Iterators++, Part 2 -- Eric Niebler

By Felix Petriconi | Feb 15, 2015 01:17 PM | Tags: c++14 advanced

Eric Niebler goes deeper into Iterator details in his new blog post

Iterators++, Pat 2

From the article:

This is the third in a series about proxy iterators, the limitations of the existing STL iterator concept hierarchy, and what could be done about it. In the first post I explained what proxy iterators are (an iterator like vector<bool>‘s that, when dereferenced, returns a proxy object rather than a real reference) and three specific difficulties they cause in today’s STL:

1. What, if anything, can we say in general about the relationship between an iterator’s value type and its reference type?
2. How do we constrain higher-order algorithms like for_each and find_if that take functions that operate on a sequence’s elements?
3. How do we implement algorithms that must swap and move elements around, like sort and reverse?

C++17 Library Papers for Cologne

By Meeting C++ | Feb 14, 2015 03:36 AM | Tags: standardization intermediate committee c++17

The first part of a mini series about the Library Papers for the LWG Meeting in Cologne:

C++17 Library Papers for Cologne

by Jens Weller

From the article:

Last fall I did the last series about the Standardization papers for C++. I didn't had the time to finish the last part for the Library subgroup, as Meeting C++ 2014 was getting close too. I'll be attending the next meeting of the Library Working Group in Cologne, which is just a few days away, so I'll do a miniseries for the LWG papers...

Default Constructed Return Value: return {} -- Adi Shavit

By Adi | Feb 13, 2015 04:52 PM | Tags: c++11

Simplify your code using a default constructed return value with `return {}`:

Default Constructed Return Value: return {}

by Adi Shavit

From the article:

It is common for C/C++ functions to return default values, for example, if some internal condition fails.
This is straightforward for native return types. This pattern is often seen in factory methods or functions that return a polymorphic type via a base handle.

If the function has multiple early return points, this default construction needs to be repeated at each such point and each time stating the explicit return type just for calling the default ctor. This is annoying because the compiler already knows the return type.

CppCon 2014 Persisting C++ Classes in Relational Databases with ODB--Boris Kolpackov

By Adrien Hamelin | Feb 13, 2015 08:00 AM | Tags: intermediate c++11

While we wait for CppCon 2015 in September, we’re featuring videos of some of the 100+ talks from CppCon 2014. Here is today’s feature:

Persisting C++ Classes in Relational Databases with ODB

by Boris Kolpackov

(watch on YouTube) (watch on Channel 9)

Summary of the talk:

ODB is an open source, cross-platform and cross-database (SQLite, PostgreSQL, MySQL, MS SQL Server, Oracle) object-relational mapping (ORM) system for C++. It allows you to persist C++ objects to a relational database without having to deal with tables, columns, or SQL, and without manually writing any mapping code.

In the first part of this two-part talk we will cover the basics of transactionally persisting, loading, updating, and deleting simple C++ classes in a database as well as querying the database for objects. We will then look into persisting C++ classes that have more interesting data members, such as containers and pointers to objects, or that form a polymorphic hierarchy. Support for C++11, Qt, and Boost value types, containers, and smart pointers will also be covered.

Targeting 5 different database systems at the same time may sound like a daunting task but as we will see it is not that hard with ODB. Life would also be a lot easier if our C++ classes never changed. The next best thing is to have comprehensive tooling support. So we will conclude the first half with a discussion of database schema evolution and its support in ODB.

Ranges and Iterators for numerical problems

By Meeting C++ | Feb 12, 2015 09:29 AM | Tags: performance intermediate c++11 boost

A guest blog post by Karsten Ahnert at Meeting C++ as a follow up of his talk!

Ranges and Iterators for numerical problems

by Karsten Ahnert

From the article:

In this blog post I am going to show some ideas how one can implement numerical algorithms with ranges. Examples are the classical Newton algorithm for finding the root of a function and ordinary differential equations. The main idea is to put the main loop of the algorithm into range such that the user can...

ODB C++ ORM 2.4.0 Released, Adds Bulk Operations Support

By Boris Kolpackov | Feb 11, 2015 11:55 AM | Tags: None

ODB C++ ORM 2.4.0 Released, Adds Bulk Operations Support

by Boris Kolpackov

ODB is an open source object-relational mapping (ORM) system for C++. It allows you to persist C++ objects to a relational database without having to deal with tables, columns, or SQL and without manually writing any of the mapping code.

Major new features in this release:

• Support for bulk operations in Oracle and SQL Server. Bulk operations can be used to persist, update, or erase a range of objects using a single database statement execution which often translates to a significantly better performance.
• Ability to join and load one or more complete objects instead of, or in addition to, a subset of their data members with a single SELECT statement execution (object loading views).
• Support for specifying object and table join types in views (LEFT, RIGHT, FULL, INNER, or CROSS).
• Support for calling MySQL and SQL Server stored procedures.
• Support for defining persistent objects as instantiations of C++ class templates.

A more detailed discussion of these features can be found on the blog. For the complete list of new features in this version see the official release announcement.

ODB is written in portable C++ (both C++98/03 and C++11 are supported) and you should be able to use it with any modern C++ compiler. In particular, we have tested this release on GNU/Linux (x86/x86-64/ARM), Windows (x86/x86-64), Mac OS X (x86/x86_64), and Solaris (x86/x86-64/SPARC) with GNU g++ 4.2.x-5.x, MS Visual C++ 2005, 2008, 2010, 2012, and 2013, Sun Studio 12u2, and Clang 3.x.

The currently supported database systems are MySQL, SQLite, PostgreSQL, Oracle, and SQL Server. ODB also provides optional profiles for Boost and Qt, which allow you to seamlessly use value types, containers, and smart pointers from these libraries in your persistent classes.

More information, documentation, source code, and pre-compiled binaries are available on the project's page.

How do I write complex initialization?--Stack Overflow

By Adrien Hamelin | Feb 11, 2015 11:21 AM | Tags: intermediate c++11

Quick A: Initialize it using a lambda that’s invoked immediately.

Recently on SO:

Best practice for declaring variable without initialising it, so auto is unavailable

I want to declare two variables of the same type, and have the compiler figure out the types. However I don't want to initialise one of the variables until later. I don't think I can use auto here, so what's the best option?

std::vector<int> v;

// `start` and `end` should be the same type
auto start = v.begin();
??? end;

// complicated code to assign a value to `end` (i.e. putting
// the code in a function or using ?: is not practical here) 
if (...) {
    end = ...;
} else {
    end = v.end();
}

What's the best way to tell the compiler that end should be the same type as start, but without having to initialise the variable?

auto start = v.begin(), end;  // Doesn't work, `end` has to be initialised
decltype(start) end;          // Does work, but not sure if it's best practice

Update

A couple of comments have suggested ways that would work in certain situations, so I am clarifying my situation here:

std::vector<int> v;
int amount = 123;

// `start` and `end` should be the same type
auto start = v.begin();
??? end;

// code to assign a value to `end`
if (amount) {
    end = start + amount;
    amount = 0;
} else {
    end = v.end();
}

I believe a lamda function would be trickier here, because amount is being reset to 0 after end is calculated, so in a lamda function that calculates a value for end, amount = 0 would have to come after the return statement. The only option would be to create more local variables, which would incur an (admittedly tiny) performance penalty.

The Rule of Zero revisited: The Rule of All or Nothing--Arne Mertz

By Adrien Hamelin | Feb 11, 2015 11:17 AM | Tags: c++11 basics

In this article you will find a new rule of thumb:

The Rule of Zero revisited: The Rule of All or Nothing

by Arne Mertz

From the article:

In 2012, Martinho Fernandes coined the Rule of Zero in a blog post. In 2014, Scott Meyers wrote a blog post about a concern with that rule and proposed a Rule of Five Defaults.

Back then, I had written a small comment on Scott’s post that deserves some further elaboration. In this post I am going to wrap up my thoughts about the two posts and propose a “Rule of All or Nothing”...

the asynchronous library - Christophe Henry @ Meeting C++ 2014

By Meeting C++ | Feb 11, 2015 10:24 AM | Tags: performance parallelism intermediate experimental c++11

A new video from Meeting C++ 2014:

the asynchronous library

by Christophe Henry

From the talk description:

An infrastructure library on which Boost Meta State Machine can build. This will be provided by the Asynchronous library: Active Objects, proxies, threadpools, parallelization algorithms, work-stealing, distributed programming...

2015-02 mid-meeting mailing available

By Blog Staff | Feb 11, 2015 09:49 AM | Tags: None

The 2015-02 mailing of new standards papers is now available.

NOTE: A number of these papers have already been publicized on this blog. This is the complete list including ones not previously publicized.