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

SG <>
Fri, 6 Feb 2009 09:41:41 -0800 (PST)
On 6 Feb., 17:54, "Alf P. Steinbach" <> 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) {
   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?


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