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Quick Q: How to initialize a const object (say vector) with complex initialization? -- StackOverflow

By Blog Staff | Mar 5, 2013 10:56 AM | Tags: basics

Quick A: With a lambda function. Try const mytype myobj{ []{ /* compute value */ return value; } }.

How would you initialize a const vector of function results using C++11?

Is it possible to use something like generate_n to create a const vector of, say, random numbers? I couldn't think of a way to do it without deriving vector and doing the assignment in the constructor.

C++ and Xcode 4.6 -- from Marshall Clow

By Eric Niebler | Mar 1, 2013 01:36 PM | Tags: basics

Marshall Clow gives some helpful tips for C++ programmers migrating to to XCode 4.6:

C++ and XCode 4.6

So, you’ve installed Xcode 4.6, and you are a C++ programmer.

You want to use the latest and greatest, so you create a new project, and add your sources to the project, and hit Build, and … guess what? Your code doesn’t build!

Continue reading...

 

No, really, moving a return value is easy -- StackOverflow

By Blog Staff | Feb 25, 2013 08:55 AM | Tags: basics

People new to C++11 often hear about move semantics, and expect that they have to do work to take advantage of it. That's often not true, and often the cleanest, simplest code that doesn't even mention move or && anywhere is just what you want -- that's C++11, clean, safe, and faster than ever.

Perhaps the most common case (and question) involves returning values from functions. The new rule for modern C++ style: Just return even big objects by value, and move Just Happens.

It just came up again on StackOverflow:

C++11 rvalues and move semantics confusion

The link skips straight to Howard Hinnant's clear and correct answer.

Sometimes we just try too hard, because we expect efficient programming not to be easy. Welcome to C++11.

Quick Q: When should I use std::function vs. make my function a template? -- StackOverflow

By Blog Staff | Feb 21, 2013 09:25 AM | Tags: intermediate

To accept a functor as a parameter, when should you:

  • accept a std::function, which adds an indirection, vs.
  • make your function a template<class Func> and accept a Func, which can bind directly to whatever is passed?

std::function vs template

Thanks to C++11 we received the std::function family of functor wrappers. Unfortunately, I keep hearing [...] that they are horribly slow. [... Is the right recommendation] that functions can be used as de facto standard of passing functors, and in places where high performance is expected templates should be used?

Quick Q: How do I move an expensive object into a map? -- StackOverflow

By Blog Staff | Feb 19, 2013 04:39 AM | Tags: basics

Quick A: Using the form of insert that takes an rvalue and passing a temporary or a std::move'd object, or calling emplace.

Moving an object into a map

The problem with this is that the huge objects will be copied into the maps

 

Huge huge1(some,args);
Huge huge2(some,args);

std::map<int,Huge> map1;
std::map<Huge,int> map2;

map1.insert({0,huge1});
map2.insert({huge2,0});

how can I guarantee a move? Will this work or is there more to it?

map1.insert({0,std::move(huge1)});
map2.insert({std::move(huge2),0});

Quick Q: Is the safe-bool idiom obsolete in C++11? -- StackOverflow

By Blog Staff | Feb 18, 2013 05:32 PM | Tags: intermediate basics

Quick A: Yes. Another way that modern C++ is safer and simpler.

(If you don't know what the safe-bool idiom is, don't worry. It's  a workaround that's now obsolete.)

Xeo asked:

Is the safe-bool idiom obsolete in C++11?

This answer of @R. Martinho Fernandes shows, that the safe-bool idiom is apperently deprecated in C++11, as it can be replaced by a simple

explicit operator bool() const;

... Is our assumption in the title correct? I hope we didn't overlook any potential drawbacks.

Preconditions, Part 2 -- Andrzej Krzemieński

By Blog Staff | Feb 15, 2013 10:38 AM | Tags: intermediate basics

Andrzej continues this month with more interesting thoughts on preconditions.

Preconditions, Part 2

by Andrzej Krzemieński

In this post I will continue sharing my thoughts on preconditions. It will cover some philosophy behind the concept of preconditions (and bugs), and investigate the possibility of employing the compiler to verify some preconditions. Many people provided a useful feedback on my previous post. I will also try to incorporate it into this post.

Note that this article diverges from recommended practice in one way... it hints at the idea of throwing exceptions to report precondition violations. Instead, per C++ Coding Standards and other established guidance, prefer to use assertions to check preconditions: precondition violations are just bugs in the caller's code that should be caught at test time, assertions cause no overhead in production, and assertions fire immediately at the line of code that contains the bug without losing the call stack and other local context. Using assertions is still considered to be a best practice.

Stroustrup’s Tour of C++: Fourth chapter posted

By Blog Staff | Feb 8, 2013 09:36 AM | Tags: basics

The final installment of Bjarne Stroustrup's four-part Tour of C++ is now available. This material is a preview draft of Chapter 5 of Stroustrup’s upcoming The C++ Programming Language, 4th Edition.

A Tour of C++, Part 4: Concurrency and Utilities

by Bjarne Stroustrup

Bjarne writes:

Describe all of C++ in 100 pages (or less). Don't just describe the language, include the standard library. Don't use "white lies" to simplify. Describe the major programming styles and techniques. Give rationale. Don't forget about concurrency. And, oh, by the way, make it readable to programmers (do not require a PhD).

That was the task I set myself when I decided to write the "Tour of C++" for TC++PL4. I suspect that succeeding perfectly is beyond me, but at least I met the first criteria: The tour is currently 98 pages, and shrinking.

This last, part 4, of the tour presents concurrency and some of the newer standard-library facilities.

Enjoy!

See the whole Tour here.