passing shared_ptr to std::fstream * EDIT

Question

I had some problem to understand the shared_ptr doc since I am newbie in c++. I hope you could help me with my example code:

#include <iomanip>
#include <string>
#include <iostream>
#include <boost/shared_ptr.hpp>
#include <boost/multi_array.hpp>
#include <boost/lexical_cast.hpp>
#include <cstdlib>
#include <fstream>   // file I/O
#include <sstream>

void open_file( boost::shared_ptr< std::fstream > &file_stream , const std::string file_name , uint8_t presition );

int main( int argc, char* argv[] ) {

    uint32_t         num_bits = 12;
    std::fstream     *ls_coeff_p;
    boost::multi_array< boost::shared_ptr< std::fstream> , 2> ls_coeff_f( boost::extents[ num_bits ][ 2 ] );

    std::string  ls_coeff_file_name = "datafiles/ls_coeff";
    std::string  stable             = "_stable_";
    std::string  unstable           = "_unstable_";
    std::string  file_name_end      = ".log";

    for ( uint8_t i = 0; i < num_bits ; i++ ) {             
            open_file( ls_coeff_f[i][0] , ls_coeff_file_name + unstable + boost::lexical_cast<std::string>(i) + file_name_end , 4 );
            open_file( ls_coeff_f[i][1] , ls_coeff_file_name +   stable + boost::lexical_cast<std::string>(i) + file_name_end , 4 );
    }

    // just as test case
    ls_coeff_p = ls_coeff_f[0][0];

    *ls_coeff_p << "Hallo world!" << std::endl;

    for ( uint8_t i = 0; i < num_bits ; i++ ) {
            ls_coeff_f[i][0]->close();
            ls_coeff_f[i][1]->close();
    }

}

void open_file( boost::shared_ptr< std::fstream > &file_stream , const std::string file_name , uint8_t presition ) {

    file_stream->open ( file_name  , std::fstream::out );
    file_stream->precision( presition );
    file_stream->setf(std::ios::fixed,std::ios::floatfield);

}

i get the following error:

 In function 'int main(int, char**)':
 Line 30: error: cannot convert 'boost::shared_ptr<std::basic_fstream<char, std::char_traits<char> > >' to 'std::fstream*' in assignment
 compilation terminated due to -Wfatal-errors.

Regards

EDIT

after appling the suggestions of @Space_C0wb0y:

#include <iomanip>
#include <string>
#include <iostream>
#include <boost/shared_ptr.hpp>
#include <boost/multi_array.hpp>
#include <boost/lexical_cast.hpp>
#include <cstdlib>
#include <fstream>   // file I/O
#include <sstream>

void open_file( boost::shared_ptr< std::fstream > &file_stream , const std::string file_name , uint8_t presition );

int main( int argc, char* argv[] ) {

     uint32_t         num_bits = 12;
     std::fstream     *ls_coeff_p;
     boost::multi_array< boost::shared_ptr< std::fstream> , 2> ls_coeff_f( boost::extents[ num_bits ][ 2 ] );

     std::string  ls_coeff_file_name = "datafiles/ls_coeff";
     std::string  stable             = "_stable_";
     std::string  unstable           = "_unstable_";
     std::string  file_name_end      = ".log";

     for ( uint8_t i = 0; i < num_bits ; i++ ) {             
          open_file( ls_coeff_f[i][0] , ls_coeff_file_name + unstable + boost::lexical_cast<std::string>(i) + file_name_end , 4 );
          open_file( ls_coeff_f[i][1] , ls_coeff_file_name +   stable + boost::lexical_cast<std::string>(i) + file_name_end , 4 );
}

    // just as test case
    ls_coeff_p = ls_coeff_f[0][0].get();

    *ls_coeff_p << "Hallo world!" << std::endl;

    for ( uint8_t i = 0; i < num_bits ; i++ ) {
        ls_coeff_f[i][0]->close();
        ls_coeff_f[i][1]->close();
    }

}

void open_file( boost::shared_ptr< std::fstream > &file_stream , const std::string file_name , uint8_t presition ) {

    file_stream->open ( file_name.c_str()  , std::fstream::out );
    file_stream->precision( presition );
    file_stream->setf(std::ios::fixed,std::ios::floatfield);

}

i get the following error:

t: /usr/local/include/boost/shared_ptr.hpp:315: T* boost::shared_ptr<T>::operator->() const [with T = std::basic_fstream<char, std::char_traits<char> >]: Assertion `px != 0' failed.

i debuged it a bit and found out the the problem is caused in

 file_stream->open ( file_name.c_str()  , std::fstream::out );

also if it is not inside the function open_file I get the same error

The answers are perfectly fine, just 2 notes: You will get runtime errors for not inizializing your pointers. And as a general guideline, try to remain consistent in what pointers you use. Using a bald pointer to a shared resource can cause trouble, if the shared pointer goes out of scope or you "delete" your pointer. — AndreasT, Mar 26 '11 at 11:22
@AndreasT, is this the inizializion line that i forgot: `ls_coeff_f[i][0] = boost::shared_ptr(new std::fstream );` is this how i was suppose to write it? or else? — Eagle, Mar 26 '11 at 16:34

Björn Pollex · Answer 1 · 2011-03-26T20:49:49.260

To access the raw pointer in a shared_ptr you have to use the get-method:

ls_coeff_p = ls_coeff_f[0][0].get();

About your edit:

You have to initialize the shared pointers in the array before you can use them. Here is how I'd do it:

boost::shared_ptr<std::fstream> open_file( const std::string file_name , 
                                           uint8_t presition ) {
    boost::shared_ptr<std::fstream> stream = boost::make_shared<std::fstream>( 
        file_name.c_str(), std::fstream::out )
    stream->precision( presition );
    stream->setf(std::ios::fixed,std::ios::floatfield);
    return stream;
}

for ( uint8_t i = 0; i < num_bits ; i++ ) {
            ls_coeff_f[i][0] = open_file( ls_coeff_file_name + unstable + 
                                          boost::lexical_cast<std::string>(i) + 
                                          file_name_end , 4 );
            ls_coeff_f[i][1] = open_file( ls_coeff_file_name + stable + 
                                          boost::lexical_cast<std::string>(i) + 
                                          file_name_end , 4 );
    }

changed my code and got: Line 43: error: 'class boost::shared_ptr > >' has no member named 'open' — Eagle, Mar 26 '11 at 11:13

score 1 · Answer 2 · answered Mar 26 '11 at 11:03

1

You can get the underlying pointer of a shared_ptr with the get member. A better option would be to make ls_coeff_p a shared_ptr as well.

You can also skip the temporary variable and do

*(ls_coeff_f[0][0]) << "Hello world" << std::endl;

answered Mar 26 '11 at 11:03

Fred Foo

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passing shared_ptr to std::fstream * EDIT

2 Answers2