Let's say I have already defined 9 macros from
ABC_1 to ABC_9
If there is another macro XYZ(num) whose objective is to call one of the ABC_{i} based on the value of num, what is a good way to do this? i.e. XYZ(num) should call/return ABC_num.
This is what the concatenation operator ## is for:
#define XYZ(num) ABC_ ## num
Arguments to macros that use concatenation (and are used with the operator) are evaluated differently, however (they aren't evaluated before being used with ##, to allow name-pasting, only in the rescan pass), so if the number is stored in a second macro (or the result of any kind of expansion, rather than a plain literal) you'll need another layer of evaluation:
#define XYZ(num) XYZ_(num)
#define XYZ_(num) ABC_ ## num
In the comments you say that num should be a variable, not a constant. The preprocessor builds compile-time expressions, not dynamic ones, so a macro isn't really going to be very useful here.
If you really wanted XYZ to have a macro definition, you could use something like this:
#define XYZ(num) ((int[]){ \
0, ABC_1, ABC_2, ABC_3, ABC_4, ABC_5, ABC_6, ABC_7, ABC_8, ABC_9 \
}[num])
Assuming ABC_{i} are defined as int values (at any rate they must all be the same type - this applies to any method of dynamically selecting one of them), this selects one with a dynamic num by building a temporary array and selecting from it.
This has no obvious advantages over a completely non-macro solution, though. (Even if you wanted to use macro metaprogramming to generate the list of names, you could still do that in a function or array definition.)
Yes, that's possible, using concatenation. For example:
#define FOO(x, y) BAR ##x(y)
#define BAR1(y) "hello " #y
#define BAR2(y) int y()
#define BAR3(y) return y
FOO(2, main)
{
puts(FOO(1, world));
FOO(3, 0);
}
This becomes:
int main()
{
puts("hello " "world");
return 0;
}
Related
There were several questions here regarding variadic macros in C. These include:
How to make a variadic macro (variable number of arguments) which explains the basics, e.g., passing a variable number of arguments to functions such as printf
Is it possible to iterate over arguments in variadic macros?, which explains how to iteratively apply a macro to each of the arguments of the variadic macro.
https://github.com/swansontec/map-macro which explains how to do so on pairs
My question is related to the iteration technique. I am interested in a macro with this generalized semantics.
ITERATE(Before, Action, Between, After, Empty, ...)
that will place Before prior to all expansions, apply Action to each argument, place Between between every two consecutive applications, and will finally place the expansion of After. Moreover, if the number of argument With such a macro, it should be possible to write
// Loop elements
#define A(x) (x)
#define Befor (
#define After )
#define Between ||
#define Empty 1
// Define an OR macro
#define OR(...) ITERATE(Before, A, Between, Empty, __VA_ARGS__)
// Use it
OR() // Expands to 1
OR(a) // Expands to ((a))
OR(a,b) // Expands to ((a)||(b))
OR(a,b,c) // Expands to to ((a)||(b)||(c))
The purpose of course is not to write an OR function. A generalized functionality could be for more intricate applications. E.g., a macro for defining classes and functions, something to print the trace, etc.
I never liked the recursive REPEAT() macro idiom - it generates horrible hour long read error messages that are.. recursive, so you don't know where the error is and it's also hard to grasp how the OBSTRUCT(REPEAT_INDIRECT) () stuff works. Overall, overloading the macro on number of arguments and using an external tool (shell script or m4 preprocessor) to generate C source code is waay easier, easier to read, maintain and fix and also you can expand the macros on the tools side removing the burden of recursive expansion on C side. With that in mind, your ITERATE can be generated with existing preprocessor libraries, P99_FOR or BOOST_FOREACH comes to mind.
Also, typing shift all the time is strange - I prefer snake case. Here's a reduced example without Before and After macros with overloading the macro on number of arguments:
#define _in_ITERATE_0(f,b,e) e()
#define _in_ITERATE_1(f,b,e,_1) f(_1)
#define _in_ITERATE_2(f,b,e,_1,...) f(_1)b()_in_ITERATE_1(f,b,e,__VA_ARGS__)
#define _in_ITERATE_3(f,b,e,_1,...) f(_1)b()_in_ITERATE_2(f,b,e,__VA_ARGS__)
// or you could expand it instead of reusing previous one with same result:
#define _in_ITERATE_4(f,b,e,_1,_2,_3,_4) f(_1)b()f(_2)b()f(_3)b()f(_4)
// etc.... generate
#define _in_ITERATE_N(_0,_1,_2,_3,_4,_5,_6,_7,_8,_9,N,...) _in_ITERATE_##N
#define ITERATE(func, between, empty, ...) \
_in_ITERATE_N(0,##__VA_ARGS__,9,8,7,6,5,4,3,2,1,0)(func, between, empty, ##__VA_ARGS__)
#define _in_OR_OP(x) (x)
#define _in_OR_EMPTY() 1
#define _in_OR_BETWEEN() ||
#define OR(...) (ITERATE(_in_OR_OP, _in_OR_BETWEEN, _in_OR_EMPTY, ##__VA_ARGS__))
// Use it
OR() // Expands to (1)
OR(a) // Expands to ((a))
OR(a,b) // Expands to ((a)||(b))
OR(a,b,c) // Expands to to ((a)||(b)||(c))
outputs:
(1)
((a))
((a)||(b))
((a)||(b)||(c))
For more examples on overloading macro on count of arguments see this thread. I am using ## GNU extension to remove the comma before __VA_ARGS__ because I am used to using it - I think __VA_OPT__(,) should be nowadays preferred, I am not sure.
I want to create parameterized preprocessor macro for IBM Metal C Prolog.
The initial unparametrized form is
#pragma prolog(Foo, " #MCPROLG MAIN=(YES,16,132)")
The real prolog is more complex, but for the sake of the question the important part is that there are values within a string.
#pragma directive itself can't be part of a macro, so I replace that with _Pragmalike that:
_Pragma("prolog(Foo, \" #MCPROLG MAIN=(YES,16,132)\")")
I can parameterize Foo like this:
#define STR(...) #__VA_ARGS__
#define PROLOG(function) _Pragma(STR(prolog( function , " #MCPROLG MAIN=(YES,16,132)")))
How can I create macro that parameterizes the value 16?
It appears that I need to concatenate strings within preprocessor, I've tried following approaches. All use this stringization macro:
#define STR(...) #__VA_ARGS__
Token that is replaced with 16 (let's name it size) can't be within a string itself so that it is replaced.
#define PROLOG(function, size) _Pragma(STR(prolog( function , " #MCPROLG MAIN=(YES, size ,132)")))
_Pragma only accepts a single string, so I can't peruse C string concatenation like this:
#define PROLOG(function, size) _Pragma(STR(prolog( function , " #MCPROLG MAIN=(YES," #size ",132)")))
I can't stringize the whole second argument of prolog like this:
#define PROLOG(function, size) _Pragma(STR(prolog( function , STR( #MCPROLG MAIN=(YES, size ,132)))))
because #MCPROLG needs to stay within a string so that its # is not treated as stringization token.
To address issue 1 (need to expand a parameter in the replacement list), you need an indirect stringify macro:
#define STR(X) STR_I(X)
#define STR_I(X) #X
You may make this variadic if you want but it's not necessary here (the commas in your string are surrounded by parentheses; the preprocessor will match those; e.g., FOO(A=(B,C,D)), given FOO is a function-like macro, has one argument).
To address issue 2, yes, you need to stringify the whole thing. Effectively there's no such thing as string literal concatenation to the preprocessor (because it runs in translation phase 4, and string literal concatenation doesn't happen until translation phase 6).
To address issue 3, just produce a hash from another macro (root it at an object-like macro, where # has no special meaning):
#define HASH #
#define HASHM() HASH
The function-like variant allows you to produce the same hash right next to something else (HASHMCPROLG does nothing useful; HASH MCPROLG produces # MCPROLG, HASHM()MCPROLG produces #MCPROLG).
Those pieces in hand, the rest is easy:
#define PROLOG(FN_,SZ_) _Pragma(STR(prolog(FN_, STR( HASHM()MCPROLG MAIN=(YES,SZ_,132)))))
Here I'm assuming you also need an end parentheses around the pragma prolog and that in the question this was a typo; that is, it should be:
_Pragma("prolog(foo, \"#MCPROLG MAIN=(YES,16,132)\")")
...not:
_Pragma("prolog(foo, \"#MCPROLG MAIN=(YES,16,132)\"")
Imagine I want to #define a macro such that it is equal to the current value of another macro (if such a concept exists).
For example:
#include "def_a.h" // defines macro A
#define B A
This defines B to be A. If A later changes definition (i.e., through a redefinition) the value of B also changes (because B expands to A at the point of use, which further expands to the new value of A).
What I'd like is some way to "bake in" the value of A into B so that B just expands to the value of A, not A itself.
For example:
#define A first
#define B BAKE_IN(A)
#undef A
#define A second
#define C BAKE_IN(A)
#undef A
#define A third
// here I want B to expand to first, and C to expand to second
Of course BAKE_IN is not a real thing, but I'm wondering if there is some way to achieve this effect.
Now, I didn't really say what should happen if A itself is defined in terms of other macros, but I'm OK both with "one level of expansion" (i.e., B gets the value of A is expanded, so further macros remain) and also "full expansion" (i.e., A is fully expanded, recursively, as it would be at a point of use).
Macros are never replaced in the body of a #define directive, so there is no way to define a macro as the current value of another macro, except for the limited case of macros whose value is a constant arithmetic expression.
In the latter case, you can use BOOST_PP_ASSIGN_SLOT from the Boost preprocessor library. (Although most of the Boost libraries are C++-specific, the Boost preprocessor library works for both C and C++, and has no dependency on any other Boost component.)
I don't think there is a clean solution. The closest thing that I found is to preserve "stringified" values within char arrays:
#include <stdio.h>
#define BAKE_IN(X, id) BAKE_IN_REAL(X ## _, X, id)
#define BAKE_IN_REAL(X, Y, id) static const char X ## id[] = #Y;
#define BAKE_OUT(X, id) X ## _ ## id
#define A first
BAKE_IN(A, 1)
#define B BAKE_OUT(A, 1)
#undef A
#define A second
BAKE_IN(A, 2)
#define C BAKE_OUT(A, 2)
#undef A
int main(void)
{
printf("%s\n", B); // prints "first"
printf("%s\n", C); // prints "second"
return 0;
}
The idea is that BAKE_IN macro declares object named as e.g. A_1, which holds the current expansion of A.
There are two major limitations:
Every pair of BAKE_IN and BAKE_OUT needs unique id
The expansion is only available in "stringified" form
I have a number of definitions consisting of two comma-separated expressions, like this:
#define PIN_ALARM GPIOC,14
I want to pass the second expression of those definitions (14 in the case above) to unary macros like the following:
#define _PIN_MODE_OUTPUT(n) (1U << ((n) * 2U))
How can I extract the second number? I want a macro, call it "PICK_RIGHT", which will do this for me:
#define PICK_RIGHT(???) ???
So that I can make a new macro that can take my "PIN" definitions:
#define PIN_MODE_OUTPUT(???) _PIN_MODE_OUTPUT(PICK_RIGHT(???))
And I can simply do:
#define RESULT PIN_MODE_OUTPUT(PIN_ALARM)
Do not use macros for this. If you must, the following will work by throwing away the left part first so just the number remains. Use with care. No guarantees.
#define PIN_ALARM GPIOC,14
#define RIGHTPART_ONLY(a,b) b
#define PIN_MODE_OUTPUT(a) RIGHTPART_ONLY(a)
#define RESULT PIN_MODE_OUTPUT(PIN_ALARM)
int main (void)
{
printf ("we'll pick ... %d\n", PIN_MODE_OUTPUT(PIN_ALARM));
printf ("or maybe %d\n", RESULT);
return 0;
}
If you want the left part as a string, you can use this (with the same warnings as above), where the left part gets converted to a string by #:
#define LEFTPART_ONLY(a,b) #a
#define PIN_MODE_NAME(a) LEFTPART_ONLY(a)
There is a practical reason this is not entirely without problems. GPIOC is a symbol and as such it is possibly defined elsewhere. Fortunately, it is not a problem if it is undefined, or it is but to a simple type - after all, first thing the macros do is "throw away the left part". But as Jonathan Leffler comments
Note that if GPIOC maps to a macro containing commas, you're likely to get compilation errors.
I was wondering is there can be a way to stringize an integer variable using stringizing compiler directive.
I tried using:
#define stringize(a) #a
#define h(a) stringize(a)
#define g(a,b) a##b
#define f(a,b) g(a,b)
int main()
{
int num = 1024;
printf("%s=%s\n",stringize(h(f(1,2))), h(f(1,2))); //1. h(f(1,2))=12
printf("%s=%s\n",h(h(f(1,2))), h(f(1,2))); //2. "12"=12
printf("%s=%d\n", h(num),num); //num=1024
return 0;
}
so as adding another level in stringize macro(#1) will make the substitution to happen first then placing it in code(#2), in similar way can variables be replaced at compile time with the values.
I mean to say if var = value; then is there some way that
some_macro(var) --> can stringize it into "value"?
There's no way of getting the value of a variable using the preprocessor - preprocessing (as its name suggests) takes place before compilation, and the variables do not exist at that stage.
No. The preprocessor is acting on tokens, it doesn't know about variables and their values. What would you want to get if the value was read from stdin?