s = std::string() vs. s = "" - C++ Language (Moderated)

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Kevin Saff
Guest

Posted: Wed Sep 29, 2004 6:07 pm Post subject: s = std::string() vs. s = ""

When should one use the default constructor for strings, versus assigning
the null literal ""? I have found that my code typically uses "" for
returns and default arguments, but I am beginning to think this is poor
style. String() seems more direct.

--
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Quote:

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Tracking/Fleet Support| * + [_(x)_] + *

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Howard Hinnant
Guest

Posted: Thu Sep 30, 2004 10:32 am Post subject: Re: s = std::string() vs. s = ""

In article <I4rwAp.C94 (AT) news (DOT) boeing.com>,
"Kevin Saff" <google.com (AT) kevin (DOT) saff.net> wrote:

Quote:

sting() can be far more efficient than string(""). The former knows at
compile time to construct a zero length string. The latter must parse
the const char* at runtime with strlen and only then decide that it
doesn't need to allocate a bunch of memory and memcpy the data in.

The Metrowerks implementation of string() (Pro 9) is only 3 or 4
non-branching assembly language statements (inlined of course).
Mentally compare that with what is needed to parse a const char*, figure
out how much capacity you need, allocate it if necessary, and memcpy the
data in.

-Howard

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Victor Bazarov
Guest

Posted: Fri Oct 01, 2004 11:44 am Post subject: Re: s = std::string() vs. s = ""

"Kevin Saff" <google.com (AT) kevin (DOT) saff.net> wrote...

Quote:

s.clear() or s.erase() seem the most direct to me.

V

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Zian Smith
Guest

Posted: Fri Oct 01, 2004 11:48 am Post subject: Re: s = std::string() vs. s = ""

"Kevin Saff" <google.com (AT) kevin (DOT) saff.net> wrote

Quote:

I think they both almost come down to the same thing, they both call
constructors..

s = std::string(); //calls the default constructor, string()
s = ""; //calls the overloaded constructor, string(const char*)

I guess the default constructor call is a little faster since it does
not involve passing around a parameter. If speed is a consideration
perhaps you want to use that?

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news
Guest

Posted: Fri Oct 01, 2004 11:52 am Post subject: Re: s = std::string() vs. s = ""

"Kevin Saff" <google.com (AT) kevin (DOT) saff.net> wrote

Quote:

It is more direct, but the truth of the matter is that a zero length string fed to
the charT* constructor isn't much of a performance penalty.

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Bob Hairgrove
Guest

Posted: Fri Oct 01, 2004 11:54 am Post subject: Re: s = std::string() vs. s = ""

On 29 Sep 2004 14:07:23 -0400, "Kevin Saff"
<google.com (AT) kevin (DOT) saff.net> wrote:

Quote:

When you write this:

std::string x = "";

you are actually constructing a temporary string and invoking the copy
constructor to construct x with the temporary. A good compiler would
probably help optimize away the temporary, but I like using the
default constructor better because it surely must be the "most
optimized" of all.

Whether or not it is better style ... I tend to think that any time I
see something written like the above example, it means the programmer
didn't quite trust (or know) what the default constructor is doing.
That doesn't ncessarily make a good impression, as a programmer, not
knowing the default behavior of an STL class like std::string.

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[email]NoSpamPlease (AT) Home (DOT) com[/email]

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Pete Becker
Guest

Posted: Fri Oct 01, 2004 3:50 pm Post subject: Re: s = std::string() vs. s = ""

Howard Hinnant wrote:

Quote:

The Metrowerks implementation of string() (Pro 9) is only 3 or 4
non-branching assembly language statements (inlined of course).
Mentally compare that with what is needed to parse a const char*, figure
out how much capacity you need, allocate it if necessary, and memcpy the
data in.

if (!*str)
3 or 4 non-branching assembly language statements (inlined of course)

That's almost certainly not going to be an application's primary
bottleneck. <g>

--

Pete Becker
Dinkumware, Ltd. (http://www.dinkumware.com)

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news
Guest

Posted: Fri Oct 01, 2004 4:03 pm Post subject: Re: s = std::string() vs. s = ""

"Howard Hinnant" <hinnant (AT) metrowerks (DOT) com> wrote

more efficient than string(""). The former knows at

Quote:

compile time to construct a zero length string. The latter must parse
the const char* at runtime with strlen and only then decide that it
doesn't need to allocate a bunch of memory and memcpy the data in.

It won't take strlen very long to determine the string size is zero.
If the string size is zero, the compiler isn't going to allocate memory or copy
anything.

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James Hopkin
Guest

Posted: Fri Oct 01, 2004 4:57 pm Post subject: Re: s = std::string() vs. s = ""

"Kevin Saff" <google.com (AT) kevin (DOT) saff.net> wrote

Quote:

I would say s.clear() is the most direct incantation.

James

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Ulrich Eckhardt
Guest

Posted: Fri Oct 01, 2004 5:11 pm Post subject: Re: s = std::string() vs. s = ""

Kevin Saff wrote:

Quote:

Two comments here:
- using string() is potentially faster. Further, using clear() to clear a
string is even more expressive and potentially another bit faster.
- in the presence of generic programming, you can't use "", e.g.
template<typename CharType>
void clear_string( std::basic_string<CharType>& bar)
{
bar = ""; // fails for CharType = wchar_t
bar = std::basic_string<CharType>(); // works, but hard to read
bar.clear(); // short, fast, works.
}

Uli

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Kevin Saff
Guest

Posted: Fri Oct 01, 2004 5:16 pm Post subject: Re: s = std::string() vs. s = ""

"Bob Hairgrove" <invalid (AT) bigfoot (DOT) com> wrote

Quote:

On 29 Sep 2004 14:07:23 -0400, "Kevin Saff"
[email]google.com (AT) kevin (DOT) saff.net[/email]> wrote:

When should one use the default constructor for strings, versus
assigning
the null literal ""? I have found that my code typically uses "" for
returns and default arguments, but I am beginning to think this is poor
style. String() seems more direct.

When you write this:

std::string x = "";

you are actually constructing a temporary string and invoking the copy
constructor to construct x with the temporary. A good compiler would
probably help optimize away the temporary, but I like using the
default constructor better because it surely must be the "most
optimized" of all.

Whether or not it is better style ... I tend to think that any time I
see something written like the above example, it means the programmer
didn't quite trust (or know) what the default constructor is doing.
That doesn't ncessarily make a good impression, as a programmer, not
knowing the default behavior of an STL class like std::string.

I think I've always found that line ugly. I should have given examples of
the particular cases where I have been likely to use "":

inline std::string using_null_literals (std::string const& argument =
"")
{
return "";
}

Compare this to:

inline std::string using_default_construction (std::string const&
argument = std::string())
{
return std::string();
}

(Or better, a typedef String somewhere at namespace level to get rid of the
ugly :: operator, and allow switching between different string
implementations, like wstring.)

The former is somewhat faster to type, and when using a non-empty literal
for a default argument or return there is no reason not to just use
"a-literal-string". So, I suppose I have in the past preferred "" for
consistency. I now think just std::string() is better since it allows the
binding of the construction of an empty object at compile time, rather than
testing at run-time.

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Kevin Saff
Guest

Posted: Fri Oct 01, 2004 5:19 pm Post subject: Re: s = std::string() vs. s = ""

"Howard Hinnant" <hinnant (AT) metrowerks (DOT) com> wrote

Quote:

In article <I4rwAp.C94 (AT) news (DOT) boeing.com>,
"Kevin Saff" <google.com (AT) kevin (DOT) saff.net> wrote:

When should one use the default constructor for strings, versus
assigning
the null literal ""? I have found that my code typically uses "" for
returns and default arguments, but I am beginning to think this is poor
style. String() seems more direct.

sting() can be far more efficient than string(""). The former knows at
compile time to construct a zero length string. The latter must parse
the const char* at runtime with strlen and only then decide that it
doesn't need to allocate a bunch of memory and memcpy the data in.

The Metrowerks implementation of string() (Pro 9) is only 3 or 4
non-branching assembly language statements (inlined of course).
Mentally compare that with what is needed to parse a const char*, figure
out how much capacity you need, allocate it if necessary, and memcpy the
data in.

Thanks, I think that convinces me; since I don't see any advantage to "",
and string() might be faster, there's no good reason to use "".

I doubt I'll be able to tell the difference speed-wise in my current
project, which is limited by virtual memory thrashing due to my data storage
design - really generic, really two orders of magnitude too large for the
application. I think it'll be awhile before considerations such as "" vs.
string() show up in my performance instrumentation.

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Kevin Saff
Guest

Posted: Sat Oct 02, 2004 1:08 am Post subject: Re: s = std::string() vs. s = ""

"James Hopkin" <jhopkin (AT) reflectionsinteractive (DOT) com> wrote

Quote:

"Kevin Saff" <google.com (AT) kevin (DOT) saff.net> wrote

When should one use the default constructor for strings, versus
assigning
the null literal ""? I have found that my code typically uses "" for
returns and default arguments, but I am beginning to think this is poor
style. String() seems more direct.

I would say s.clear() is the most direct incantation.

I'm sorry, I meant constructing, not assigning. I agree s.clear() is better
than either s = string() or s = "".

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news
Guest

Posted: Tue Oct 05, 2004 6:17 am Post subject: Re: s = std::string() vs. s = ""

"Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote

Quote:

"Kevin Saff" <google.com (AT) kevin (DOT) saff.net> wrote...
When should one use the default constructor for strings, versus assigning
the null literal ""? I have found that my code typically uses "" for
returns and default arguments, but I am beginning to think this is poor
style. String() seems more direct.

s.clear() or s.erase() seem the most direct to me.

It would if he was trying to erase an string. What he is trying to do is
pass an empty string to another function.

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Ben Hutchings
Guest

Posted: Tue Oct 05, 2004 10:05 pm Post subject: Re: s = std::string() vs. s = ""

Howard Hinnant wrote:

Quote:

VC++ 7.1 can evaluate strlen(string-literal) at compile-time. You
need to work harder on your optimiser. ;-)

--
Ben Hutchings
Nothing is ever a complete failure; it can always serve as a bad example.

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