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Open Multi-Methods for C++11, Part 1 -- Jean-Louis Leroy

By Blog Staff | Aug 12, 2013 04:00 PM | Tags: None

jean-louis-leroy.pngNew on Code Project:

Open Multi-Methods for C++11, Part 1

by Jean-Louis Leroy

Note: We recommend first reading this C++ multimethods paper coauthored by Bjarne Stroustrup for more background.

From the article:

This article is the first in a series about open multi-methods for C++11. In this installment, I will explain what they are, how they fit in the object-oriented paradigm, and make controversial statements.

Subsequent articles will present a new library that implements open multi-methods, using the facilities provided by C++11 (in particular, variadic templates). The library's salient features are: fast, constant time dispatch using compact tables; arbitrary number of virtual and non virtual arguments; access to the next most specific specialization; and support for shared libraries and dynamic loading. The series will conclude with an in-depth presentation of the internals of the library. ...

Some Optimizations Are More Important Than Others -- Andrew Koenig

By Blog Staff | Aug 9, 2013 01:27 AM | Tags: None

From the desk of ARK:

Some Optimizations Are More Important Than Others

by Andrew Koenig

From the article:

[...] In short, the key to finding effective ways to speed up a program is to look at the parts of the program that dominate its execution time and find ways of speeding up those parts that require relatively little programmer effort to implement.

With this background in mind, let's think about moving data rather than copying it. Suppose, for example, that we have two functions, each of which takes a string argument: ...

Universal References and the Copy Constructor -- Eric Niebler

By Blog Staff | Aug 8, 2013 07:41 AM | Tags: advanced

The "universal references" term is getting traction:

Universal References and the Copy Constructor

by Eric Niebler

From the article:

At the most recent NWCPP meeting in Redmond, WA, the always-entertaining Scott Meyers shared his latest insights about so-called “universal references” and their pitfalls. In particular, he was warning about the hazards of overloading on universal references. His advice was good, I thought, but missed some important corner cases about the interactions between universal references and the special member functions. In this article, I show what the “special” problems are with the special member functions and universal references, and some ways to avoid the problems. ...

... 

Scott’s advice is simple and sound: avoid overloading on universal references. By which he means, don’t do this: 
template<typename T>
void foo( T const & t )
  {/*...*/}

template<typename T>
void foo( T && t )
  {/*...*/}

In the code above, the author presumably wanted all lvalues to go to the first and all rvalues to go to the second. But that’s not what happens. What happens is this: ...

Quick Q: Why do I have to write 'mutable' on a lambda? -- StackOverflow

By Blog Staff | Aug 5, 2013 11:04 AM | Tags: intermediate

Inquiring minds want to know:

Why does C++0x's lambda require “mutable” keyword for capture-by-value, by default?

Short example:

#include <iostream>

int main()
{
    int n;
    [&](){n = 10;}();             // OK
    [=]() mutable {n = 20;}();    // OK
    // [=](){n = 10;}();          // Error: a by-value capture cannot be modified in a non-mutable lambda
    std::cout << n << "\n";       // "10"
}

The question: Why do we need the mutable keyword? It's quite different from traditional parameter passing to named functions. What's the rationale behind?

I was under the impression that the whole point of capture-by-value is to allow the user to change the temporary -- otherwise I'm almost always better off using capture-by-reference, aren't I?

Any enlightenments?

(I'm using MSVC2010 by the way. AFAIK this should be standard)

Quick Q: Where do I have to write 'template' and 'typename'? -- StackOverflow

By Blog Staff | Aug 4, 2013 09:19 PM | Tags: intermediate

This question is an oldie but goodie. Note that in C++11 some of this becomes clearer, because it's now recommended to use constexpr and using template aliases instead of "traits" styles, and traits are one of the bigger (but not only) reasons to have a template refer to another template.

Where and why do I have to put the “template” and “typename” keywords?

In templates, where and why do I have to put typename and template on dependent names? What exactly are dependent names anyway? I have the following code: ...

Why Would You Ever Pass a Container By Value? -- Andrew Koenig

By Blog Staff | Jul 31, 2013 03:54 PM | Tags: basics

From the desk of ARK:

Why Would You Ever Pass a Container By Value?

by Andrew Koenig

From the article:

Consider two fundamental features of C++: functions and references. Which shall we teach first?

If we teach references first, there is the problem of coming up with interesting example programs that use references but completely avoid user-defined functions. This is hard to do because the most common use of references is as function parameters — so it's probably easier to teach functions first.

However, if we teach functions before we teach references, then every function we write must accept its arguments by value — references not yet being available as an alternative...

Contextually Converted to Bool -- Chris Sharpe

By Blog Staff | Jul 31, 2013 09:03 AM | Tags: intermediate

A nice writeup of when this "it just works the way you expect" feature kicks in:

Contextually converted to bool

by Chris Sharpe

From the article:

Something I found mildly surprising about C++11 is that this works:

#include <iostream>

struct Testable
{
    explicit operator bool() const { return true; }
};

int main()
{
    Testable t;
    if (t)
        std::cout << "Converted to true!\n";
}

That is, it compiles and prints Converted to true!.

The new bit here is the explicit keyword. When I first saw an example like this, I expected to have to write

if (bool( t )) // ...

[...]