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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>uSTL - the small STL library</title>
<link rel="stylesheet" type="text/css" href="style/default.css" />
<meta http-equiv="Content-Type" content="text/xhtml+xml; charset=ISO-8859-1" />
<meta name="Description" content="API and usage description for uSTL, a size-optimized STL implementation" />
<meta name="Keywords" content="C++, STL, template, bloat, optimization" />
<meta name="author" content="Mike Sharov" />
<meta name="date" content="2011-03-16" />
</head>
<body>
<div class="banner">
<h1>uSTL</h1>
<div class="motto">Candy for the optimization nut in you</div>
</div>
<hr class="banner" />
<h2><a name="Introduction">Introduction</a></h2>
<p>
The C++ standard template library (STL) is a collection of common
containers and algorithms in template form. Unfortunately its standard
incarnation shipped with gcc is implemented without much concern for code
size. Not only is the library itself large, the current version being
over a megabyte in size, but with all the code you instantiate by using a
vector for each of your containers, it is easy to become fearful and opt
for using static arrays instead or, worse yet, abandon C++ altogether
for C. This is especially painful to former DOS assembly programmers
like myself, who fret endlessly when the size of the executable crosses
the magic 64k boundary, forgetting that nobody cares about memory anymore.
</p><p>
Of course, these days everyone has gigabytes of RAM and has no compunction
about loading up OpenOffice, whose source tree is over a gigabyte in
size. Why then bother with saving a kilobyte of code here and there? I
can't really say. Maybe it's that warm fuzzy knowledge that you are making
maximum possible use of your computer's resources. Maybe it's that thrill
you get after expressing your program's functionality in the fewest
possible instructions and the minimum imaginable overhead. Or maybe it
really is of no importance and any code bloat will be easily overcome
by faster processors in some near future. I just know what I like, and
it's the sight of clean, concise, and fast code. Therefore this library.
</p>
<h2>Contents</h2>
<ul>
<li><a href="#Installation">Installation</a></li>
<li><a href="#Containers">Containers and Iterators</a></li>
<li><a href="#Strings">Strings</a></li>
<li><a href="#Algorithms">Algorithms</a></li>
<li><a href="#Memblocks">Memblock and Memlink</a></li>
<li><a href="#Streams">Streams</a></li>
<li><a href="#Tuples">Tuples</a></li>
<li><a href="#Exceptions">Exceptions</a></li>
<li><a href="#Savings">Template Bloat Be Gone</a></li>
<li><a href="#Contact">Bug reporting</a></li>
<li><a href="html/index.html">Doxygen reference</a></li>
</ul>
<h2><a name="Installation">Installation</a></h2>
<p>
To start with you'll need a decent compiler. Although uSTL will compile
under gcc 2.95, some features require at least gcc 3.4 and are simply
turned off with an older version. C++ support is vastly improved in
the recent compiler versions, and I strongly recommend gcc 4 for best
possible code.
</p><p>
The latest version of uSTL can always be downloaded from its SourceForge
<a href="https://sourceforge.net/project/showfiles.php?group_id=76798">project files page</a>.
If you like living dangerously, you can pull the working branch directly from
<a href="http://ustl.git.sourceforge.net/git/gitweb.cgi?p=ustl">git://ustl.git.sourceforge.net/gitroot/ustl/ustl</a>.
The mainline source should build on any unix-based system, including Linux,
BSD, MacOS, SunOS, and Solaris. A separate port for Symbian OS is maintained by
<a href="http://www.penrillian.com/index.php?option=com_content&amp;task=view&amp;id=82&amp;Itemid=73">Penrillian</a>.
Windows-based systems and weird embedded platforms, are not, and will
not be supported by the mainline. However, if you make a port, I'll be
happy to mention it here. After unpacking:
</p><pre>
./configure
make install
</pre><p>
<kbd>./configure --help</kbd> lists available build options.
You might want to specify a different installation prefix with
<kbd>--prefix=/usr</kbd>; the default destination is /usr/local.
Developers will want to build with <kbd>--with-debug</kbd> to get a
lot of assert error checking, which I highly recommend. If you have
gcc 4.4 or later, you may want to also use <kbd>--force-inline</kbd>
(see the bottom of this page for a fuller explanation). If you are the
type to edit configuration manually, it's in Config.mk and config.h. When
it's built, you can run the included tests with <kbd>make check</kbd>.
Finally, here's a simple hello world application:
</p><pre>
#include &lt;ustl.h&gt;
using namespace ustl;
int main (void)
{
cout &lt;&lt; "Hello world!\n";
return (EXIT_SUCCESS);
}
</pre><p>
If you have at least gcc 3.4, uSTL is built as a standalone library,
without linking to libstdc++ (except on BSD platforms where gcc does not
support it) Because g++ links to it by default, you'll need to link your
applications with gcc, or to pass <kbd>-nodefaultlibs -lc</kbd> to g++
if you want to use uSTL to completely replace libstdc++. This is where
the actual space savings happen. (If you want to see just how much you can
save, skip to the <a href="#Savings">Template Bloat Be Gone</a> section)
</p>
<h2><a name="Containers">Containers and Iterators</a></h2>
<p>
STL containers provide a generic abstraction to arrays, linked lists,
and other methods of memory allocation. They offer the advantages
of type-safety, the peace of mind that comes from never having to
malloc anything again, and a standard access API called iterators. Each
container's API is equivalent to that of a simple array, with iterators
being the equivalent of pointers into the array. The uniform access
API allows creation of standardized algorithms, discussed futher down,
that work on any container. Here are some examples of using vector,
the container representing a simple array:
</p><pre>
vector&lt;int&gt; v;
v.push_back (1);
v.push_back (2);
v[1] = 0;
v.erase (v.begin() + 1);
v.pop_back();
v.insert (v.begin(), 4);
v.resize (15);
</pre><p>
As you can see, a vector is basically the same thing as the arrays
you use now, except that it is resizable. The function names
ought to be self-explanatory with the exception of the addressing
arguments. You can do index addressing and get free bounds checking
with asserts. Incidentally, I highly recommend you work with a debug
build when writing code; uSTL is chock full of various asserts checking
for error conditions. In the optimized build, most such errors will be
silently ignored where possible and will cause crashes where not. That
is so because they are programmer errors, existing because you have a
bug in your code, not because the user did something wrong, or because
of some system failure. Programmer errors assert. User or system errors
throw exceptions.
</p><p>
Vectors are addressed with iterators, which are just like pointers (and
usually are). Calling begin() gives you the pointer to the first element,
calling end() gives you the pointer to the end of the last element. No,
not the last element, the end of it, or, more accurately, the end of the
array. It's that way so you can keep incrementing an iterator until it
is equal to the end() value, at which point you know you have processed
all the elements in the list. This brings me to demonstrate how you
ought to do that:
</p><pre>
foreach (vector&lt;int&gt;::iterator, i, v)
if (*i &lt; 5 || *i &gt; 10)
*i = 99;
</pre><p>
Although the foreach macro is a uSTL-only extension, it is a one-liner
you can easily copy out of uutility.h if you ever want to switch back to
regular STL. It is a great way to ensure you don't forget to increment
the counter or run past the end of the vector. The only catch to be aware
of, when inside an iterator loop, is that if you modify the container,
by adding or removing entries, you have to update the iterator, since the
container memory storage may have moved when resized. So, for example,
if you wish to remove certain types of elements, you'd need to do use
an index loop or something like:
</p><pre>
foreach (vector&lt;CEmployee&gt;::iterator, i, employees)
if (i-&gt;m_Salary &gt; 50000 || i-&gt;m_Performance &lt; 100)
--(i = employees.erase (i));
</pre><p>
This is pretty much all there is to say about containers. Create them,
use them, resize them, that's what they are for. There are other
container types, but you will probably not use them much. There's
<code>set</code>, which is a perpetually sorted vector, useful when you
want to binary_search a large collection. There's <code>map</code> which
is an associative container where you can look up entries by key. Its
utility goes down drastically when you have complex objects that need to
be searched with more than one parameter, in which cast you are better
off with vector and foreach. I have never needed the others, and do
not recommend their use. Their implementations are fully functional,
but do not conform to STL complexity guarantees and are implemented as
aliases to vector, which naturally changes their performance parameters.
</p>
<h2><a name="Strings">Strings</a></h2>
<p>
Every program uses strings, and STL was kind enough to provide a
specification. uSTL deviates a bit from the standard by not implementing
wchar strings. There is only one <code>string</code> class, which assumes
all your strings will be UTF8-encoded, and provides some additional
functionality to make working with those easier. I did that for the same
reason I dropped the locale classes; bloat. It is simply too expensive to
implement the standard locale classes, as the enormous size of libstdc++
illustrates. If you need them, you can still include them from libstdc++,
but it may be just as simple to use the locale support provided by libc
through printf, which may be called through <code>format</code> functions
in string and ostringstream.
</p><p>
Anyway, back to strings. You can think of the string object as a
char vector with some additional operations built-in, like searching,
concatenation, etc.
</p><pre>
string s ("Hello");
s += ' ';
s += "world?";
s.replace (s.find ('?'), 1, "!");
s[3] = s[s.find_first_of("lxy")];
s[s.rfind('w')] = 'W';
s.format ("A long %zd number of 0x%08lX\n", 345u, 0x12345);
cout &lt;&lt; s &lt;&lt; endl;
</pre><p>
A nonstandard behaviour you may encounter is from linked strings created
by the string constructor when given a null-terminated const string. In
the above example, the constructor links when given a const string and
stays as a const link until the space is added. If you try to write to it,
you'll get an assert telling you to use copy_link first to convert the
link into a copy. Resizing the linked object automatically does that for
you, so most of the time it is transparent. You may also encounter another
instance of this if you try getting iterators from such an object. The
compiler uses the non-const accessors by default for local objects,
so you may need to declare it as a const string if you don't wish to
copy_link. Why does uSTL string link instead of copying? To save space
and time. All those strings are already in memory, so why waste heap
space and processor time to copy them if you just want to read them? I
thought it a good tradeoff, considering that it is trasparent for the
most common uses.
</p><p>
Other nonstandard extensions include a <code>format</code> function to
give you the functionality of sprintf for string objects. Another is
the UTF8 stuff. Differing a bit from the standard, <code>size</code>
returns the string length in bytes, <code>length</code> in characters.
You can iterate by characters instead of bytes with a special utf8
iterator:
</p><pre>
for (string::utf8_iterator i = s.utf8_begin(); i &lt; s.utf8_end(); ++ i)
DrawChar (*i);
</pre><p>
or just copy all the chars into an array and iterate over that:
</p><pre>
vector&lt;wchar_t&gt; result (s.length());
copy (s.utf8_begin(), s.utf8_end(), result.begin());
</pre><p>
To write wide characters to the string, wchar_t values can be directly
given to push_back, insert, append, or assign, in the same way as the
char ones.
</p><p>
A few words must be said regarding reading wide characters. The shortest
possible rule to follow is "don't!" I have received a few complaints about
the fact that all offsets given to and returned by string functions are
byte offsets and not character offsets. The problem with modifying or even
looking for specific wide characters is that you are not supposed to know
what they are. Your strings will be localized into many languages and it
is impossible for you to know how the translation will be accomplished.
As a result, whenever you are hardcoding a specific character value,
or a specific character length (like a three-character extension),
you are effectively hardcoding yourself into a locale. The only valid
operation on localized strings is parsing it via standard delimiters,
treating anything between those delimiters as opaque blocks. For this
reason, whenever you think you need to do something at a particular
character offset, you should recognize it as a mistake and find the
offset by the content that is supposed to be there.
</p><p>
If this philosophy is consistently followed, it becomes clear that
actual character boundaries are entirely irrelevant. There are only
two exceptions to this: first occurs if you are writing a text editor
and want to insert user data at a character position, the second occurs
if you are writing a font renderer and want to translate characters to
glyphs. In both cases you should make use of the utf8_iterator to find
character boundaries and values. Given that these two cases apply to
just a handful of people who are involved in implementing user interface
frameworks, I believe that the opacity restriction is well justified by
the amount of code space it saves for the vast majority of library users.
</p>
<h2><a name="Algorithms">Algorithms</a></h2>
<p>
Algorithms are the other half of STL. They are simply templated common
tasks that take iterator arguments, and as a result, work with any
container. Most will take an iterator range, like (v.begin(), v.end()),
but you can, of course operate on a subset of a container by giving a
different one. Because the usual operation is to use the whole container,
uSTL provides versions of most algorithms that take container arguments
instead of the iterator range. Here are the algorithms you will actually
find useful:
</p><pre>
copy (v1, v2.begin()); // Copies vector v1 to vector v2.
fill (v, 5); // Fills v with fives.
copy_n (v1, 5, v2.begin()); // Copies first five elements only.
fill_n (v.begin() + 5, 10, 5); // Fills elements 5-15 with fives.
sort (v); // Sorts v.
find (v, 14); // Finds 14 in v, returning its iterator.
binary_search (v, 13); // Looks up 13 with binary search in a sorted vector.
lower_bound (v, 13); // Returns the iterator to where you want to insert 13.
iota (v.begin(), v.end(), 0); // Puts 0,1,2,3,4,... into v.
reverse (v); // Reverses all the elements in v.
</pre><p>
The rest you can discover for yourself. There are obscure mathematical
operations, like inner_product, set operations, heap operations, and
lots and lots of predicate algorithms. The latter are algorithms that
take a functor (an object that can be called like a function) and were
supposed to help promote code reuse by encapsulating common operations.
For example, STL expects you to use the <code>for_each</code> algorithm and
write a little functor for all your iterative tasks:
</p><pre>
class CCompareAndReplace {
public:
CCompareAndReplace (int minValue, int maxValue, int badValue)
: m_MinValue (minValue), m_MaxValue (maxValue), m_BadValue (badValue) {}
void operator (int&amp; v) {
if (v &lt; m_MinValue || v &gt; m_MaxValue)
v = m_BadValue;
}
private:
int m_MinValue;
int m_MaxValue;
int m_BadValue;
};
for_each (v.begin(), v.end(), CCompareAndReplace (5, 10, 99));
</pre><p>
And yes, it really does work. Doesn't always generate much bloat either,
since the compiler can often see right through all this trickery and
expand the for_each into a loop without actually creating the functor
object. However, the compiler has a much harder time when you start
using containers of complex objects or operating on member variables
and member functions. Since that is what you will most likely have in
any real code outside the academic world, the utility of predicate
algorithms is questionable. Their readability is even more so,
considering that the above fifteen line example can be written as a
three line iterative foreach loop. Finally, there is the problem of
where to put the functor. It just doesn't seem to "belong" anywhere in
the object-oriented world. (C++0x changes that somewhat with lambda
functions) Sorry, Stepanov, I just don't see how these things can be
anything but an ugly, bloated hindrance.
</p>
<h2><a name="Memblocks">Memblocks and Memlinks</a></h2>
<p>
The STL specification is only about containers and algorithms, the stuff
described from here on is totally non-standard, so by using them you'll
have to stick with uSTL as your STL implementation. I think it's worth
it, but, of course, the choice is up to you.
</p><p>
The major difference between the standart STL implementation and uSTL is
that the former has memory management stuff all over the place, while
the latter keeps it all together in the <code>memblock</code> class. Normally
STL containers are resized by calling <code>new</code> to create more storage
and then copying the elements there from the old one. This method wastes
space by fragmenting memory, wastes time by copying all the existing data
to the new location, and wastes codespace by having to instantiate all
the resizing code for each and every container type you have. This method
is also absolutely necessary to do this resizing in a perfectly object-safe
way. The uSTL way is to manage memory as an opaque, typeless block, and
then use the container templates to cast it to an appropriate pointer type.
</p><p>
This works just fine, except for one little catch: there is one type
of object you can't store in uSTL containers -- the kind that has pointers
to itself. In other implementations, resizing actually creates new objects
in the new location and destroys them in the old location. uSTL simply
memcpys them there without calling the copy constructor. In other words,
the object can not rely on staying at the same address. Most objects really
don't care. Note that this is not the same thing as doing a bitwise copy,
that you were rightly warned against before! It's a bitwise <em>move</em>
that doesn't create a new object, but simply relocates an existing one.
</p><p>
What this one small concession does is allow aggregation of all memory
management in one place, namely, the <code>memblock</code> class. All the
containers are thus converted mostly into typecasting wrappers that
exist to ensure type safety. Look at the assembly code and you'll see
mostly calls to memblock's functions. This is precisely the feature
that allows reduction in code instantiated by container templates.
</p><p>
However, memblock's usefulness doesn't end there! It can now replace
all your dynamically allocated buffers that you use for unstructured
data. Need to read a file? Don't use new to allocate memory; use a
memblock! It even has a friendly read_file member function for just
that purpose. Need to write a file? Use the write_file call! Unless
you are working with a database or some really large archive, you
should be able to load all your files this way. Imagine, not having
to worry about file I/O again! It's much nicer to work with data in
memory; you know how long it is, so you know when to stop. You can
seek with impunity, and any operations have the cost of a memcpy.
</p><p>
Memblock is derived from memlink, an object for linking to a memory
block. Now you get to store a pointer and the size of whatever it
points to, but with uSTL you can use a memlink object to keep them
together, reducing source clutter and making your code easier to
read and maintain. You can link to constant blocks too with cmemlink,
from which memlink is derived. Because all three are in a single
hierarchy, you never need to care whether you're working on an
allocated block or on somebody else's allocated block. Pointers are
kept together with block sizes, memory is freed when necessary,
and you never have to call new or delete again. Who needs garbage
collection? Memblocks give you the same functionality at a fraction
of the cost.
</p><p>
Linking is not limited to memlink. You can link memblock objects.
You can link string objects. You can even link containers! Now
you can use alloca to create a vector on the stack; use the
<code>typed_alloca_link(v,int,99)</code> macro. All linked objects
will allocate memory and copy the linked data when you increase their
size. You can also do it explicitly by calling <code>copy_link</code>.
Why link? It's cheaper than copying and easier than keeping track
of pointers. For example, here's a line parser:
</p><pre>
string buf, line;
buf.read_file ("some_config_file.txt");
for (uoff_t i = 0; i &lt; buf.size(); i += line.size() + 1) {
line.link (buf.iat(i), buf.iat (buf.find ('\n',i)));
process_line (line);
}
</pre><p>
This way process_line gets a string object instead of a pointer and
a size. If you don't rely on the string being null-terminated, which
basically means not using libc functions on it, this is all you need.
Otherwise buf will have to be writable and you can replace the newline
with a null. In either case you are using no extra heap. The overhead
of link is negligible in most cases, but if you really want to do this
in a tight loop, you can use relink call, which expands completely
inline into one or two instructions, avoiding the virtual unlink() call.
</p>
<h2><a name="Streams">Streams</a></h2>
<p>
The C++ standard library provides global stream objects called cin,
cout, and cerr to replace printf and friends for accessing stdin, stdout,
and stderr, respectively. uSTL versions work mostly the same as the
standard ones (yes, the <code>format</code> call is a uSTL extension). Most
calls use snprintf for output and thus use whatever locale libc uses.
</p><pre>
cout &lt;&lt; "Hello world!" &lt;&lt; endl;
cout &lt;&lt; 456 &lt;&lt; ios::hex &lt;&lt; 0x1234 &lt;&lt; endl;
cerr.format ("You objects are at 0x%08X\n", &amp;o);
</pre><p>
String-writing streams are also available:
</p><pre>
ostringstream os;
os &lt;&lt; "Writing " &lt;&lt; n &lt;&lt; " objects somewhere" &lt;&lt; endl;
cout &lt;&lt; os.str() &lt;&lt; endl;
</pre><p>
fstream is a file access interface with exception handling for errors:
</p><pre>
fstream f;
// C++ standard says that fstream does not throw by default,
f.exceptions (fstream::allbadbits); // so this enables throwing.
f.open ("file.dat", ios::in | ios::out); // throws file_exception
f.read (buf, bufSize); // let's read something
f.seek (334455); // go somewhere
f.write (buf2, buf2Size); // and write something
f.fnctl (FCNTLID(F_SETFL), O_NONBLOCK); // yup, ALL file operations
memlink l = f.mmap (bufSize, offset); // even mmap
fill (l, 0);
f.msync (l);
f.munmap (l);
f.close(); // also throws file_exception (with filename!)
</pre><p>
istream and ostream, which are not really usable by themselves in the
standard implementation, are hijacked by uSTL to implement binary data
input and output:
</p><pre>
const size_t writtenSize =
Align (stream_size_of(number) +
stream_size_of(ctr)) +
stream_size_of(n) +
stream_size_of(v);
memblock buf (writtenSize);
ostream os (buf);
os &lt;&lt; number &lt;&lt; ctr;
os.align();
os &lt;&lt; n &lt;&lt; v;
</pre><p>
These operations are all very efficient, approaching a straight memcpy
in performance. ostream will not resize the buffer, hence the necessity
to estimate the final size. Most stream_size_of calls are computed at
compile time and thus produce no code. Because the data is written as
is, it is necessary to consider proper data alignment; for example,
a 4 byte int can not be written at stream offset 2. Some architectures
(Macs) actually crash when doing it; Intel processors just do it slowly.
Hence the need to pack the data to a proper "grain". The default align
call will pack to the maximum necessary grain, but can be given an
argument to change that. In case you're wondering, the reason for all
these idiosyncracies is optimization. The smallest and fastest possible
code to dump your stuff into a binary file is produced by this method.
uSTL defines flow operators to write integral values, strings, and
containers, but you can custom-serialize your objects like this:
</p><pre>
namespace myns {
/// Some class I want to serialize
class CMyClass {
public:
void read (istream&amp; is);
void write (ostream&amp; os) const;
size_t stream_size (void) const;
private:
vector&lt;int&gt; m_Elements; ///&lt; A bunch of elements.
size_t m_SomeSize; ///&lt; Some integral value.
MyObject m_SomeObject; ///&lt; Some other streamable object.
}
/// Reads the object from stream \p is.
void CMyClass::read (istream&amp; is)
{
is &gt;&gt; m_Elements &gt;&gt; m_SomeSize &gt;&gt; m_SomeObject;
}
/// Writes the object to stream \p os.
void CMyClass::write (ostream&amp; os) const
{
os &lt;&lt; m_Elements &lt;&lt; m_SomeSize &lt;&lt; m_SomeObject;
}
/// Returns the size of the written object.
size_t CMyClass::stream_size (void) const
{
return (stream_size_of (m_Elements) +
stream_size_of (m_SomeSize) +
stream_size_of (m_SomeObject));
}
} // namespace myns
</pre>
<h2><a name="Tuples">Tuples</a></h2>
<p>
One last container I'll mention is a <code>tuple</code>, which is a
fixed-size array of identical elements. No, it's not the same as the tuple
in boost, which is more like a template-defined struct. This one should
have been named "array", which is what it will be called in the next STL
standard, but I guess I'm stuck with the name now. What are they good
for? Graphical objects. Points, sizes, rectangles, triangles, etc. As a
bonus, operations on tuples can automatically use SIMD instructions if
they are available. Any fixed size-array also works better as a tuple,
since it becomes a standard STL container, which you can use with any
algorithm, copy by assignment, initialize in the constructor, etc.
</p><pre>
typedef int32_t coord_t;
typedef tuple&lt;2, coord_t&gt; Point2d;
typedef tuple&lt;2, coord_t&gt; Size2d;
typedef tuple&lt;2, Point2d&gt; Rect;
Rect r (Point2d (1,2), Point2d (3,4));
r += Size2d (4, 4);
r[1] -= Size2d (1, 1);
foreach (Rect::iterator, i, r)
TransformPoint (*i);
Point2d pt (1, 2);
pt += r[0];
pt *= 2;
</pre>
<h2><a name="Exceptions">Exceptions</a></h2>
<p>
uSTL implements all the standard exception classes defined by the
C++ standard. The exception tree is standalone, but is derived
from std::exception when compiling with libstdc++ for ease of
catching everything. uSTL exceptions implement some additional useful
features. First, they are completely serializable. You can write them
as a binary blob into a file, send them over a network, and handle them
somewhere else. Each exception will print an informative error message
directly to a text stream, reducing your try/catch block to:
</p><pre>
try {
DoSomething();
} catch (exception&amp; e) {
cerr &lt;&lt; "Error: " &lt;&lt; e &lt;&lt; endl;
#ifndef NDEBUG
cerr &lt;&lt; e.backtrace();
#endif
} catch (...) {
cerr &lt;&lt; "Unexpected fatal error has occured.\n";
}
</pre><p>
Second, each exception stores a backtrace (callstack) at the time
of throwing and can print that backtrace as easily as the above
example illustrates. While it is indeed a good practice to design
your exceptions so that you should not care where it was thrown from,
situations occasionally arise while debugging where knowing the thrower
is useful to fix the bug a little faster than otherwise.
</p><p>
Finally, there are additional exception classes for dealing with libc
function errors, file errors, and stream classes. libc_exception can
be thrown whenever a libc function fails, immediately telling you
what the function call was and the errno description of the failure.
file_exception, thrown by fstream operations, also contains the file name,
which can be pretty darn useful. stream_bounds_exception is extremely
useful in debugging corrupted data, as it tells you exactly where the
corruption starts and what you were trying to read there.
</p>
<h2><a name="Savings">Template Bloat Be Gone</a></h2>
<p>
So how much space are you going to save and where? Allow me to demonstrate with
the following small program. I'm basically creating a vector and exercise the
most common operations. Those are resize, push_back, insert, and erase, which
you use pretty much every time you have a vector.
</p><pre>
#if USING_USTL
#include &lt;ustl.h&gt;
using namespace ustl;
#else
#include &lt;vector&gt;
using namespace std;
#endif
int main (void)
{
vector&lt;int&gt; v;
v.resize (30);
for (size_t i = 0; i &lt; v.size(); ++ i)
v[i] = i;
v.push_back (57);
v.insert (v.begin() + 20, 555);
v.erase (v.begin() + 3);
return (EXIT_SUCCESS);
}
</pre><p>
Feel free to compile it and see for yourself. I'm compiling on a Core
i7 with gcc 4.5.2 and <kbd>-Os -DNDEBUG=1</kbd>. The libstdc++ version
is linked implicitly with it, and uSTL version is linked with gcc
(instead of g++) and <kbd>-lustl</kbd>. Both executables are stripped.
The libstdc++ version looks like this:
</p><pre>
% ls -l std/tes
7096 tes
% size std/tes
text data bss dec hex filename
3780 632 16 4428 114c std/tes
% size -A std/tes.o
std/tes.o :
section size
.group 8
.group 8
.group 8
.group 8
.group 8
.group 8
.group 8
.group 8
.group 8
.group 8
.group 8
.text 256
.data 0
.bss 0
.text._ZNSt6vectorIiSaIiEED2Ev 8
.text._ZNKSt6vectorIiSaIiEE12_M_check_lenEmPKc 68
.text._ZNSt11__copy_moveILb0ELb1ESt26random_access_iterator_tagE8__copy_mIiEEPT_PKS3_S6_S4_ 51
.text._ZSt14__copy_move_a2ILb0EN9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEES6_ET1_T0_S8_S7_ 14
.text._ZSt4copyIN9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEES6_ET0_T_S8_S7_ 14
.text._ZNSt20__copy_move_backwardILb0ELb1ESt26random_access_iterator_tagE13__copy_move_bIiEEPT_PKS3_S6_S4_ 63
.rodata.str1.1 45
.text._ZNSt6vectorIiSaIiEE14_M_fill_insertEN9__gnu_cxx17__normal_iteratorIPiS1_EEmRKi 355
.text._ZNSt6vectorIiSaIiEE6resizeEmi 64
.text._ZNSt6vectorIiSaIiEE13_M_insert_auxEN9__gnu_cxx17__normal_iteratorIPiS1_EERKi 195
.text._ZNSt6vectorIiSaIiEE9push_backERKi 46
.text._ZNSt6vectorIiSaIiEE6insertEN9__gnu_cxx17__normal_iteratorIPiS1_EERKi 79
.gcc_except_table 14
.comment 40
.note.GNU-stack 0
.eh_frame 576
Total 1976
</pre><p>
The uSTL version looks like this:
</p><pre>
% ls -l ustl/tes
5720 tes
% size ustl/tes
text data bss dec hex filename
2435 616 16 3067 bfb ustl/tes
% size -A ustl/tes.o
ustl/tes.o :
section size addr
.text 327 0
.data 0 0
.bss 0 0
.gcc_except_table 19 0
.comment 40 0
.note.GNU-stack 0 0
.eh_frame 88 0
Total 474
</pre><p>
Let's see what's going on here. The .text size in the std version is
smaller, indicating less inlined functionality. This version of gcc
libstdc++ instantiates additional eleven functions totalling 953 bytes
just for this one vector type. These functions will become larger for
containers with objects, but about 1k in savings that you see as the
difference in execuable size is a good measure. The uSTL version inlines
everything and calls memblock functions instead.
</p><p>
1k doesn't seem like much, but consider that you get it for <em>every
type of container you instantiate</em>! An int vector here, a float
vector here, a bunch of object containers there, and before you know it
you are using half your executable just for container overhead.
</p><p>
But wait, there is more! Let's look at the total memory footprint:
</p><pre>
% footprint std/tes
text data bss dec hex filename
3780 632 16 4428 114c tes
84445 928 632 86005 14ff5 libgcc_s.so.1
527390 720 72 528182 80f36 libm.so.6
962481 34816 84664 1081961 108269 libstdc++.so.6
1404487 18024 20032 1442543 1602ef libc.so.6
2982583 55120 105416 3143119 2ff5cf (TOTALS)
% footprint ustl/tes
text data bss dec hex filename
2435 616 16 3067 bfb tes
84445 928 632 86005 14ff5 libgcc_s.so.1
152800 10408 73248 236456 39ba8 libustl.so.1.5
1404487 18024 20032 1442543 1602ef libc.so.6
1644167 29976 93928 1768071 1afa87 (TOTALS)
</pre><p>
As you can see, the footprint for the uSTL version is 44% smaller,
saving 1375048 bytes. If you don't count libc, measuring only the
C++-specific overhead, libstdc++ loads 1696148 while libustl only 322461,
<em>five times less</em>! Finally, most of uSTL's size comes from gcc's
support libraries; if you compile uSTL configured <kbd>--with-libstdc++</kbd>
option, then you'll see that it only takes up 72322 bytes, of which only
23350 are used by .text, meaning that only about a third of the library
size is my fault. gcc developers will have to reduce the size of libsupc++
before any further size reduction would be practical.
</p><p>
One final note concerns the current gcc versions, 4.4 and later. gcc
developers have decided, for various reasons, to treat the inline keyword
as nothing more than a hint to the optimizer, resulting in a lot less
inlining for uSTL-using code. You can see which functions fail to inline
if you turn on -Winline warning. Back in gcc 3 days there were various
parameters that could be tweaked to get the inlining to happen. These no
longer seem to work. Because it takes quite a bit of code to make these
failures happen, I am unable to submit a gcc bug for it. Simple examples
don't exhibit inlining failures and submitting the entire uSTL codebase
seems inappropriate. Therefore I've pretty much given up on it. The only
solution I can come up with is to just clobber the optimizer over the head
with <code>#define inline __attribute__((always_inline)) inline</code>. It
is enabled if you configure with <kbd>--force-inline</kbd>. The above
examples are compiled with this option.
</p>
<h2><a name="Contact">Bug reporting</a></h2>
<p>
Report bugs through the SourceForge.net
<a href="http://sourceforge.net/projects/ustl">uSTL project page</a> with the
standard bugtracker.
</p>
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