alt Hacker NewsNever quite understood why people are so obsessed with meta programming capabilities in a language, be it templates, comptime, macros, whatever.
I program mostly in C, if I need 'meta' programming I just write another C program that processes C source code (I've written a simple C parser), then in my build script I build in two stages, build meta program, run it, build rest of program.
Simple, effective, debuggable (the meta program is just normal C), infinite capabilities - can nest this to arbitritary depths, need meta-meta programming? Make a program that generates a meta program.
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Actually why even specify metaprogram as C like source code? It must be convenience. But there is little practical use, like a good program always models a lot of different representations of more or less the same things, just recombined and processed a little differently. Why would we want to deal with semantics of C types for example, if we can model a much clearer and better constrained universe of types used in e.g. a de/serialization framework? Even only pointers are quite special, and often only of very immediate use, but there is no point in e.g. persisting them to disk or sending them over the network.
This works for extreme needs.
But you're probably not doing s ton of metaprogramming all the time like you should be, and would with a language that allows it.
The lack of metaprogramming is also why C is so slow compared to C++
Writing a C++ parser is much harder than a C parser to the point there had been just 3 parsers used among all C++ compilers for quite a while. So you'd need to use some library for parsing. So now you are looking into the library's parser compatibility with the compiler you are using (it might not support the C++ standard you are on at all, it can have bugs preventing it from parsing the code that the compiler parses just fine) and not just on your code but on the library headers you include in your code. What are you going to do when cindex/libclang or whatever chokes on a libstdc++ header? You also have the issue with builtin macros: are they are the same in your library parser? Most likely not. Good luck testing all that.
Two-stage compilation is just a bonus on top: you add a sequential dependency in your build graph and if you have enough of these parsing programs you are going to wait till they are all built before your build can go wide.
Meta programming in C++ can enable you to remove lots of runtime branching in your code at the cost binary size.
Absolutely spot on, easier, and way more effective
Why would I write a parser that almost-but-not-quite matches the compiler's own parser, when I could just use the compiler's parser directly? I don't want to write a parser, and I especially don't want to debug weird corner cases where my implementation diverges somehow. I just want to write some code that goes like, for each field in T, do X.
C++ metaprogramming is bad, but the problem there is the C++ part, not the metaprogramming-in-the-language part.
One obvious answer is that people probably don’t want to write a whole parser and wire up new steps in their build pipeline just to do something simple like get the name of enum cases as a string.
Without taking a stance on whether in-language meta programming facilities are good or bad, it’s not hard to find examples of cases where people find it useful to have them.