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?

Concatenation Operator

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.

Additionally, if one has an array of floats, is ‘+’ overloaded to be the same as a vector addition, or an array concatenation?

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.)

Interoperability With C String Syntax

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:

const char abc[5] = "world";
string str = "hello" + abc;

That isn't going to work. But it does work when the core language knows about strings:

const char[5] abc = "world";
char[] str = "hello" ~ abc;

Consistency With C String Syntax

There are three ways to find the length of a string in C++:

const char abc[] = "world";	:	sizeof(abc)/sizeof(abc[0])-1
				:	strlen(abc)
string str;			:	str.length()

That kind of inconsistency makes it hard to write generic templates. Consider D:

char[5] abc = "world";	:	abc.length
char[] str		:	str.length

Checking For Empty Strings

C++ strings use a function to determine if a string is empty:

string str;
if (str.empty())
	// string is empty

In D, an empty string has zero length:

char[] str;
if (!str.length)
	// string is empty

Resizing Existing String

C++ handles this with the resize() member function:

string str;
str.resize(newsize);

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;

Slicing a String

C++ slices an existing string using a special constructor:

string s1 = "hello world";
string s2(s1, 6, 5);		// s2 is "world"

D has the array slice syntax, not possible with C++:

string s1 = "hello world";
string s2 = s1[6 .. 11];	// s2 is "world"

Slicing, of course, works with any array in D, not just strings.

Copying a String

C++ copies strings with the replace function:

string s1 = "hello world";
string s2 = "goodbye      ";
s2.replace(8, 5, s1, 6, 5);	// s2 is "goodbye world"

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"

The .dup is needed because string literals are read-only in D, the .dup will create a copy that is writable.

Conversions to C Strings

This is needed for compatibility with C API's. In C++, this uses the c_str() member function:

void foo(const char *);
string s1;
foo(s1.c_str());

In D, strings can be converted to char* using the .ptr property:

void foo(char*);
char[] s1;
foo(s1.ptr);

although for this to work where foo expects a 0 terminated string, s1 must have a terminating 0. Alternatively, the function std.string.toStringz will ensure it:

void foo(char*);
char[] s1;
foo(std.string.toStringz(s1));

Array Bounds Checking

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.

String Switch Statements

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":
	...
}

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.

Filling a String

In C++, this is done with the replace() member function:

string str = "hello";
str.replace(1,2,2,'?');		// str is "h??lo"

In D, use the array slicing syntax in the natural manner:

char[5] str = "hello";
str[1..3] = '?';		// str is "h??lo"

Value vs Reference

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 ‘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.

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 ‘owner’ of the string data.

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.

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.

Benchmark

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;
}

(An alternate implementation that uses buffered file I/O to handle larger files.)

Two people have written C++ implementations using the C++ standard template library, wccpp1 and wccpp2. The input file alice30.txt is the text of "Alice in Wonderland." The D compiler, dmd, and the C++ compiler, dmc, 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.

Program	Compile	Compile Time	Run	Run Time
D wc	dmd wc -O -release	0.0719	wc alice30.txt >log	0.0326
C++ wccpp1	dmc wccpp1 -o -I\dm\stlport\stlport	2.1917	wccpp1 alice30.txt >log	0.0944
C++ wccpp2	dmc wccpp2 -o -I\dm\stlport\stlport	2.0463	wccpp2 alice30.txt >log	0.1012

The following tests were run on linux, again comparing a D compiler (gdc) and a C++ compiler (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 (dmd) is included for comparison.

Program	Compile	Compile Time	Run	Run Time
D wc	gdc -O2 -frelease -o wc wc.d	0.326	wc alice30.txt > /dev/null	0.041
D wc	dmd wc -O -release	0.235	wc alice30.txt > /dev/null	0.041
C++ wccpp1	g++ -O2 -o wccpp1 wccpp1.cc	2.874	wccpp1 alice30.txt > /dev/null	0.086
C++ wccpp2	g++ -O2 -o wccpp2 wccpp2.cc	2.886	wccpp2 alice30.txt > /dev/null	0.095

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.)

Program	Compile	Compile Time	Run	Run Time
D wc	gdc -O2 -frelease -o wc wc.d	0.28	wc alice30.txt > /dev/null	0.03
C++ wccpp1	g++ -O2 -o wccpp1 wccpp1.cc	1.90	wccpp1 alice30.txt > /dev/null	0.07
C++ wccpp2	g++ -O2 -o wccpp2 wccpp2.cc	1.88	wccpp2 alice30.txt > /dev/null	0.08

wccpp2 by Allan Odgaard

#include <algorithm>
#include <cstdio>
#include <fstream>
#include <iterator>
#include <map>
#include <vector>

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<string, int> dict;
    int tLines = 0, tWords = 0, tBytes = 0;
    for(int i = 1; i < argc; i++)
    {
	ifstream file(argv[i]);
	istreambuf_iterator<char> from(file.rdbuf()), to;
	vector<char> v(from, to);
	vector<char>::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 > 2)
	    printf("-----------------------\n%5d %5d %5d\n", tLines, tWords, tBytes);
    printf("-----------------------\n\n");

    for(map<string, int>::const_iterator it = dict.begin(); it != dict.end(); ++it)
	    printf("%5d %s\n", it->second, it->first.c_str());

    return 0;
}

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