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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).

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    Ok, I was not sure ... does someone know how to mark a question as duplicate without closing it? Commented Feb 7, 2017 at 17:48
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    As far as I know, those are mutually exclusive. If a question is marked as duplicate, it gets closed with the little banner at the top directing you to the old question. Commented Feb 7, 2017 at 17:51
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    @Sabrina - I want to #include a header file with specific macros defined/not-defined and then "capture" the value of a macro that this header file defines. Later, the macro that I captured from may change value, but I want the original value. Commented Feb 7, 2017 at 18:23
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    @Sabrina: That is no way related to C# Commented Feb 7, 2017 at 18:29
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    @beeonrope: basically, this isn't possible with the preprocessor and even if it were, it wouldn't help with your use case. I put a suggestion about the assert use case as an answer to your other question; in this context, I'm going to stick with the answer I provided: you can freeze a macro only if it's value is an integer, and only with the black pp magic of boost-pp. Commented Feb 7, 2017 at 23:26

2 Answers 2

Reset to default
2

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
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1 Comment

Thanks! This is quite useful when you need a string version of the macro, which is actually the case for some type of diagnostics, so I'll probably use this in the future. I accepted the other answer in this case because it was more directly applicable (since I definitely don't need a stringified form for my use case), but this is a good contribution.
1

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.)

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8 Comments

Your "exception" is none actually. And this is about C, not C++.
@olaf: the boost preprocessor library works in both C and C++.
@Olaf - does the underlying technique BOOST_PP_ASSIGN_SLOT uses work in C then, or are the preprocessor rules different?
@BeeOnRope: Boost is a C++ library. And it cannot change how cpp works.
@olaf: boost-pp is not a c++ library. It is true that it does not change how cpp works -- but it has lots of tricks to make it do things you might not expect are possible.
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