Re: strings to construct a system call.

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Thu, 16 Oct 2008 02:07:54 -0700 (PDT)
Message-ID:
<212816d2-eafd-48e9-b54c-d6b17f5f1a61@u28g2000hsc.googlegroups.com>
On Oct 16, 1:43 am, Atropo <lxvasq...@gmail.com> wrote:

Having several strings how do I construct variable to pass to
system():
Lets say the date command

 string str = "14/10/08 19:06:09";
        strDD = str.substr(0,2);
        strMM = str.substr(3,2);
        strYY = str.substr(6,2);
        strhh = str.substr(9,2);
        strmm = str.substr(12,2);
        strss = str.substr(15,2);


Hmmm. I'd use boost::regex for this, checking the format at the
same time.

Also, you don't show the definitions; I'm supposing that strDD,
et al. are all std::string.

// I would like contruct date -u 1014190608.09 2>&1 > /dev/
null

strT<<"date -u "<<strMM<<strDD<<strhh<<strmm<<strYY<<"."<<strss<< "
2>&1 >/dev/null;"<<endl;


Again, you don't show the definition of strT. Given the use,
I'll suppose it's an std::ostringstream...

             system(strT);


Which isn't an appropriate type for system(). You need a char
const*. If strT is an std::ostringstream, this means:

    system( strT.str().c_str() ) ;

when I compile it throws

rtchk.cpp: In function `int main (int, char **)':
rtchk.cpp:59: no match for `string & << const char[9]'


Not having the actual sources, it's very difficult to know
where line 59 is. But since the error message is complaining
about an << operator into string (and not std::string? curious),
I'll guess that you declared strT as a string (whatever that
is), and not as an std::ostringstream.

so i could be a syntax error, but I'm newbie and don't know.


Not a syntax error, but a type error. You need to learn about
data types.

The classical way of writing this would be something like:

    void
    setDate( std::string const& userDateString )
    {
        static boost::regex const
                                matcher(
            "^(\\d\\d)/(\\d\\d)/(\\d\\d) (\\d\\d):(\\d\\d):(\\d\\d)
$" ) ;
        enum DateFmt { date, day, month, year, hour, minute,
second } ;
        boost::match_results< std::string::const_iterator >
                                fields ;
        if ( ! boost::regex_match( userDateString, fields, matcher ) )
{
            throw std::runtime_error( "Illegal date" ) ;
        }
        std::ostringstream builder ;
        builder.fill( '0' ) ;
        builder << "echo date -u "
                << std::setw( 2 ) << fields[ month ]
                << std::setw( 2 ) << fields[ day ]
                << std::setw( 2 ) << fields[ hour ]
                << std::setw( 2 ) << fields[ minute ]
                << std::setw( 2 ) << fields[ year ]
                << '.' << std::setw( 2 ) << fields[ second ] ;
        if ( system( builder.str().c_str() ) != 0 ) {
            throw std::runtime_error( "Attempt to set date failed" ) ;
        }
    }

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
                   Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34

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