Doing non-trivial anything you slow down compilation, make debugging harder and increase fragility and complexity. Sometimes that's a mistake, sometimes not.

Getting rid of something because someone sometimes abuses it, is short-sighted.

You don't need to get rid of it to leave it unused. Macro wizardry leads to poor code.

> slow compilation, hard to debug code, opaque and fragile code, and incredible complexity

Most people describe C++ in those terms.

Exactly. So if you're willing to accept that downside, C++ does a better job of letting you express that kind of cleverness.

I am not sure. I agree one should minimize macros in C, but for some things a small macro layer still beats the complexity of C++ and is not inferior in my opinion.

C++ templates are far cleaner, easier to debug, and produce more comprehensible errors than C macros. And C++ templates are rightly famous for being terrible at all of the above.

Hey, I agree about your second sentence but I do not really see the first. Alone that I can directly inspect the result of the preprocessor makes macros easier to debug than templates in my experience.

> I can directly inspect the result of the preprocessor makes macros easier to debug

Tangential note but: why does this require a separate re-invocation of the compiler with the '-E' flag? Say I have this silly example with a comma instead of the semicolon inside W() macro:

    #define W(a, b)  b, int a
    #define X(a, b)  W(b, a)

    int kekw(int x) {
        int z = X(23, k) = x;
        return z + k;
    }

When I shove it into GCC without -E, it says

    <source>: In function 'int kekw(int)':
    <source>:1:21: error: expected unqualified-id before 'int'
        1 | #define W(a, b)  b, int a
          |                     ^~~
    <source>:2:18: note: in expansion of macro 'W'
        2 | #define X(a, b)  W(b, a)
          |                  ^
    <source>:5:13: note: in expansion of macro 'X'
        5 |     int z = X(23, k) = x;
          |             ^
    <source>:6:16: error: 'k' was not declared in this scope
        6 |     return z + k;
          |       

instead of, you know, showing me the expanded text so that I could see what's actually wrong with the generated variable declarations? Something like

    <source>: In function 'int kekw(int)':
    <source>:1:21: error: expected unqualified-id before 'int'
        1 | #define W(a, b)  b, int a
          |                     ^~~
    <source>:2:18: note: in expansion of macro 'W'
        2 | #define X(a, b)  W(b, a)
          |                  ^
    <source>:5:13: note: in expansion of macro 'X'
        5 |     int z = X(23, k) = x;
          |             ^
    <source>:5:-: note: the expanded source text is
          |     int z = 23, int k = x;
          |                 ^~~
    <source>:6:16: error: 'k' was not declared in this scope
        6 |     return z + k;
          |       

perhaps? I understand that tracking the lines/columns inside the expanded text is highly non-trivial (although collapsing them all onto the position of the top-level macro is already just as non-trivial), but still. Show me what the source that you actually can't compile, not the three-steps removed precursor to it — I sure as hell can't expand macros in my head.

Also, what is this first error message about in the first place? Why the hell does it expect unqualified-id before "int"? If you put "int" after the unqualified-id, it still would be illegal; and an unqualified-id is not the only thing that can follow the comma in this context anyway.

During translation of source text to a C program, the source text goes through several phases. It turns raw bytes into preprocessor tokens, then preprocessor tokens get expanded into more preprocessor tokens and then preprocessor tokens get translated into C tokens, which are fed into the C parser. There is no “expanded text”, only expanded tokens. Generally this is implemented as a state machine that produces tokens on demand, so there is no expanded text. When GCC shows you that error message, it uses the recorded source location and looks it up in the original source file. This is trivial to do. Expanding just one line of the source file would either require re-invoking the preprocessor from the beginning or storing already consumed preprocessor tokens.

Obviously this is possible, but no one has gotten around to implementing this feature and it’s not clear you would want either solution: buffering preprocessor tokens in case you need to expand a line for an error message greatly increases memory usage and the vast majority of compilations are successful ones with no errors, slowing down the normal case. And re-invoking the preprocessor to show you one line on every error message makes error reporting slow.

> There is no “expanded text”, only expanded tokens.

If the compiler supports "-E", there absolutely is some equivalent of the expanded text, with (somewhat limited) whitespace info available: if you insert whitespace into between "b" and the comma in "b,", then "-E" invocation will dutifully show it. Yes, multiple spaces are collapsed into one, but their presence/absence is tracked already.

> Expanding just one line of the source file would either require re-invoking the preprocessor from the beginning or storing already consumed preprocessor tokens.

Yeah, keep the whole expansion of the latest top-level (not nested) macro. I believe some bytecode-like compression scheme (a replay of that state machine you mentioned) à la LZ would work fairly well, with minor additional memory necessary. Hell, scratch that: since #define/#undef are impossible inside a macro, you don't even need to remember the expansion of the macro, you simply need to keep the current macro definitions and remember what was the last macro you tried to expand — if your error happened inside its expansion, just re-expand again. Zero additional memory needed, since this information is already stored for the current way of error reporting — you can see in the current output that it remembers what macros it expanded and where, the whole path down to the offending token: at 1:21 "in expansion of macro 'W'" which was at 2:18 "in expansion of macro 'X'" which was at 5:13.

> And re-invoking the preprocessor to show you one line on every error message makes error reporting slow.

Eh, it's on the whole faster than making the programmer re-run with "-E" option manually.

> If the compiler supports "-E", there absolutely is some equivalent of the expanded text, with (somewhat limited) whitespace info available: if you insert whitespace into between "b" and the comma in "b,", then "-E" invocation will dutifully show it. Yes, multiple spaces are collapsed into one, but their presence/absence is tracked already.

Whitespace is significant in the preprocessor so of course it tracks whitespace. Whitespace isn’t dropped until phase 7 when pp-tokens are converted to C tokens.

As I said it’s not impossible, write the patch and see if GCC accepts it.

Also the position in the original source code is tracked.

In fact, this enhancement should be doable. Please file a bug. But the answer why something is not done in a C compiler is usually: Nobody did substantial investments on it in the last two decades. So if you want to see improvements, usually one has to submit a patch.

What you're saying is like saying "C can be expanded to assembly, and that makes it easier to debug". If you need to do that, the entire process has failed.

I don't think so. In any case, templates are still worse to debug, so if the entire process has failed, then even more so for templates.

Yes and no. If you want to make something like a generic dynamic array, you can do it with type erasure (`void*`), but then your code is no longer typesafe. I would find that more fragile. The preprocessor allows you to add constructs that make it behave like any other modern language.

As an aside, clangd can dereference a macro in place to make debugging easy.

And for generic data structures one can also a very thin layer of macros around type-erased implementations.

You're usually better off just using realloc, or an abort-on-oom wraper of it, in my experience.

Okay smarty-pants. What if I want a generic map then ;)