Current Proposals for C++17

http://meetingcpp.com/index.php/br/items/current-proposals-for-c17.html

by meetingcpp • 10 years ago

66 36 10 years ago

vmorgulis4• 10 years ago

With Concepts, we will also have "auto" in function arguments. It avoids "template <typename ...>" for simple functions.

I already use it as a "catch all" over specialized functions (gcc -std=c++17).

repsilat• 10 years ago

Why prefer

    int foo(auto a, auto &&b);

    int foo(a, &&b);

? Too big a change? Too much magic? Maybe more difficult to read or parse. The argument "It's not clear that the function is a template" isn't there any more, though, so it seems reasonable to me.

kevinnk4• 10 years ago

You actually hit on the exact reason; your change is significantly harder to parse. C++ allows you to alias typenames and variable names, so you wouldn't know if `int foo(T);` was a declaration of a function that take a type `T` or a function that has an implicitly typed parameter called `T` until much later in the parse. Adding auto makes it clear this is a declaration of a variable. That kind of ambiguity already exists in C++[1], but is widely regarded as one of the things that makes parsing C++ extremely hairy.

Interestingly, I remember hearing about this exact problem in the context of Swift's closures; if you want type inference, you need to leave off the parens around the parameters. This is mainly done (IIRC) because it's easier for the parser.

[1] http://blog.reverberate.org/2013/08/parsing-c-is-literally-u...

vmorgulis4• 10 years ago

Do you mean?

    template <typename T1,typename T2>
    int foo(T1 a, T2 &&b);

"auto" is shorter.

perone• 10 years ago

This is moving fast, a moratorium for some time would be beneficial for community. Like the one that happened in Python sometime ago.

GFK_of_xmaspast2• 10 years ago

The last big change was c++11, and that was already five years ago.

snuxoll• 10 years ago

Forgetting about C++14?

vmorgulis4• 10 years ago

Stroustup is pushing a lot forward:

"C++ use is on the increase. We need to improve C++ to sustain the momentum, not better serve the C++ programmers, not out of fear of competition."

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n449...

Rexxar• 10 years ago

I'm not a native English speaker and I'm no sure I understand this part correctly : "..., not better serve the C++ programmers, ...". Does it means that improving c++ is not done to help c++ programmers ?

scott_s• 10 years ago

I think this is rather unlike the Python 3 situation. My guess for the resistance to Python 3 adoption is that it was, by design, not backwards-compatible with Python 2. C++ versions are not completely backwards-compatible, but I think they are more than the Python 2 to 3 switch. What is certain is that lots of other languages release new versions a few times a decade. See, for example, version histories of:

https://en.wikipedia.org/wiki/Java_version_history

https://en.wikipedia.org/wiki/C_Sharp_(programming_language)...

https://en.wikipedia.org/wiki/ANSI_C

meetingcpp4• 10 years ago

um, nope, not going to happen...

C++ did that in the 2000's, in that time there wasn't much news about standardization, because of this C++ was seen as legacy, people moved to other languages. The new languages such as Rust, Go etc. are also a result of this.

The C++ committee has now a process of ongoing standardization established, which will improve the language further, and make it easier and more efficient to write C++. Also, the standards are usually backward compatible for at least 2 standards. What gets removed is often obscure features like auto_ptr, random_shuffle etc. which are superseeded by better alternatives. Clang modernize can even get your code base automatically updated to a new standard.

I know, that some of you are left behind, as you are stuck with the traditional "almost never update the toolchain" model, but clang and other tools are such a leap forward, that this is not a model for the future anymore.

So, C++ ecosystem evolves to become better, and make you as a programmer more productive and lets you write easier and safer code.

hexagonc• 10 years ago

While I appreciate the effort to make things easier, as someone who is trying to move from C++98 to C++11, I'm not sure the new features make the language easier to use so long as you still have to learn the old ways of doing things. Speaking for myself, at least, this just causes interference between competing habits. What would be better for me and maybe others, would be if some of the unsafe or deprecated features in C++ were disallowed or if there were a compiler option to disallow them. I know this would break C89 compatibility but so what if you know you only care about the newer and more efficient features of C++?

maxxxxx3• 10 years ago

I have been away from C++ for a while. Has there been any progress in making it easier to link different libraries together? In C# it's so nice that you can just reference another library and you are all done.

When I used C++ last time this was always a huge ordeal.

kevinnk4• 10 years ago

Do you mean link as in "linker"? Or are you asking if it's possible to do "using LibX" like in C#?

You can already do "using ClassX", but yeah, the package management story in C++ isn't great.

There's a proposal for modules in C++[1], which should help, but as far as I know that's not going to be in C++17. Clang already has a (different) version implemented as well[2].

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n446... [2] http://clang.llvm.org/docs/Modules.html

maxxxxx3• 10 years ago

I meant compiling C++ code into a shared library like a DLL on Windows and be able to use the classes in that DLL from other code. The same way you can easily call C code in different libraries.

Razengan• 10 years ago

I assume performance is one of the main reasons that C/C++ are still relevant today, and the reason why modern languages like Swift and Go can't be as fast is because of their "runtime" software layer which makes their fancy features possible.

My honest if somewhat noobish question is: Why isn't support for features like type safety and so on being incorporated into CPUs themselves, like they did with SIMD instruction sets?

I mean, if so many languages require "runtimes" now, why haven't the features that are common across those runtimes, been factored out into hardware already?

dottrap• 10 years ago

That's not a fair characterization of Swift's runtime. Swift is similar to C++ in that it compiles down a lot of stuff so it doesn't need to make runtime decisions.

Swift could theoretically be as fast or faster than C/C++, but C/C++ have decades compiler optimizations that are going to be hard to beat. I say Swift has the potential to be faster because of the pointer aliasing problems of C/C++, which Swift can potentially optimize (like why Fortran still can beat C).

But your question about runtimes and performance are kind of a red herring. High performance means utilizing the underlying hardware well. A lot of the popular modern languages were designed for convenience over speed and were designed in an era of ever-increasing single processor CPU speeds. Very few languages map to how hardware actually works so they give up speed. Few mainstream languages automatically vectorize _well_ and _reliably_ to automatically exploit SIMD (C/C++ don't really do this...we generally have to write the intrinsics ourselves if we care). Few mainstream languages do a good job exploiting the multiple cores/CPUs available to us (don't say threads...they are pretty poor. This is one interesting feature of Go). Cache locality is now extremely important to high performance and most languages don't let us really control this. (C/C++ give us some say on our memory layouts.)

Current Proposals for C++17

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