Re: Is this the correct way to do this template?

From:

"Jim Langston" <tazmaster@rocketmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 3 Jun 2006 14:27:24 -0700

Message-ID:

<29ngg.70$4l4.35@fe04.lga>

"Kai-Uwe Bux" <jkherciueh@gmx.net> wrote in message
news:1283tncifq54639@corp.supernews.com...

Jim Langston wrote:

"Kai-Uwe Bux" <jkherciueh@gmx.net> wrote in message
news:1283oqn1ptr1n6d@corp.supernews.com...

Jim Langston wrote:

"Kai-Uwe Bux" <jkherciueh@gmx.net> wrote in message
news:1283mflrghr5e76@corp.supernews.com...

Jim Langston wrote:

"Kai-Uwe Bux" <jkherciueh@gmx.net> wrote in message
news:1283jn2thc3n315@corp.supernews.com...

Jim Langston wrote:

I have a template I call StrmConvert, which uses std::stringstream
to convert from any type to any other type that can be used by
stringstring. This is what it looks like:

template<typename T, typename F > T StrmConvert( F from )
{
    std::stringstream temp;
    temp << from;
    T to = T();
    temp >> to;
    return to;
}

[snip]

The use for double does concern me, given your comment about the
precision
problems, which is why I asked how to fix it. There are some cases
where I'm transfering a float value with a number of decimal digits I
need to
preserve. So far it hasn't caused any problems, but later on it will
so
I
will need to fix it.

Giving an optional 3rd paramter for places of precision would probably
be my best bet.

Ok, here is a hack that I use to determine how many digits I need to
print in order to preserve the accuracy.

#include <iostream>
#include <limits>
#include <cmath>

template < typename T >
unsigned short max_prec ( void ) {
return
   2 +
   static_cast< unsigned short >
   ( - std::log( std::numeric_limits<T>::epsilon() ) / std::log( 10.0 )
   );
}

int main ( void ) {
std::cout << "float: " << max_prec< float >() << '\n'
           << "double: " << max_prec< double >() << '\n'
           << "long double: " << max_prec< long double >() << '\n';
}

If you use max_prec() digits, you should be fine -- if it wasn't for the
usual pitfalls of floats, that is.

Bah, std::stringstream ignores precision on text input.

Huh?

That is, the
follow program outputs:
1.2345678901229999
1.23457

Ok, let's see.

I guess std::stringstream can't convert from a text string to a floating
point value with any type of accuracy.

It is supposed to.

#include <iostream>
#include <string>
#include <sstream>

int main ( void )
{
    std::stringstream MyStream;
    MyStream.precision( 18 );

    double DoubleVal = 1.234567890123;
    std::string Text;

    MyStream << DoubleVal;
    MyStream >> Text;
    std::cout << Text << std::endl;

    MyStream << Text;
    MyStream >> DoubleVal;
    std::cout << DoubleVal << std::endl;

Aha! Here, you print DoubleVal to std::cout with 6 digits of precision!

std::string wait;
std::cin >> wait;
}

Well, now I really feel like a n00b. lol.

Thanks. I've settled on always setting the precision to 17 since I was
having trouble calling your template if the F wasn't a float or double, and
even inside an if block if (typeid(F) == typeid(double) ).. it wouldn't
compile because even though it would never run that code it insisted on
compiling it anyway.

template<typename T, typename F > T StrmConvert( F from )
{
    std::stringstream temp;
    temp.precision( 17 );
    temp << from;
    T to = T();
    temp >> to;
    return to;
}

template<typename F> std::string StrmConvert( F from )
{
    return StrmConvert<std::string>( from );
}