Quick Q: When should you make a type non-movable in C++11? -- StackOverflow

By Blog Staff | Jan 13, 2013 04:48 AM | Tags: basics

So C++11 has these cool move semantics... but when would you not implement move construction and move assignment for your type?

When to make a type non-movable in C++11?

I was surprised this didn't show up in my search results, I thought someone would've asked this before, given the usefulness of move semantics in C++11:

When do I have to (or is it a good idea for me to) make a class non-movable in C++11?

(Reasons other than compatibility issues with existing code, that is.)

Quick Q: What does T&& mean in C++11? -- StackOverflow

By Blog Staff | Jan 12, 2013 10:07 AM | Tags: None

This question is so common it immediately got triple-digit upvotes on the question and answer:

What does T&& mean in C++11?

I've been looking into some of the new features of C++11 and one I've noticed is the double ampersand in declaring variables, like T&& var.

 

For a start, what is this beast called? I wish Google would allow us to search for punctuation like this.

What exactly does it mean?

 

At first glance, it appears to be a double reference (like the C-style double pointers T** var), but I'm having a hard time thinking of a use case for that.

When does a constexpr function get evaluated at compile time? -- StackOverflow

By Blog Staff | Jan 11, 2013 10:14 AM | Tags: intermediate basics

Here's a common question about constexpr...

A suggestion: As of this writing the more correct and useful (and simpler) answer K-ballo's, which was not selected as best -- please upvote K-ballo and help approve the pending edit that improves it. Thanks.

When does a constexpr function get evaluated at compile time?

Since it is possible that a function declared as constexpr can be called during run-time, under which criteria does the compiler decide whether to compute it at compile-time or during runtime?

 

template<typename base_t, typename expo_t>
constexpr base_t POW(base_t base, expo_t expo)
{
    return (expo != 0 )? base * POW(base, expo -1) : 1;
}

int main(int argc, char** argv)
{
    int i = 0;
    std::cin >> i;
    std::cout << POW(i, 2) << std::endl;
    return 0;
}

 

In this case, i is unknown at compile-time, which is probably the reason why the compiler treats POW() as a regular function which is called at runtime. This dynamic however, as convenient as it may appear to be, has some impractical implications. For instance, could there be a case where I would like the compiler to compute a constexpr function during compile-time, where the compiler decides to treat it as a normal function instead, when it would have worked during compile-time as well? Are there any known common pitfalls?

Quick Q: Is it still bad practice to return a vector from a function?

By Blog Staff | Jan 11, 2013 10:04 AM | Tags: intermediate basics

Here's another FAQ about modern C++11 style, and how C++11 is simpler than classic C++, including that this affects how we design our interfaces to make them simpler and easier to read and use.

However, be sure to read through the comments, because they cover several considerations including when it's safe to start relying on the simpler C++11 semantics as you migrate a code base from C++98 to C++11 and may still have to support C++98 clients for a while.

In C++, is it still bad practice to return a vector from a function?

Short version: It's common to return large objects—such as vectors/arrays—in many programming languages. Is this style now acceptable in C++0x if the class has a move constructor, or do C++ programmers consider it weird/ugly/abomination?

Long version: In C++0x is this still considered bad form?

std::vector<std::string> BuildLargeVector();

...

std::vector<std::string> v = BuildLargeVector();

 

[...]

Quick Q: C++ template typedef -- StackOverflow

By Blog Staff | Jan 11, 2013 09:46 AM | Tags: basics

In the "look how simple this is now in C++11" department, this just in on SO:

C++ template typedef

I have a class

template<size_t N, size_t M>

class Matrix {

    // ....

};

I want to make a typedef which creates a Vector (column vector) which is equivalent to a Matrix with sizes N and 1. Something like that:

typedef Matrix<N,1> Vector<N>;

Which produces compile error. The following creates something similar, but not exactly what I want:

template <int N>

class Vector: public Matrix<N,1>

{ };

Is there a solution or a not too expensive workaround / best-practice for it? Thanks in advance!

Quick Q: Does the range-based for loop make std algorithms obsolete? -- StackOverflow

By Eric Niebler | Jan 10, 2013 02:56 PM | Tags: basics

Here's a fine question from StackOverflow[C++11]. Click through for some fine answers.

Does the Range-based for Loop Make std Algorithms Obsolete?

Stroustrup's Tour of C++: Third chapter posted

By Blog Staff | Jan 9, 2013 10:22 AM | Tags: basics

Part 3 of Bjarne Stroustrup's draft Tour of C++ is now available. This material is a preview draft of Chapter 4 of Stroustrup's upcoming The C++ Programming Language, 4th Edition.

A Tour of C++, Part 3: Containers and Algorithms

by Bjarne Stroustrup

Stroustrup writes:

No significant program is written in just a bare programming language,
it would be too tedious.

However, just about any task can be rendered simple by the use of good libraries.

This third chapter of my tour of C++ begins the presentation of the standard library, which is about half of the C++ standard.

Constructive comments would be most welcome.

C++ Concurrency - Herb Sutter

By Blog Staff | Jan 8, 2013 01:42 PM | Tags: intermediate

Another C++ and Beyond 2012 talk is now available online on Channel 9.

[Ed.: Note that the talk title and abstract should read as below. The initial Channel 9 video posting used an outdated title and abstract, and will be corrected soon.]

C++ and Beyond 2012: C++ Concurrency

by Herb Sutter

Herb says:

I've spoken and written on these topics before. Here's what's different about this talk:

 

• Brand new: This material goes beyond what I've written and taught about before in my Effective Concurrency articles and courses.
• Cutting-edge current: It covers the best-practices state of the art techniques and shipping tools, and what parts of that are standardized in C++11 already (the answer to that one may surprise you!) and what's en route to near-term standardization and why, with coverage of the latest discussions.
• Blocking vs. non-blocking: What's the difference between blocking and non-blocking styles, why on earth would you care, which kinds does C++11 support, and how are we looking at rounding it out in C++1y?

The answers all matter to you – even the ones not yet in the C++ standard – because they are real, available in shipping products, and affect how you design your software today.

Preconditions, Part 1 -- Andrzej Krzemieński

By Blog Staff | Jan 7, 2013 11:43 AM | Tags: intermediate

On preconditions, and their compile time enforcement with static_assert and a dash of regex.

Preconditions, Part 1

by Andrzej Krzemieński

In this post, I want to share my thoughts about the notion of precondition. In “Design by Contract” philosophy, preconditions are always mentioned along postconditions and invariants, and in the context of OO design. In this post I focus only on preconditions and not necessarily related to any class. For instance, the following function specifies a precondition on its argument:

double sqrt(double x);

// precondition: x >= 0

Note that the function specifies the precondition even though there is no language feature for this purpose (at least in C++). A precondition is a “concept” or an “idea” rather than a language feature. This is the kind of preconditions that this post is about.

Continue reading...

C++ and Beyond 2012: Panel - Convincing your Colleagues

By Blog Staff | Jan 4, 2013 06:57 AM | Tags: basics

A new C++ and Beyond 2012 panel is now available:

C++ and Beyond 2012: Panel - Convincing your Colleagues

From C++ and Beyond 2012, Andrei, Herb and Scott present Convincing Your Colleagues - an interactive panel.

Abstract:

You can't do a better job if you don't change what you're doing, but change is hard.  It's especially hard when what needs to change is your colleagues' approach to software development. Moving your team forward often requires persuading your peers to change their behavior, sometimes to do something they're not doing, other times to stop doing something they've become accustomed to.  Whether the issue is to embrace or avoid C++ language features, to adopt new development tools or abandon old ones, to increase use of or scale back on overuse of design patterns, to adhere to coding standards, or any of the plethora of other matters that affect software creation, moving things forward typically requires getting your colleagues to buy into the change you're proposing.  But how can you do that?

In this panel session, Andrei, Herb, and Scott share how they go about convincing their colleagues to change and take questions from the audience.