Re: Any possiblity to avoid costy construction of temporary?

From:

Oncaphillis <oncaphillis@snafu.de>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 23 Mar 2008 23:22:39 CST

Message-ID:

<47E7077B.3060404@snafu.de>

Sebastian Karlsson wrote:

Hash("SomeLengthyString") is expensive to construct and will stay
constant for all calls to SomeFunc. I understand that the compiler
isn't allowed to cache Hash("SomeLengthyString") through calls since
it's a performance / memory trade off, and from the compilers pov the
constructor of Hash might have additional dependencies. I would get
the expected behaviour if I would make Hash("SomeLengthyString")
static, which led me to the following code, which even compiles under
MSVC2005:

[code]
void SomeFunc( Hash someHash )
{
     if( static Hash("SomeLengthyString") == someHash )
     {
          ...
     }
}
[/code]

However the constructor of Hash is still called once for every
comparsion.

g++ does not accept that construct at all. Complaining:

<snip>
error: expected primary-expression before ?static?
</snip>

Seems like you're constructing a static temporary variable,
since there is no object name only the constructor Hash(...).

Anyway. If I do:

<snip>
....
static Hash h("SomeLengthyString");
if( h == someHash) {
....
}
</snip>

it behaves like expected. Sometimes dense expressions
like declaration and comparison in one line is evil.
Although I like it too :-).

Hope that helps.

O.

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