:
--WIN_TYPES.H
typedef ...some struct
--POSIX_TYPES.h
typedef ...some another struct
--program.h
#ifdef WIN32
#define TYPES_H "WINTYPES.H"
#else
#define TYPES_H "POSIX_TYPES.H"
#endif
#include TYPES_H
Much readable than implementing it in other ways, to my opinion.