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Some Subtleties of Aliasing -- Andrew Koenig

By Blog Staff | May 12, 2013 04:59 PM | Tags: intermediate

Why "aliasing can cause paradoxical behavior"...

Some Subtleties of Aliasing

by Andrew Koenig

 

... As an example of a problem with this code, I suggested that it might be called by writing

 

scale(v, v[n]);

with the aim of dividing every element of v by v[n], but that this call would actually do something else entirely. From the comments on the article, I think that some readers were unclear about what that something else might be. To understand this call's behavior in detail, we have to look at the code more closely. ...

Quick Q: What's the best way to return multiple values? -- StackOverflow

By Blog Staff | May 12, 2013 01:55 PM | Tags: basics

Another example of how adding a feature, in this case move semantics, makes using the language simpler:

Interface for returning a bunch of values

I have a function that takes a number and returns up to that many things (say, ints). What's the cleanest interface?

  1. Return a vector<int>. The vector would be copied several times, which is inefficient.
  2. Return a vector<int>*. My getter now has to allocate the vector itself, as well as the elements. There are all the usual problems of who has to free the vector, the fact that you can't allocate once and use the same storage for many different calls to the getter, etc. This is why STL algorithms typically avoid allocating memory, instead wanting it passed in.
  3. Return a unique_ptr<vector<int>>. It's now clear who deletes it, but we still have the other problems.
  4. Take a vector<int> as a reference parameter. The getter can push_back() and the caller can decide whether to reserve() the space. However, what should the getter do if the passed-in vector is non-empty? Append? Overwrite by clearing it first? Assert that it's empty? It would be nice if the signature of the function allowed only a single interpretation.
  5. Pass a begin and end iterator. Now we need to return the number of items actually written (which might be smaller than desired), and the caller needs to be careful not to access items that were never written to.
  6. Have the getter take an iterator, and the caller can pass an insert_iterator.
  7. Give up and just pass a char *. smile

Quick Q: How can I make my type construct from { 1, 2, 3, 4 } like vector? -- StackOverflow

By Blog Staff | May 9, 2013 06:56 AM | Tags: basics

Quick A: Write a constructor that takes a std::initializer_list.

See the link for the nice longer answer:

how to implement an initializer list for user defined type? (analogus to std::vector initializer list)

std::vector can be initialized as

 

std::vector<std::string> words1 {"the", "frogurt", "is", "also", "cursed"};
ref

Now if want to achieve the similar functionality for one of my types how do I go about doing it? How should I implement the constructor for this functionality?

Quick Q: How does return by rvalue reference work? -- StackOverflow

By Blog Staff | May 8, 2013 01:27 PM | Tags: basics

Quick A: So easily that it's automatic... just return a value, not a reference.

How does return by rvalue reference work?

Just when I thought I kind of understand rvalue reference, I ran into this problem. The code is probably unnecessarily long, but the idea is quite simple. There is a main() function, and returnRValueRef() function.

[...]

AClass && returnRValueRef() {
  AClass a1(4);
  return move(a1);
}

int main() {
  AClass a;
  a = returnRValueRef();
}

Quick Q: What's the difference between vector<T...> and vector<T>...? -- StackOverflow

By Blog Staff | May 8, 2013 09:22 AM | Tags: intermediate

SO's version of the question added one layer of wrapping:

What's the difference between doing vector<vector<T...>> and vector<vector<T>...>

I saw code like this earlier:

 

using A = std::vector<std::vector<T>...>

where T is a variadic list of template arguments. I wanted to know what the difference is between putting the parameter pack at the end of the last angle bracket and the first. For example:

using B = std::vector<std::vector<T...>>;

Both of these two compile fine but I don't know what the difference is.

Can someone explain? Thanks.

Introduction To the C++11 Feature: Delegating Constructors -- sumi_cj

By Blog Staff | May 7, 2013 11:35 AM | Tags: basics

Head on over to the C/C++ Cafe for:

Introduction to the C++11 feature: delegating constructors

by sumi_cj

Excerpt:

In C++98, if a class has multiple constructors, these constructors usually perform identical initialization steps before executing individual operations. In the worst scenario, the identical initialization steps are copied and pasted in every constructor. See the following example: ...

Introduction To the C++11 Feature: Extended friend Declaration -- FangLu

By Blog Staff | May 6, 2013 11:26 PM | Tags: intermediate basics

Head on over to the C/C++ Cafe for:

Introduction to the C++11 feature: extended friend declaration

by FangLu

Excerpt:

The extended friend declaration feature is newly introduced in the C++11 standard. In this article, I will introduce this feature and provide some examples on how to use this feature.

 

Firstly, let's see why this feature is added into C++11. ...

 

GotW #1: Variable Initialization—or Is It? -- Herb Sutter

By Blog Staff | May 6, 2013 06:46 PM | Tags: intermediate basics

Herb Sutter is resuming his Guru of the Week series of problem-and-solution articles about coding in C++, with the intent to gradually update the 88 existing issues and write a few more along the way.

The first problem was posted today:

GotW #1: Variable Initialization -- or Is It? (3/10)

by Herb Sutter

This first problem highlights the importance of understanding what you write. Here we have a few simple lines of code — most of which mean something different from all the others, even though the syntax varies only slightly.

The solution is coming "soon"...