How do I make a C macro behave like a function

Question

Let s say that for some reason you need to write a macro  MACRO X Y     Let s assume there s a good reason you can t use an inline function    You want this macro to emulate a call to a function with no return value     Example 1  This should work as expected   if  x  gt  y    MACRO x  y   do something      Example 2  This should not result in a compiler error   if  x  gt  y    MACRO x  y   else   MACRO y - x  x - y     Example 3  This should not compile   do something    MACRO x  y  do something        The na  ve way to write the macro is like this    define MACRO X Y                          cout  lt  lt   1st arg is    lt  lt   X   lt  lt  endl       cout  lt  lt   2nd arg is    lt  lt   Y   lt  lt  endl       cout  lt  lt   Sum is    lt  lt    X   Y    lt  lt  endl    This is a very bad solution which fails all three examples  and I shouldn t need to explain why   Ignore what the macro actually does  that s not the point     Now  the way I most often see macros written is to enclose them in curly braces  like this    define MACRO X Y                                                                           cout  lt  lt   1st arg is    lt  lt   X   lt  lt  endl         cout  lt  lt   2nd arg is    lt  lt   Y   lt  lt  endl         cout  lt  lt   Sum is    lt  lt    X   Y    lt  lt  endl         This solves example 1  because the macro is in one statement block   But example 2 is broken because we put a semicolon after the call to the macro   This makes the compiler think the semicolon is a statement by itself  which means the else statement doesn t correspond to any if statement   And lastly  example 3 compiles OK  even though there is no semicolon  because a code block doesn t need a semicolon     Is there a way to write a macro so that it pass all three examples     Note  I am submitting my own answer as part of the accepted way of sharing a tip  but if anyone has a better solution feel free to post it here  it may get more votes than my method

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C  11 brought us lambdas  which can be incredibly useful in this situation    define MACRO X Y                                       amp   x   y                                             cout  lt  lt   1st arg is    lt  lt  x   lt  lt  endl               cout  lt  lt   2nd arg is    lt  lt  y   lt  lt  endl               cout  lt  lt   Sum is    lt  lt   x    y    lt  lt  endl           X    Y     You keep the generative power of macros  but have a comfy scope from which you can return whatever you want  including void   Additionally  the issue of evaluating macro parameters multiple times is avoided

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Here is an answer coming right from the libc6  Taking a look at  usr include x86 64-linux-gnu bits byteswap h  I found the trick you were looking for   A few critics of previous solutions    Kip s solution does not permit evaluating to an expression  which is in the end often needed  coppro s solution does not permit assigning a variable as the expressions are separate  but can evaluate to an expression  Steve Jessop s solution uses the C  11 auto keyword  that s fine  but feel free to use the known expected type instead    The trick is to use both the  expr expr  construct and a    scope    define MACRO X Y                        register int   x   static cast lt int gt  X     y   static cast lt int gt  Y           std  cout  lt  lt   1st arg is    lt  lt    x  lt  lt  std  endl          std  cout  lt  lt   2nd arg is    lt  lt    y  lt  lt  std  endl          std  cout  lt  lt   Sum is    lt  lt     x     y   lt  lt  std  endl            x     y                  Note the use of the register keyword  it s only a hint to the compiler  The X and Y macro parameters are  already  surrounded in parenthesis and casted to an expected type  This solution works properly with pre- and post-increment as parameters are evaluated only once   For the example purpose  even though not requested  I added the   x     y  statement  which is the way to make the whole bloc to be evaluated as that precise expression   It s safer to use void    if you want to make sure the macro won t evaluate to an expression  thus being illegal where an rvalue is expected   However  the solution is not ISO C   compliant as will complain g   -pedantic   warning  ISO C   forbids braced-groups within expressions  -pedantic    In order to give some rest to g    use    extension   OLD WHOLE MACRO CONTENT HERE  so that the new definition reads    define MACRO X Y         extension                     register int   x   static cast lt int gt  X     y   static cast lt int gt  Y           std  cout  lt  lt   1st arg is    lt  lt    x  lt  lt  std  endl          std  cout  lt  lt   2nd arg is    lt  lt    y  lt  lt  std  endl          std  cout  lt  lt   Sum is    lt  lt     x     y   lt  lt  std  endl            x     y                   In order to improve my solution even a bit more  let s use the   typeof   keyword  as seen in MIN and MAX in C    define MACRO X Y         extension                       typeof   X    x    X             typeof   Y    y    Y           std  cout  lt  lt   1st arg is    lt  lt    x  lt  lt  std  endl          std  cout  lt  lt   2nd arg is    lt  lt    y  lt  lt  std  endl          std  cout  lt  lt   Sum is    lt  lt     x     y   lt  lt  std  endl            x     y                   Now the compiler will determine the appropriate type  This too is a gcc extension   Note the removal of the register keyword  as it would the following warning when used with a class type   warning  address requested for      x     which is declared    register     -Wextra

User · Answer

There is a rather clever solution    define MACRO X Y                            do                                             cout  lt  lt   1st arg is    lt  lt   X   lt  lt  endl         cout  lt  lt   2nd arg is    lt  lt   Y   lt  lt  endl         cout  lt  lt   Sum is    lt  lt    X   Y    lt  lt  endl       while  0    Now you have a single block-level statement  which must be followed by a semicolon   This behaves as expected and desired in all three examples

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Your answer suffers from the multiple-evaluation problem  so  eg   macro  read int file1   read int file2       will do something unexpected and probably unwanted

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I know you said  ignore what the macro does   but people will find this question by searching based on the title  so I think discussion of further techniques to emulate functions with macros are warranted   Closest I know of is    define MACRO X Y    do         auto MACRO tmp 1    X         auto MACRO tmp 2    Y         using std  cout        using std  endl        cout  lt  lt   1st arg is    lt  lt   MACRO tmp 1   lt  lt  endl           cout  lt  lt   2nd arg is    lt  lt   MACRO tmp 2   lt  lt  endl           cout  lt  lt   Sum is    lt  lt   MACRO tmp 1   MACRO tmp 2   lt  lt  endl      while 0    This does the following    Works correctly in each of the stated contexts  Evaluates each of its arguments exactly once  which is a guaranteed feature of a function call  assuming in both cases no exceptions in any of those expressions   Acts on any types  by use of  auto  from C  0x  This is not yet standard C    but there s no other way to get the tmp variables necessitated by the single-evaluation rule  Doesn t require the caller to have imported names from namespace std  which the original macro does  but a function would not    However  it still differs from a function in that    In some invalid uses it may give different compiler errors or warnings  It goes wrong if X or Y contain uses of  MACRO tmp 1  or  MACRO tmp 2  from the surrounding scope  Related to the namespace std thing  a function uses its own lexical context to look up names  whereas a macro uses the context of its call site  There s no way to write a macro that behaves like a function in this respect  It can t be used as the return expression of a void function  which a void expression  such as the comma solution  can  This is even more of an issue when the desired return type is not void  especially when used as an lvalue  But the comma solution can t include using declarations  because they re statements  so pick one or use the           GNU extension

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As others have mentioned  you should avoid macros whenever possible   They are dangerous in the presence of side effects if the macro arguments are evaluated more than once   If you know the type of the arguments  or can use C  0x auto feature   you could use temporaries to enforce single evaluation   Another problem  the order in which multiple evaluations happen may not be what you expect   Consider this code    include  lt iostream gt  using namespace std   int foo  int  amp  i     return i    10    int bar  int  amp  i     return i    100      define BADMACRO  X  Y   do         cout  lt  lt   X    lt  lt   X   lt  lt     Y    lt  lt   Y   lt  lt     X Y    lt  lt    X   Y    lt  lt  endl          while  0    define MACRO  X  Y   do         int x   X  int y   Y        cout  lt  lt   X    lt  lt  x  lt  lt     Y    lt  lt  y  lt  lt     X Y    lt  lt    x   y    lt  lt  endl          while  0   int main         int a   1  int b   1      BADMACRO  foo a   bar b         a   1  b   1      MACRO  foo a   bar b         return 0      And it s output as compiled and run on my machine    X 100  Y 10000  X Y 110 X 10  Y 100  X Y 110

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Macros should generally be avoided  prefer inline functions to them at all times  Any compiler worth its salt should be capable of inlining a small function as if it were a macro  and an inline function will respect namespaces and other scopes  as well as evaluating all the arguments once   If it must be a macro  a while loop  already suggested  will work  or you can try the comma operator    define MACRO X Y            cout  lt  lt   1st arg is    lt  lt   X   lt  lt  endl        cout  lt  lt   2nd arg is    lt  lt   Y   lt  lt  endl        cout  lt  lt   3rd arg is    lt  lt    X     Y    lt  lt  endl        void 0        The  void 0 causes the statement to evaluate to one of void type  and the use of commas rather than semicolons allows it to be used inside a statement  rather than only as a standalone  I would still recommend an inline function for a host of reasons  the least of which being scope and the fact that MACRO a    b    will increment a and b twice

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If you re willing to adopt the practice of always using curly braces in your if statements   Your macro would simply be missing the last semicolon    define MACRO X Y                          cout  lt  lt   1st arg is    lt  lt   X   lt  lt  endl       cout  lt  lt   2nd arg is    lt  lt   Y   lt  lt  endl       cout  lt  lt   Sum is    lt  lt    X   Y    lt  lt  endl   Example 1   compiles   if  x  gt  y        MACRO x  y     do something      Example 2   compiles   if  x  gt  y        MACRO x  y     else       MACRO y - x  x - y       Example 3   doesn t compile   do something    MACRO x  y  do something

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Create a block using    define MACRO      do         while false    Do not add a   after the while false

[c++] How do I make a C++ macro behave like a function?

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