February 2016

One of C++ most underrated features: Namespace aliases—Jonathan Müller

A nice story with a goal:

One of C++ most underrated features: Namespace aliases

by Jonathan Müller

From the article:

About two months ago I wrote the following r/cpp comment:

[compte supprimé]

  • You could always just alias the namespace like namespace fnc = FunctionalPlus.


  • Namespace aliases are one of C++ most underrated features.

In the thread a new library was presented. One user complained about the long namespace name, (s)he got the above replies. Judging by the number of upvotes, people seemed to agree with my comment. In this blog post I am going to elaborate it...

A bit of background for the unified call proposal

C++ provides two calling syntaxes, x.f(y) and f(x,y). This has bothered me for a long time. I discussed the problem in the context of multimethods in D&E in 1994, referring back to a proposal by Doug Lea from 1991, and again in 2007. Each of the syntaxes has its virtues, but they force design decisions on people, especially library designers. When you write a library, which syntax do you use for operations on objects passed to you by your users? The STL insists on the traditional functional syntax, f(x,y), whereas many OO libraries insist on the dot notation, x.f(y). In turn, libraries force these decisions on their users.

Interestingly, I solved the problems for operators: x+y doesn’t care whether you provide operator+(T,T) or T::operator+(T). The problem is getting more noticeable as we get more generic libraries. Note begin(c) and c.begin() for range-for loops and in general code. Why do we/someone have to write both? If c.begin() exists, begin(c) should find it, just as x+y finds the right implementation. Such call-syntax adapters are pure overhead and different people’s adapters can clash.

In early 2014, Herb Sutter and I each independently decided to propose a unified syntax. Herb suggested unification based on allowing x.f(y) to find a non-member function, giving preference to the x.f(y) syntax, whereas my ideal was that x.f(y) and f(x,y) should mean exactly the same. After a quick discussion, we joined forces. Based on real input from code and users, I reluctantly agreed that for compatibility reasons, x.f(y) and f(x,y) could not mean exactly the same. The only feasible way forward was to do a traditional lookup based on the syntax used, and then try the other syntax if the first one failed. Stability – backwards compatibility – is an important feature, overruling my desire for perfection.

To my surprise, many people came out strongly against x.f(y) finding f(x,y) – even if member functions were preferred over free-standing functions by the lookup rules. I received email accusing me of “selling out to the OO crowd” and people whose experience and opinion I respect insisted that x.f(y) finding f(x,y) would seriously compromise their ability to design stable interfaces. I think those fears are greatly exaggerated, but I could be wrong. Also, I prefer the dot syntax in some cases; for example, I find x.f(y).g(z) more readable than g(f(x,y),z). However, there was no chance of acceptance of a proposal that included x.f(y) finding f(x,y). Maybe modules will eventually help here. Furthermore, David Vandevoorde pointed out that because of the two-phase lookup rules having x.f(y) find f(x,y) would complicate the task for compiler writers and possibly be expensive in compile time.

So, we are left with a simple proposal to allow f(x,y) to find x.f(y) where f(x,y) wouldn’t work today. This solves the problem for library writers and their users along the lines of the STL:

  • Library designers don’t have to force users to use one preferred syntax or to duplicate the implementation to handle both.
  • It allows us to write simple concepts for new libraries.
  • We no longer have to write call-syntax adapters.
  • We can in many cases add functionality to an abstraction without modifying a class
  • We no longer have to bother the users with the distinction between member functions and helper functions.

The proposal was approved by the Evolution Working Group. Faisal Vali did an experimental implementation in Clang. Now we just have to hope that we can agree on exact wording in time for C++17. I consider it a small proposal that will significantly simplify the way we design and use libraries.

Compiler improvements in VS 2015 Update 2—Andrew Pardoe

The C++ compiler team is excited for you to try out the compiler in Visual Studio 2015 Update 2 CTP 1. Let’s dive into some of the feature improvements they have made in the compiler for Update 2:

Compiler improvements in VS 2015 Update 2

by Andrew Pardoe

From the article:

Since Update 1 we’ve made progress on being Standards-conformant for lot of C++11 and C++14 features...

New papers: Iterator facade for ranges; adopt File System TS for C++17; unified call syntax wording

Note: This is the last batch of individually- or small-group-posted papers this week. Tomorrow is the pre-Jacksonville paper mailing deadline, so we expect a bunch of papers to come in all at once, and instead of blogging about them individually, those will all advertised here next week as part of the mailing blog post.


New WG21 papers are available. If you are not a committee member, please use the comments section below or the std-proposals forum for public discussion.


P0186R0: Iterator Facade Library Proposal for Ranges (Beman Dawes, Eric Niebler, Casey Carter)

Proposes a library component for easily creating conforming iterators. Based on existing practice. Depends only on the C++17 working paper plus Concepts TS and Ranges TS. Breaks no existing code or ABI's. Two open-source implementations including test suites available. Proposed wording provided.


P0218R0: Adopt the File System TS for C++17 (Beman Dawes)

Technical work on N4100, File System Technical Specification, ISO/IEC TS 18822:2015, was completed in July 2014, and published by ISO in July 2015. There are three shipping implementations and one soon-to-ship implementation. Two of the shipping implementations have been have been in use for several years. This document proposes adopting the File System Technical Specification, with corrections, for C++17. The alternative to this proposal is to start work on version two of the File System TS. Doing nothing is not an alternative.


P0251R0: Unified Call Syntax Wording (Bjarne Stroustrup, Herb Sutter)

This is the proposed wording for a unified call syntax based on the idea that f(x,y) can invoke a member function, x.f(y), if there are no f(x,y). The inverse transformation, from x.f(y) to f(x,y) is not proposed. [...] As agreed in EWG at the Kona meeting, the intent of the wording is: For f(x,y), see if f(x,y) is valid and if so do that call; otherwise try x.f(y).


A bit of background for the structures bindings proposal

[This note from Bjarne provides some background for the updated structured bindings proposal posted yesterday. --Ed.]


In the context of the C++ Core Guidelines, we (Herb Sutter, Gabriel Dos Reis, me, and few others) were discussing sources of bugs and inefficiencies. We noticed that returning multiple values (e.g., from a tuple) was one of the two remaining sources of uninitialized variables. We noticed that there were real cases of bugs and inefficiency (from default initialization followed by assignment rather than initialization) stemming from separating variable introduction from initialization. Use of tie() is at best a partial solution that doesn’t get to the root of the problem.

The next day or so, Herb had a first draft of a proposal. We refined it, adding use cases and implications. I noted that we had a syntax in concepts for the introduction of multiple names that we could borrow. We noted that whatever we came up with had to fit into our ideas for a possible future far-more-general pattern matching design. We always try to ensure that a proposal doesn’t block an important potential evolution path. The design document for the proposal was refined and extended.

It was now obvious that the proposal completed C++’s mechanism for returning multiple values from a function: With structured binding, we can initialize a set of values, {x,y,z},  just as we  can return a set of values, {e1,e2,e3}.

In web discussions, and especially at the Kona standards meeting, many alternatives were discussed. We focused on how to deal with user-defined types with private members. The new feature mustn’t discourage encapsulation by providing a convenient notation for structs only. We also considered whether explicit declaration of types were needed and how to achieve conversions. A conversion from a char[] in a return value to a string was considered particularly desirable by several people. These two needs were both addressed by adding the ability for users to define get<N> functions for a class. Jens Maurer refined the proposed wording and in doing so found a few weaknesses in the design.

For the details and code examples, you can find the current proposal and wording papers here for the upcoming meeting in Jacksonville.

Searching and replacing in strings with boost

My series on building applications with Qt an boost continues:

Searching and replacing in strings with boost

by Jens Weller

From the article:

The next big milestone for my CMS is to actually generate HTML files, and I'm almost there. I'll reach it in the next two weeks, most code is written, just a little bit of refactoring is needed. This blog post is about searching and replacing in strings. As I started last week with implementing the functionality, that turns the data in my CMS into an HTML website.

There needs to be a lot of text transformed, in order to turn a shared structure like a cross page layout into a single, special HTML file, one of those transformations is, to replace the internal links with the correct links. A link to a different page in the same website cannot be represented as a text link, instead it is represented by a linkid, which corresponds to the Page it points to. This is to have still the correct link, if the page is renamed or moved...

Overload 131 is now available

ACCU’s Overload journal of February 2016 is out. It contains the following C++ related articles.

Overload 131

From the journal:

Defining Concepts
Concepts provide a new way of constraining code. Andrew Sutton shows us how to define and use them. by Andrew Sutton

On Zero-Side-Effect Interactive Programming, Actors, and FSMs
Functional programming is alien to many programmers. Sergey Ignatchenko considers parallels between actors and finite state machines.

Template Programming Compile Time Combinations & Sieves
Functional style frequently uses sequences. Nick Weatherhead applies these ideas to combinations in C++. by Nick Weatherhead

Classdesc: A Reflection System for C++11
C++ lacks direct support for reflection. Russell Standish brings an automated reflection system for C++, Classdesc, up to date. by Russell Standish

QM Bites : Maximising Discoverability of Virtual Methods
C++11 introduced override as a contextual keyword. Matthew Wilson encourages us to use it. by Matthew Wilson

So Why is Spock Such a Big Deal?
Spock testing in a Java environment is all the rage. Russel Winder talks through the history of testing on the JVM and demonstrates why Spock is so groovy. by Russel Winder

Modern C++ features: in-place construction—Arne Mertz

And emplace_back function:

Modern C++ features: in-place construction

by Arne Mertz

From the article:

Move constructors are often cheaper than copy constructors, which makes the construction and immediate relocation of objects in modern C++ more effective than in C++03. However, just moving the parts needed to construct the object in the right place can be even more effective. Several standard library functionalities use perfect forwarding to construct objects right where they are needed.