Re: std::string and std::ostringstream performances

"Jim Langston" <>
Wed, 31 Oct 2007 09:09:58 -0700
"Bala2508" <> wrote in message


I have a C++ application that extensively uses std::string and
std::ostringstream in somewhat similar manner as below

std::string msgHeader;

msgHeader = "<";
msgHeader += a;
msgHeader += "><";

msgHeader += b;
msgHeader += "><";

msgHeader += c;
msgHeader += ">";

Similarly it uses ostringstream as well and the function that uses
this gets called almost on every message that my application gets on
the socket. I am using this to precisely construct a XML Message to
be sent to another application.

What we observed when we ran a collect/analyzer on the application is
that it shows majority of the CPU spent in trying to deal with these 2
datatypes, their memory allocation using std::allocator and other
stuff. The CPU goes as high as 100% sometimes.

I would like to get an advice/suggestion on the following points
1. Is there a better way to use std::string / std::ostringstream than
the way I have been using it?
2. AM I using the wrong datatype for such kind of operations and
should move on to use something else? Any suggestions what the
datatype should be?

I eventually need these datatypes because the external library that I
am using to send this data out needs it in std::string /
std::ostringstream formats.

Would like to have some suggestions to bring down the CPU utilization.

One suggestion would be .reserve(). I E.
std::string msgHeader;
msgHeader.reserve( 100 );

That way the string msgHeader wouldn't need to try to allocate more memory
until it has used the initial 100 characters allocated. Some compilers are
better at preallocating a default number of bytes than others. Sometimes
they have to be given a hint. Figure out a good size to reserve (one big
enough where you won't need to be doing reallocatings, one small enough that
you're not running out of memory) and then try profiling it again and see if
it helps.

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