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$(COMMUNITY D Strings vs C++ Strings,
Why have strings built-in to the core language of D rather than entirely in
a library as in C++ Strings? What's the point? Where's the improvement?
<h4>Concatenation Operator</h4>
$(P C++ Strings are stuck with overloading existing operators. The
obvious choice for concatenation is += and +.
But someone just looking at the code will see + and think "addition".
He'll have to look up the types (and types are frequently buried
behind multiple typedef's) to see that it's a string type, and
it's not adding strings but concatenating them.
)
$(P Additionally, if one has an array of floats, is $(SINGLEQUOTE +) overloaded to
be the same as a vector addition, or an array concatenation?
)
$(P In D, these problems are avoided by introducing a new binary
operator ~ as the concatenation operator. It works with
arrays (of which strings are a subset). ~= is the corresponding
append operator. ~ on arrays of floats would concatenate them,
+ would imply a vector add. Adding a new operator makes it possible
for orthogonality and consistency in the treatment of arrays.
(In D, strings are simply arrays of characters, not a special
type.)
)
<h4>Interoperability With C String Syntax</h4>
$(P Overloading of operators only really works if one of the operands
is overloadable. So the C++ string class cannot consistently
handle arbitrary expressions containing strings. Consider:
)
$(CCODE
const char abc[5] = "world";
string str = "hello" + abc;
)
$(P That isn't going to work. But it does work when the core language
knows about strings:
)
$(CCODE
const char[5] abc = "world";
char[] str = "hello" ~ abc;
)
<h4>Consistency With C String Syntax</h4>
$(P
There are three ways to find the length of a string in C++:
)
$(CCODE
const char abc[] = "world"; : sizeof(abc)/sizeof(abc[0])-1
: strlen(abc)
string str; : str.length()
)
$(P
That kind of inconsistency makes it hard to write generic templates.
Consider D:
)
-----------------------
char[5] abc = "world"; : abc.length
char[] str : str.length
-----------------------
<h4>Checking For Empty Strings</h4>
$(P
C++ strings use a function to determine if a string is empty:
)
$(CCODE
string str;
if (str.empty())
// string is empty
)
$(P
In D, an empty string has zero length:
)
-----------------------
char[] str;
if (!str.length)
// string is empty
-----------------------
<h4>Resizing Existing String</h4>
$(P
C++ handles this with the resize() member function:
)
$(CCODE
string str;
str.resize(newsize);
)
$(P
D takes advantage of knowing that str is an array, and
so resizing it is just changing the length property:
)
-----------------------
char[] str;
str.length = newsize;
-----------------------
<h4>Slicing a String</h4>
$(P
C++ slices an existing string using a special constructor:
)
$(CCODE
string s1 = "hello world";
string s2(s1, 6, 5); // s2 is "world"
)
$(P
D has the array slice syntax, not possible with C++:
)
-----------------------
string s1 = "hello world";
string s2 = s1[6 .. 11]; // s2 is "world"
-----------------------
$(P
Slicing, of course, works with any array in D, not just strings.
)
<h4>Copying a String</h4>
$(P
C++ copies strings with the replace function:
)
$(CCODE
string s1 = "hello world";
string s2 = "goodbye ";
s2.replace(8, 5, s1, 6, 5); // s2 is "goodbye world"
)
$(P
D uses the slice syntax as an lvalue:
)
-----------------------
char[] s1 = "hello world".dup;
char[] s2 = "goodbye ".dup;
s2[8..13] = s1[6..11]; // s2 is "goodbye world"
-----------------------
$(P The $(CODE .dup) is needed because string literals are
read-only in D, the $(CODE .dup) will create a copy
that is writable.
)
<h4>Conversions to C Strings</h4>
$(P
This is needed for compatibility with C API's. In C++, this
uses the c_str() member function:
)
$(CCODE
void foo(const char *);
string s1;
foo(s1.c_str());
)
$(P
In D, strings can be converted to char* using the .ptr property:
)
-----------------------
void foo(char*);
char[] s1;
foo(s1.ptr);
-----------------------
$(P although for this to work where $(TT foo) expects a 0 terminated
string, $(TT s1) must have a terminating 0. Alternatively, the
function $(TT std.string.toStringz) will ensure it:)
-----------------------
void foo(char*);
char[] s1;
foo(std.string.$(B toStringz)(s1));
-----------------------
<h4>Array Bounds Checking</h4>
$(P
In C++, string array bounds checking for [] is not done.
In D, array bounds checking is on by default and it can be turned off
with a compiler switch after the program is debugged.
)
<h4>String Switch Statements</h4>
$(P
Are not possible in C++, nor is there any way to add them
by adding more to the library. In D, they take the obvious
syntactical forms:
)
-----------------------
switch (str)
{
case "hello":
case "world":
...
}
-----------------------
$(P
where str can be any of literal "string"s, fixed string arrays
like char[10], or dynamic strings like char[]. A quality implementation
can, of course, explore many strategies of efficiently implementing
this based on the contents of the case strings.
)
<h4>Filling a String</h4>
$(P
In C++, this is done with the replace() member function:
)
$(CCODE
string str = "hello";
str.replace(1,2,2,'?'); // str is "h??lo"
)
$(P
In D, use the array slicing syntax in the natural manner:
)
-----------------------
char[5] str = "hello";
str[1..3] = '?'; // str is "h??lo"
-----------------------
<h4>Value vs Reference</h4>
$(P
C++ strings, as implemented by STLport, are by value and are
0-terminated. [The latter is an implementation choice, but
STLport seems to be the most popular implementation.]
This, coupled with no garbage collection, has
some consequences. First of all, any string created must make
its own copy of the string data. The $(SINGLEQUOTE owner) of the string
data must be kept track of, because when the owner is deleted
all references become invalid. If one tries to avoid the
dangling reference problem by treating strings as value types,
there will be a lot of overhead of memory allocation,
data copying, and memory deallocation. Next, the 0-termination
implies that strings cannot refer to other strings. String
data in the data segment, stack, etc., cannot
be referred to.
)
$(P
D strings are reference types, and the memory is garbage collected.
This means that only references need to be copied, not the
string data. D strings can refer to data in the static data
segment, data on the stack, data inside other strings, objects,
file buffers, etc. There's no need to keep track of the $(SINGLEQUOTE owner)
of the string data.
)
$(P
The obvious question is if multiple D strings refer to the same
string data, what happens if the data is modified? All the
references will now point to the modified data. This can have
its own consequences, which can be avoided if the copy-on-write
convention is followed. All copy-on-write is is that if
a string is written to, an actual copy of the string data is made
first.
)
$(P
The result of D strings being reference only and garbage collected
is that code that does a lot of string manipulating, such as
an lzw compressor, can be a lot more efficient in terms of both
memory consumption and speed.
)
<h2>Benchmark</h2>
$(P
Let's take a look at a small utility, wordcount, that counts up
the frequency of each word in a text file. In D, it looks like this:
)
-----------------------
import std.file;
import std.stdio;
int main (char[][] args)
{
int w_total;
int l_total;
int c_total;
int[char[]] dictionary;
writefln(" lines words bytes file");
for (int i = 1; i < args.length; ++i)
{
char[] input;
int w_cnt, l_cnt, c_cnt;
int inword;
int wstart;
input = cast(char[])std.file.read(args[i]);
for (int j = 0; j < input.length; j++)
{ char c;
c = input[j];
if (c == '\n')
++l_cnt;
if (c >= '0' && c <= '9')
{
}
else if (c >= 'a' && c <= 'z' ||
c >= 'A' && c <= 'Z')
{
if (!inword)
{
wstart = j;
inword = 1;
++w_cnt;
}
}
else if (inword)
{ char[] word = input[wstart .. j];
dictionary[word]++;
inword = 0;
}
++c_cnt;
}
if (inword)
{ char[] w = input[wstart .. input.length];
dictionary[w]++;
}
writefln("%8s%8s%8s %s", l_cnt, w_cnt, c_cnt, args[i]);
l_total += l_cnt;
w_total += w_cnt;
c_total += c_cnt;
}
if (args.length > 2)
{
writefln("--------------------------------------%8s%8s%8s total",
l_total, w_total, c_total);
}
writefln("--------------------------------------");
foreach (char[] word1; dictionary.keys.sort)
{
writefln("%3d %s", dictionary[word1], word1);
}
return 0;
}
-----------------------
$(P (An $(LINK2 wc.html, alternate implementation) that
uses buffered file I/O to handle larger files.))
$(P
Two people have written C++ implementations using the C++ standard
template library,
<a href="http://groups.google.com/groups?q=g:thl953709878d&dq=&hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=bjacrl%244un%2401%241%40news.t-online.com">wccpp1</a>
and
$(LINK2 #wccpp2, wccpp2).
The input file
$(LINK2 http://www.gutenberg.org/dirs/etext91/alice30.txt, alice30.txt)
is the text of "Alice in Wonderland."
The D compiler,
<a HREF="http://ftp.digitalmars.com/dmd.zip" title="download D compiler">dmd</a>,
and the C++ compiler,
<a HREF="http://ftp.digitalmars.com/dmc.zip" title="download dmc.zip">dmc</a>,
share the same
optimizer and code generator, which provides a more apples to
apples comparison of the efficiency of the semantics of the languages
rather than the optimization and code generator sophistication.
Tests were run on a Win XP machine. dmc uses STLport for the template
implementation.
)
$(TABLE1
$(TR
$(TH Program)
$(TH Compile)
$(TH Compile Time)
$(TH Run)
$(TH Run Time)
)
$(TR
$(TD D wc)
$(TD dmd wc -O -release)
$(TD 0.0719)
$(TD wc alice30.txt &gt;log)
$(TD 0.0326)
)
$(TR
$(TD C++ wccpp1)
$(TD dmc wccpp1 -o -I\dm\stlport\stlport)
$(TD 2.1917)
$(TD wccpp1 alice30.txt &gt;log)
$(TD 0.0944)
)
$(TR
$(TD C++ wccpp2)
$(TD dmc wccpp2 -o -I\dm\stlport\stlport)
$(TD 2.0463)
$(TD wccpp2 alice30.txt &gt;log)
$(TD 0.1012)
)
)
$(P
The following tests were run on linux, again comparing a D compiler
($(LINK2 http://home.earthlink.net/~dvdfrdmn/d, gdc))
and a C++ compiler ($(B g++)) that share a common optimizer and
code generator. The system is Pentium III 800MHz running RedHat Linux 8.0
and gcc 3.4.2.
The Digital Mars D compiler for linux ($(B dmd))
is included for comparison.
)
$(TABLE1
$(TR
$(TH Program)
$(TH Compile)
$(TH Compile Time)
$(TH Run)
$(TH Run Time)
)
$(TR
$(TD D wc)
$(TD gdc -O2 -frelease -o wc wc.d)
$(TD 0.326)
$(TD wc alice30.txt &gt; /dev/null)
$(TD 0.041)
)
$(TR
$(TD D wc)
$(TD dmd wc -O -release)
$(TD 0.235)
$(TD wc alice30.txt &gt; /dev/null)
$(TD 0.041)
)
$(TR
$(TD C++ wccpp1)
$(TD g++ -O2 -o wccpp1 wccpp1.cc)
$(TD 2.874)
$(TD wccpp1 alice30.txt &gt; /dev/null)
$(TD 0.086)
)
$(TR
$(TD C++ wccpp2)
$(TD g++ -O2 -o wccpp2 wccpp2.cc)
$(TD 2.886)
$(TD wccpp2 alice30.txt &gt; /dev/null)
$(TD 0.095)
)
)
$(P
These tests compare gdc with g++ on a PowerMac G5 2x2.0GHz
running MacOS X 10.3.5 and gcc 3.4.2. (Timings are a little
less accurate.)
)
$(TABLE1
$(TR
$(TH Program)
$(TH Compile)
$(TH Compile Time)
$(TH Run)
$(TH Run Time)
)
$(TR
$(TD D wc)
$(TD gdc -O2 -frelease -o wc wc.d)
$(TD 0.28)
$(TD wc alice30.txt &gt; /dev/null)
$(TD 0.03)
)
$(TR
$(TD C++ wccpp1)
$(TD g++ -O2 -o wccpp1 wccpp1.cc)
$(TD 1.90)
$(TD wccpp1 alice30.txt &gt; /dev/null)
$(TD 0.07)
)
$(TR
$(TD C++ wccpp2)
$(TD g++ -O2 -o wccpp2 wccpp2.cc)
$(TD 1.88)
$(TD wccpp2 alice30.txt &gt; /dev/null)
$(TD 0.08)
)
)
<hr>
<h4><a name="wccpp2">wccpp2 by Allan Odgaard</a></h4>
$(CCODE
#include &lt;algorithm&gt;
#include &lt;cstdio&gt;
#include &lt;fstream&gt;
#include &lt;iterator&gt;
#include &lt;map&gt;
#include &lt;vector&gt;
bool isWordStartChar (char c) { return isalpha(c); }
bool isWordEndChar (char c) { return !isalnum(c); }
int main (int argc, char const* argv[])
{
using namespace std;
printf("Lines Words Bytes File:\n");
map&lt;string, int&gt; dict;
int tLines = 0, tWords = 0, tBytes = 0;
for(int i = 1; i &lt; argc; i++)
{
ifstream file(argv[i]);
istreambuf_iterator&lt;char&gt; from(file.rdbuf()), to;
vector&lt;char&gt; v(from, to);
vector&lt;char&gt;::iterator first = v.begin(), last = v.end(), bow, eow;
int numLines = count(first, last, '\n');
int numWords = 0;
int numBytes = last - first;
for(eow = first; eow != last; )
{
bow = find_if(eow, last, isWordStartChar);
eow = find_if(bow, last, isWordEndChar);
if(bow != eow)
++dict[string(bow, eow)], ++numWords;
}
printf("%5d %5d %5d %s\n", numLines, numWords, numBytes, argv[i]);
tLines += numLines;
tWords += numWords;
tBytes += numBytes;
}
if(argc &gt; 2)
printf("-----------------------\n%5d %5d %5d\n", tLines, tWords, tBytes);
printf("-----------------------\n\n");
for(map&lt;string, int&gt;::const_iterator it = dict.begin(); it != dict.end(); ++it)
printf("%5d %s\n", it-&gt;second, it-&gt;first.c_str());
return 0;
}
)
)
Macros:
TITLE=D Strings vs C++ Strings
WIKI=CPPstrings
CATEGORY_OVERVIEW=$0
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