basic question about "clearing" a string....

[Avalon1178]

Hi,

I have an application that periodically uses a std::string variable
which is assigned a VERY VERY large string (15000000+ bytes long).
This application is essentially a daemon, and it polls a data set
which can have a lot of information and it is concatenated in this
single string variable object. When the daemon finishes its job, it
goes to sleep, but before doing so, it "clears" this variable so it
can be reused again in the next poll.

Currently, when I say clear, all I'm doing to the variable is setting
it to an emtpy string (var = ""), rather than calling the .clear()
member function because of my concern with std::string performance of
actually zeroing out this very large buffer. My question is, is this
wise? Despite this large record, is it advised to use .clear()
regardless rather than setting it to empty string? Any potential
implications on this? I ran valgrind on my application and it doesn't
report a memory leak if I just set the variable to empty string....

Avalon1178

[Gennaro Prota]

On Wed, 13 Jun 2007 01:33:54 -0700, James Kanze wrote:

>The "standard" idiom for completely clearing a standard
container is to swap it with a just constructed instance, e.g.:

template< typename Container >
void
reset( Container& c )
{
Container().swa p( c ) ;
}

>As far as I know, this is the only way to get certain containers
(including std::basic_stri ng and std::vector) to free all of the
memory they might hold.

>However one shouldn't fall into the trap of believing that the "reset"
container will not hold any memory: even a just-constructed container
may have some excess capacity (it's up to the implementation) .

You'll notice I put "standard" in quotes. I meant "standard" in
the usual, everyday sense, and not as a reference to ISO 14882.

Sure, I got that. The remark wasn't addressed to you; and your post
contained no statement that I saw as incorrect. My only intent was to
add more info on the subject. BTW, a common name for the idiom is
"shrink-to-fit"; I'm not sure whether that name was
invented/popularized by Scott Meyers or Herb Sutter.

--
Gennaro Prota -- Need C++ expertise? I'm available
https://sourceforge.net/projects/breeze/
(replace 'address' with 'name.surname' to mail)

[Avalon1178]

Wow, I wasn't aware this thread is still alive! Thanks for all your
help!

So I take it that "clearing" a string via setting the variable to ""
or calling .clear() has the same effect. What is usually the practice
that is followed if I want to clear the string (not necessarily free
the memory buffer, but just to "clear" it?) Set the string variable
to "" or call clear()?

[James Kanze]

On Jun 14, 7:20 am, Avalon1178 <Avalon1...@gma il.comwrote:

Wow, I wasn't aware this thread is still alive! Thanks for all your
help!

Somebody woke it up again, with a rant and an additional
question:-).

So I take it that "clearing" a string via setting the variable to ""
or calling .clear() has the same effect. What is usually the practice
that is followed if I want to clear the string (not necessarily free
the memory buffer, but just to "clear" it?) Set the string variable
to "" or call clear()?

Depends on your personal feeling, I think. If I'm thinking in
terms of "clearing" or emptying the string, I'll call clear().
If I'm thinking in terms of an empty string, I'll assign "".

--
James Kanze (GABI Software, from CAI) email:ja******* **@gmail.com
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