Complex Initialization for a Const Variable -- Herb Sutter

By Blog Staff | Apr 5, 2013 01:55 PM | Tags: intermediate

How do you declare a const int variable when you have still have to do some computation to initialize it (so it shouldn't be const at first) but then want it be const after that?

Complex Initialization for a Const Variable

by Herb Sutter

 

On std-discussion, Shakti Misra asked:

> I have seen in a lot of places code like

int i;
if(someConditionIstrue)
{
    Do some operations and calculate the value of i;
    i = some calculated value;
}
use i; //Note this value is only used not changed. It should not be changed.

 

Olaf nailed it: The way to do it is with a lambda. ...

Ten C++11 Features Every C++ Developer Should Use -- Marius Bancila

By Blog Staff | Apr 1, 2013 05:28 PM | Tags: basics

Ignoring the dangers of linking to items published on April 1, we offer:

Ten C++11 Features Every C++ Developer Should Use

by Marius Bancila

This article discusses a series of features new to C++11 that all developers should learn and use. There are lots of new additions to the language and the standard library, and this article barely scratches the surface. However, I believe some of these new features should become routine for all C++ developers. You could probably find many similar articles evangelizing different C++11 features. This is my attempt to assemble a list of C++ features that should be a norm nowadays. Table of contents:

• auto
• nullptr
• Range-based for loops
• Override and final
• Strongly-typed enums
• Smart pointers
• Lambdas
• non-member begin() and end()
• static_assert and type traits
• Move semantics

Quick Q: How to accept lambdas as callbacks? -- StackOverflow

By Blog Staff | Apr 1, 2013 04:29 AM | Tags: intermediate

The poster is definitely thinking along the right lines -- anything callable that would have accepted a pointer to function and/or functor in C++98 should be written to be able to accept a lambda function in modern C++.

So what about callbacks as a specific example?

Passing and storing lambda function as callbacks

I was wondering if this would be an accepted approach to writing callbacks:

Storing callbacks:

struct EventHolder {
    std::function<void()> Callback;
    EventTypes::EventType Type;
};
std::vector<Events::EventHolder> EventCallbacks;

Method definition:

void On(EventType OnEventType,std::function<void()>&& Callback)
{
    Events::EventHolder NewEvent;
    NewEvent.Callback=std::move(Callback);
    NewEvent.Type=OnEventType;
    EventCallbacks.push_back(std::move(NewEvent));
}

Binding event:

Button->On(EventType::Click,[]{
    // ... callback body
});

My biggest question would be regarding passing the Callback by value. Is this a valid approach?

Quick Q: What's the right way to use && parameters? -- StackOverflow

By Blog Staff | Mar 29, 2013 11:09 AM | Tags: intermediate

From SO:

Correct usage of rvalue references as parameters

Let's take the following method as an example: [...]

void Asset::Load( const std::string& Path )
{
    // complicated method....
}
void Asset::Load( std::string&& Path )
{
     Load(Path); // call the above method
}

Is this a correct approach to writing rvalue versions of methods?

Quick Q: What's the difference between result_of and decltype? -- StackOverflow

By Blog Staff | Mar 29, 2013 08:04 AM | Tags: intermediate

From SO:

What's the difference between result_of<F(Args…> and decltype<f(args…)>?

I see that std::async is specified as follows:

template <class F, class... Args>                   // copied out of the standard
future<typename result_of<F(Args...)>::type>
async(F&& f, Args&&... args);

I had expected it to be declared like this:

template <class F, class... Args>
auto async(F&& f, Args&&... args) ->
  future<decltype(f(forward<Args>(args)...)>;

Would that be equivalent, or is there some way in which the use of result_of is preferable to the use of decltype? (I understand that result_of works with types, while decltype works with expressions.)

Preface for The C++ Programming Language 4th Ed. now available

By Blog Staff | Mar 29, 2013 07:37 AM | Tags: None

The Preface for The C++ Programming Language, 4th Ed., is now available on InformIT and also appears in full on this site's Tour of C++ page.

Preface to TC++PL4e

All problems in computer science can be solved by another level of indirection, except for the problem of too many layers of indirection.

            -- David J. Wheeler

C++ feels like a new language. That is, I can express my ideas more clearly, more simply, and more directly in C++11 than I could in C++98. Furthermore, the resulting programs are better checked by the compiler and run faster. ...

The use of C++ has changed dramatically over the years and so has the language itself. From the point of view of a programmer, most of the changes have been improvements. The current ISO standard C++ (ISO/IEC 14882:2011, usually called C++11) is simply a far better tool for writing quality software than were previous versions. How is it a better tool? What kinds of programming styles and techniques does modern C++ support? What language and standard-library features support those techniques? What are the basic building blocks of elegant, correct, maintainable, and efficient C++ code? Those are the key questions answered by this book. Many answers are not the same as you would find with 1985, 1995, or 2005 vintage C++: progress happens.

Continue reading...

Quick Q: So what's "lite" about "concepts lite" vs. full concepts?

By Blog Staff | Mar 28, 2013 03:47 PM | Tags: experimental

There's a Q&A on StackOverflow, but see also the discussion about this in the March 12 Concepts conference call minutes that were posted here on the same day as the call. It's really more than minutes, it's also a record of discussion that answers this and other questions.

From SO:

What are the differences between concepts and template constraints?

I want to know what are the semantic differences between the C++ full concepts proposal and template constraints (for instance, constraints as appeared in Dlang or the new concepts-lite proposal for C++1y).

What are full-fledged concepts capable of doing than template constraints cannot do?

Quick Q: How should you use the standard smart pointers as members? -- StackOverflow

By Blog Staff | Mar 26, 2013 10:40 PM | Tags: basics

From StackOverflow:

Using smart pointers for class members

I'm having trouble understanding the usage of smart pointers as class members in C++11. I have read a lot about smart pointers and I think I do understand how unique_ptr and shared_ptr/weak_ptr work in general. What I don't understand is the real usage. It seems like everybody recommends using unique_ptr as the way to go almost all the time. But how would I implement something like this: ...

Quick Q: Why does emplace_back need a (copy or) move constructor? -- StackOverflow

By Blog Staff | Mar 25, 2013 03:04 PM | Tags: None

Quick A: It needs it when the container is a vector or similar, because the container may need to grow and reallocate which includes moving or copying the existing contents to the new location.

why does emplace_back need move constructor?

I have the following code... But the emplace_back doesn't use the move constructor. Why does the initialization require a move constructor in this instance?

Quick Q: How do you enable C++11 support on Android? -- StackOverflow

By Blog Staff | Mar 25, 2013 11:08 AM | Tags: None

C++11 support is already available by default in many environments and platforms, but as it reaches new ones you may still need to turn it on explicitly. Android is an example:

Enable C++11 support on Android

How do I integrate C++11 in Android in order to use unordered_map (the new version of hash_map)?