Channels ▼

Andrew Koenig

Dr. Dobb's Bloggers

Elegance or Trickery: How To Free Memory

October 12, 2011

I'm continuing the elegance or trickery theme with an example that I think I originally got from Herb Sutter.

Suppose v is a vector<T>, and you want to free all of the memory that v is using. The obvious way to do so is


Alas, as so often happens in programming, the obvious solution doesn't do quite what we expect. This statement is equivalent to

v.erase(v.begin(), v.end());

which does, indeed, destroy all of v's elements. What it does not do is free all of the memory that those elements occupied. In fact, the library is prohibited from doing so, because calling v.erase is not allowed to change v's capacity, which is the number of elements that v can hold without reallocation.

One way to free that memory is to create a temporary, empty vector, and swap that vector's contents with v's elements. Then destroying the temporary vector will free the memory that v formerly used:

{ vector<T> temp;
   v.swap(temp); }

One would think that it would be possible to avoid having to make up a name for this vector by writing


instead of defining a named variable. Unfortunately, this example does not work because vector<T>() is an rvalue, and the parameter of v.swap expects an lvalue. What does work, however, is to swap the arguments of swap, so to speak, and write it this way:


The point is that even though vector<T>() is an rvalue, one can still call its member functions. In particular, it is possible to call its swap member function, so long as you give that member function an lvalue — in this case, v — as its argument.

As before, I invite discussion: Is this usage elegant, tricky, or both? And what characteristics of the code lead you to that conclusion?

Related Reading

More Insights

Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.