[hellofunk]

Strange thing to say, to be honest. Many of the most performance-critical industries are heavy on C++, not C. Games, finance, high-frequency trading, where every microsecond is worth huge money -- these are typically C++ shops. The strictness and ability to move things to compile time are features not available in C.

[jstimpfle]

Any examples of using compile time features that make a difference, instead of making code harder to maintian and increasing compile times significantly?

[hellofunk]

You’re talking about really two different issues here. I don’t think anyone would suggest templates can’t get complicated. But they are certainly the doorway into maximum performance over C.

[jstimpfle]

No I don't think I'm talking about something else. And I wouldn't submit to the opinion that performance cricitical code needs to be written in C++.

For example, Games are heavy in C++ where there are a lot of abstractions. Engine code is mostly C style as far as I can tell. Optimized code is code that exploits specific properties of systems, hence it is significantly less abstracted than higher-level code.

The stereotypical std::sort example doesn't count, really. There are many reasons why std::sort vs qsort (which are both generic sorting implementations - i.e. not optimized ones) is not significant in practice, but it's still among the most frequently cited examples - hinting that there might be few real wins (with regards to performance) from all this compile time and templates stuff.

[hellofunk]

Hm, I’m not sure where to start with your assessment here. Your comments on sorting don’t really seem relevant, I don’t think you’re referring to the same kind of compile time programming we’ve been discussing. There’s a difference between generic code, and code that runs at compile time, even though both might actually use the C++ feature of templates. C++ templates are way to write code that supports generics, but they are also a way to do a style of programming referred to as “meta-programming“, these are really not the same thing at all.

While you can certainly do without generics in the language, as proven by the success of many many languages, perhaps at the inconvenience of some developers, compile time meta programming is an entirely different area that only few languages really support. The high frequency trading firms and the game studios I have partnered with are typically using C++ specifically for those compile time programming features. They make dramatic performance differences in a wide range of highly specialized algorithms.

[jstimpfle]

It wasn't me who said "templates" first. (I don't think of them as a practical way to do compile time computation).

I was mostly looking at constexpr and whatever similar things have appeared in C++ lately. And I wanted to know about actual applications of them in the wild that make a difference.

Because, yeah I can precompute a 100K hash table or whatever at compile time, but I can also just do it at program startup (would anyone notice?) which is by far the simplest thing to do. Or I could just generate the data in a separate build step which is probably more hassle compared to constexpr but also probably friendlier in terms of build times in practice.

[sgtnoodle]

In embedded systems, often times not all memory is equal. Pre-computing a lookup table at runtime may not be practical due to the limited amount of RAM vs. flash memory. A constexpr or template meta program is, as you touched on, a nice way to do calculations at compile time in the existing language without having to add an explicit autocoding build step. An explicit build step eventually makes sense for sufficiently complex algorithms, but it can be a lot of build system maintenance overhead for small to medium complexity stuff. Implementing esoteric code using obscure syntax may be bad for readability, but keeping it "in the language" has a benefit of limiting the amount of project specific knowledge required to understand it.

[ChrisLomont]

Matrix compile time templates like Eigen result in vastly faster code than doing it in C, since many operations can be compile time simplified. C has no way do do this at compile time.

This is just the tip of the iceberg on using templates and classes to make faster, cleaner code.

[hellofunk]

> It wasn't me who said "templates" first.

But you were talking about generics, which implied you were incorrectly conflating that with template meta-programming, since generics are done with templates.

> I don't think of them as a practical way to do compile time computation

Templates are however the most flexible, advance technique for C++ compile-time programming, though the C++ standard is evolving to bring more and more of those features into the language without using templates, i.e. "constexpr if".

My recommendation would be to avoid speculating on what the benefits are of a language or its features if you clearly don't have serious experience using them. It's fine if C++ is not for you.