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I think this is.. quite wrong. I don't think C++ is an amazing language (it's not in my top 3 personally, but I do work with it daily), but templates are one of

by _se 6y ago

I think this is.. quite wrong. I don't think C++ is an amazing language (it's not in my top 3 personally, but I do work with it daily), but templates are one of the best things about the language IMO. I have a lot of experience with many different types of metaprogramming (cl macros, Rust macros, template Haskell (and Typeable/Generic), ... even CLR code generation) and C++ templates are better than everything I've used other than Common Lisp once you really understand them. A really common C++ theme I see is people that don't understand things railing against them (I used to be in this camp myself, so I can identify with the sentiment). Templates allow you to choose at will between nominal/structural subtyping (in a way), perform type-level logic, generate code that can be thread-safe or lock-free at the caller's whim... They are very powerful, much more than most devs realize, and if you understand C++, writing them is not difficult. The "really understand them" part is, of course, the hard part. You need to understand the core language and the template language to be effective with them. I can see why even some experienced C++ developers shy away from templates.

Dylan16807 6y ago
Even if they're a good general design, half the uses of SFINAE are absolutely insane and from what I understand it's significantly harder than it should be to make a fully generic template across all kinds of edge case and corner case types. The attitude with templates seems to be that if something is at all possible with hacks then there doesn't need to be any language support added, even if it would be vastly simpler to read and write and probably to compile. I don't think that works out well.
- northwindfoo 6y ago
  I wouldn't call myself a c++ expert but I have used the language for a long time. In my opinion, templates have a lot of awesome uses outside of SFINAE and if you find yourself reaching for SFINAE, that's probably code smell and you can probably simplify the design elsewhere to avoid it. Generally I find the best use of templates is the most basic: you want to write some code that operates on duck-types. As long as one is able to control themselves and pick the right use cases for maintainability, templates are simple enough. In my experience, this is very doable. Concepts are also a really cool way to rein in which types are accepted as a template parameter and have a lot of potential to make templates easier to understand and teach. To me, this is the most exciting c++20 feature.
- _se 6y ago
  With C++17, if constexpr removed most of the necessity for crazy SFINAE uses. Before that it was definitely worse. C++20 adds even more to make things more "semantic"; they're constantly adding better language support for templates.
qppo 6y ago
No one disputes that meta programming is useful. What is disputed is C++ templates being a "good" way to meta program. I certainly understand metaprogramming, but navigating very dense C++ templates is one of the most complicated things I've had to do in my life. One or two levels of abstraction and you will lose your ability to hold everything you need to solve a problem in your head, and after a few dozen lines the source code is unreadable. > You need to understand the core language and the template language to be effective with them. this is a bug.
- _se 6y ago
  I guess that just hasn't been my experience at all. I don't think a nested C++ template is any more complicated than a CL macro with a corresponding level of nested `. I also don't really see how requiring understanding of a language to generate that language could be considered a bug - every macro system carries that requirement. C++ just happens to be (much) more complex than most languages (and I do agree that a large amount of that complexity is probably not necessary).
  - qppo 6y ago
    I'd argue it's significantly more complex because you don't have access to the C++ AST, templates are ultimately non-hygienic macros, their results are invisible to both developer and user, and potential errors are inscrutable. They're difficult to abstract over, and you will wind up repeating yourself quite often since you can't just write normal C++ code with C++ includes for helper functions and data structures. I should be more clear about what I meant on the bug. C++ templates force you to understand the entirety of the language at once and hack around its edges because it doesn't support actual meta programming. It's hard for me to come up with a concrete example, but my experience writing some advanced compile-time evaluation with SFINAE patterns left me forgetting how things worked within minutes of typing them. Compare to say Rust, where the generics implementation is much cleaner and macro system better flushed out and easier to read/understand. Or the LISPs where your macro is just more LISP code. And not for nothing, templates destroy compile times. They take a long time to write and edit and make everyone else using them work slower. All told, templates do not make it easier to write C++ except in trivial cases and first-order abstractions.
ssvss 6y ago
Any recommendation for books/blogs/videos/code to learn about this ?

by _se 6y ago

Dylan16807 6y ago

Even if they're a good general design, half the uses of SFINAE are absolutely insane and from what I understand it's significantly harder than it should be to make a fully generic template across all kinds of edge case and corner case types. The attitude with templates seems to be that if something is at all possible with hacks then there doesn't need to be any language support added, even if it would be vastly simpler to read and write and probably to compile. I don't think that works out well.

northwindfoo 6y ago
I wouldn't call myself a c++ expert but I have used the language for a long time. In my opinion, templates have a lot of awesome uses outside of SFINAE and if you find yourself reaching for SFINAE, that's probably code smell and you can probably simplify the design elsewhere to avoid it. Generally I find the best use of templates is the most basic: you want to write some code that operates on duck-types. As long as one is able to control themselves and pick the right use cases for maintainability, templates are simple enough. In my experience, this is very doable. Concepts are also a really cool way to rein in which types are accepted as a template parameter and have a lot of potential to make templates easier to understand and teach. To me, this is the most exciting c++20 feature.
_se 6y ago
With C++17, if constexpr removed most of the necessity for crazy SFINAE uses. Before that it was definitely worse. C++20 adds even more to make things more "semantic"; they're constantly adding better language support for templates.

qppo 6y ago

No one disputes that meta programming is useful. What is disputed is C++ templates being a "good" way to meta program. I certainly understand metaprogramming, but navigating very dense C++ templates is one of the most complicated things I've had to do in my life. One or two levels of abstraction and you will lose your ability to hold everything you need to solve a problem in your head, and after a few dozen lines the source code is unreadable. > You need to understand the core language and the template language to be effective with them. this is a bug.

_se 6y ago
I guess that just hasn't been my experience at all. I don't think a nested C++ template is any more complicated than a CL macro with a corresponding level of nested `. I also don't really see how requiring understanding of a language to generate that language could be considered a bug - every macro system carries that requirement. C++ just happens to be (much) more complex than most languages (and I do agree that a large amount of that complexity is probably not necessary).
- qppo 6y ago
  I'd argue it's significantly more complex because you don't have access to the C++ AST, templates are ultimately non-hygienic macros, their results are invisible to both developer and user, and potential errors are inscrutable. They're difficult to abstract over, and you will wind up repeating yourself quite often since you can't just write normal C++ code with C++ includes for helper functions and data structures. I should be more clear about what I meant on the bug. C++ templates force you to understand the entirety of the language at once and hack around its edges because it doesn't support actual meta programming. It's hard for me to come up with a concrete example, but my experience writing some advanced compile-time evaluation with SFINAE patterns left me forgetting how things worked within minutes of typing them. Compare to say Rust, where the generics implementation is much cleaner and macro system better flushed out and easier to read/understand. Or the LISPs where your macro is just more LISP code. And not for nothing, templates destroy compile times. They take a long time to write and edit and make everyone else using them work slower. All told, templates do not make it easier to write C++ except in trivial cases and first-order abstractions.

ssvss 6y ago

Any recommendation for books/blogs/videos/code to learn about this ?