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reading a file into std::string

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  reading a file into std::string arnuld <sunrise@invalid.address> - 2011-08-08 13:15 +0000
    Re: reading a file into std::string Marc <marc.glisse@gmail.com> - 2011-08-08 13:40 +0000
    Re: reading a file into std::string Sam <sam@email-scan.com> - 2011-08-08 08:58 -0500
      Re: reading a file into std::string Sam <sam@email-scan.com> - 2011-08-08 09:05 -0500
        Re: reading a file into std::string red floyd <no.spam.here@its.invalid> - 2011-08-08 13:23 -0700
          Re: reading a file into std::string arnuld <sunrise@invalid.address> - 2011-08-09 05:13 +0000
            Re: reading a file into std::string red floyd <no.spam.here@its.invalid> - 2011-08-08 23:12 -0700
              Re: reading a file into std::string arnuld <sunrise@invalid.address> - 2011-08-09 06:45 +0000
                Re: reading a file into std::string red floyd <no.spam.here@its.invalid> - 2011-08-09 09:26 -0700
                  Re: reading a file into std::string arnuld <sunrise@invalid.address> - 2011-08-10 10:58 +0000
                    Re: reading a file into std::string "Bo Persson" <bop@gmb.dk> - 2011-08-10 16:19 +0200
                    Re: reading a file into std::string red floyd <no.spam.here@its.invalid> - 2011-08-10 10:06 -0700
                Re: reading a file into std::string red floyd <no.spam.here@its.invalid> - 2011-08-09 09:28 -0700
                Re: reading a file into std::string red floyd <no.spam.here@its.invalid> - 2011-08-09 09:33 -0700
                  Re: reading a file into std::string arnuld <sunrise@invalid.address> - 2011-08-10 12:13 +0000
                    Re: reading a file into std::string Nobody <nobody@nowhere.com> - 2011-08-11 08:57 +0100
                      Re: reading a file into std::string "Bo Persson" <bop@gmb.dk> - 2011-08-11 12:51 +0200
                        Re: reading a file into std::string Nobody <nobody@nowhere.com> - 2011-08-12 05:00 +0100
        Re: reading a file into std::string Nana J. Osei <mppccf0987@att.net> - 2011-08-13 23:18 +0000
          Re: reading a file into std::string Sam <sam@email-scan.com> - 2011-08-13 20:58 -0500
      Re: reading a file into std::string Juha Nieminen <nospam@thanks.invalid> - 2011-08-09 11:26 +0000
        Re: reading a file into std::string Marc <marc.glisse@gmail.com> - 2011-08-09 12:54 +0000
          Re: reading a file into std::string Juha Nieminen <nospam@thanks.invalid> - 2011-08-09 16:33 +0000
            Re: reading a file into std::string Marc <marc.glisse@gmail.com> - 2011-08-09 16:47 +0000
              Re: reading a file into std::string red floyd <no.spam.here@its.invalid> - 2011-08-09 11:27 -0700
                Re: reading a file into std::string Joshua Maurice <joshuamaurice@gmail.com> - 2011-08-11 15:34 -0700
                  Re: reading a file into std::string Juha Nieminen <nospam@thanks.invalid> - 2011-08-12 05:47 +0000
        Re: reading a file into std::string arnuld <sunrise@invalid.address> - 2011-08-09 12:57 +0000
          Re: reading a file into std::string Joshua Maurice <joshuamaurice@gmail.com> - 2011-08-11 15:40 -0700
    Re: reading a file into std::string Jean-Luc Romano <jl_post@hotmail.com> - 2011-08-12 11:28 -0700
      Re: reading a file into std::string Lynn McGuire <lmc@winsim.com> - 2011-08-12 15:56 -0500

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#8563 — reading a file into std::string

I want to read a file into std::string. I am basically a C Programmer so 
it was quite hard for me to understand how to do it in C++. I did C++ 
long time back (if you guys remember my name but I do remember Shiva and 
Victor Bazarov and others). 

I googled for it and this is the best what I could come up with. Do you 
guys have any suggestion for improvement ?  and whether this is really a 
correct C++ program. (Compiled with "g++ -ansi -pedantic -Wall -Wextra")



#include <iostream>
#include <fstream>
#include <string>


int main()
{
  std::string my_contents, tmp_contents;
  std::ifstream my_file("reference.cpp");
  if(!my_file)
    {
      std::cerr << "Error Opening file" << std::endl;
      exit(EXIT_FAILURE);
    }

   while(my_file)
    {
     std::getline(my_file, tmp_contents);
     my_contents += tmp_contents;
     my_contents += "\n";
    }

  std::cout << "String contents are: "<< "\n"
	    << my_contents << std::endl;

  my_file.close();

  return 0;
}




 
-- arnuld
www.LispMachine.Wordpress.com

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#8564

arnuld  wrote:

> I want to read a file into std::string. I am basically a C Programmer so 
> it was quite hard for me to understand how to do it in C++. I did C++ 
> long time back (if you guys remember my name but I do remember Shiva and 
> Victor Bazarov and others). 
[...]
>   std::string my_contents, tmp_contents;
>   std::ifstream my_file("reference.cpp");
>   if(!my_file)
>     {
>       std::cerr << "Error Opening file" << std::endl;
>       exit(EXIT_FAILURE);
>     }
> 
>    while(my_file)
>     {
>      std::getline(my_file, tmp_contents);
>      my_contents += tmp_contents;
>      my_contents += "\n";
>     }

You might as well use std::getline(my_file, my_contents, '\0'),
assuming there is no null character in your file. Not that it is a
good solution, but at least you don't have a loop.

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#8565

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arnuld writes:

> I want to read a file into std::string. I am basically a C Programmer so
> it was quite hard for me to understand how to do it in C++. I did C++
> long time back (if you guys remember my name but I do remember Shiva and
> Victor Bazarov and others).
>
> I googled for it and this is the best what I could come up with. Do you
> guys have any suggestion for improvement ?  and whether this is really a
> correct C++ program. (Compiled with "g++ -ansi -pedantic -Wall -Wextra")

Your program is basically a C program. Here's a C++ program.

#include <iostream>
#include <fstream>
#include <sstream>

int main()
{
	std::ostringstream o;

	o << std::ifstream("reference.cpp").rdbuf();

	std::string my_contents= o.str();

	return 0;
}

>
>
>
> #include <iostream>
> #include <fstream>
> #include <string>
>
>
> int main()
> {
>   std::string my_contents, tmp_contents;
>   std::ifstream my_file("reference.cpp");
>   if(!my_file)
>     {
>       std::cerr << "Error Opening file" << std::endl;
>       exit(EXIT_FAILURE);
>     }
>
>    while(my_file)
>     {
>      std::getline(my_file, tmp_contents);
>      my_contents += tmp_contents;
>      my_contents += "\n";
>     }
>
>   std::cout << "String contents are: "<< "\n"
> 	    << my_contents << std::endl;
>
>   my_file.close();
>
>   return 0;
> }
>
>
>
>
>
> -- arnuld
> www.LispMachine.Wordpress.com

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#8566

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Sam writes:

> arnuld writes:
>
>> I want to read a file into std::string. I am basically a C Programmer so
>> it was quite hard for me to understand how to do it in C++. I did C++
>> long time back (if you guys remember my name but I do remember Shiva and
>> Victor Bazarov and others).
>>
>> I googled for it and this is the best what I could come up with. Do you
>> guys have any suggestion for improvement ?  and whether this is really a
>> correct C++ program. (Compiled with "g++ -ansi -pedantic -Wall -Wextra")
>
> Your program is basically a C program. Here's a C++ program.
>
> #include <iostream>
> #include <fstream>
> #include <sstream>
>
> int main()
> {
> 	std::ostringstream o;
>
> 	o << std::ifstream("reference.cpp").rdbuf();
>
> 	std::string my_contents= o.str();
>
> 	return 0;
> }

… Or, here's a slightly more efficient version.

#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>

int main()
{
	std::string my_contents;

	std::copy(std::istreambuf_iterator<char>(std::ifstream("reference.cpp").rdbuf()),
		  std::istreambuf_iterator<char>(),
		  std::back_insert_iterator<std::string>(my_contents));

	return 0;
}

>
>>
>>
>>
>> #include <iostream>
>> #include <fstream>
>> #include <string>
>>
>>
>> int main()
>> {
>>   std::string my_contents, tmp_contents;
>>   std::ifstream my_file("reference.cpp");
>>   if(!my_file)
>>     {
>>       std::cerr << "Error Opening file" << std::endl;
>>       exit(EXIT_FAILURE);
>>     }
>>
>>    while(my_file)
>>     {
>>      std::getline(my_file, tmp_contents);
>>      my_contents += tmp_contents;
>>      my_contents += "\n";
>>     }
>>
>>   std::cout << "String contents are: "<< "\n"
>> 	    << my_contents << std::endl;
>>
>>   my_file.close();
>>
>>   return 0;
>> }
>>
>>
>>
>>
>>
>> -- arnuld
>> www.LispMachine.Wordpress.com

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#8570

On 8/8/2011 7:05 AM, Sam wrote:
> Sam writes:
>
>> arnuld writes:
>>
>>> I want to read a file into std::string. I am basically a C Programmer so
>>> it was quite hard for me to understand how to do it in C++. I did C++
>>> long time back (if you guys remember my name but I do remember Shiva and
>>> Victor Bazarov and others).
>>>
>>> I googled for it and this is the best what I could come up with. Do you
>>> guys have any suggestion for improvement ? and whether this is really a
>>> correct C++ program. (Compiled with "g++ -ansi -pedantic -Wall -Wextra")
>>
>> Your program is basically a C program. Here's a C++ program.
>>
>> #include <iostream>
>> #include <fstream>
>> #include <sstream>
>>
>> int main()
>> {
>> std::ostringstream o;
>>
>> o << std::ifstream("reference.cpp").rdbuf();
>>
>> std::string my_contents= o.str();
>>
>> return 0;
>> }
>
> … Or, here's a slightly more efficient version.
>
> #include <iostream>
> #include <fstream>
> #include <iterator>
> #include <algorithm>
>
> int main()
> {
> std::string my_contents;
>
> std::copy(std::istreambuf_iterator<char>(std::ifstream("reference.cpp").rdbuf()),
>
> std::istreambuf_iterator<char>(),
> std::back_insert_iterator<std::string>(my_contents));
>
> return 0;
> }
>
>


Avoid the copy.


#include <iostream>
#include <fstream>
#include <string>
#include <iterator>

int main()
{
    // note: extra parens on the constructor args to
    //   avoid potential "Most Vexing Parse" issues
    std::string my_contents(
       std::istreambuf_iterator<char>(
          (std::ifstream("reference.cpp").rdbuf())),
          (std::istreambuf_iterator<char>()));
}

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#8577

> On Mon, 08 Aug 2011 13:23:49 -0700, red floyd wrote:

> Avoid the copy.
> 
> 
> #include <iostream>
> #include <fstream>
> #include <string>
> #include <iterator>
> 
> int main()
> {
>     // note: extra parens on the constructor args to 
>     // avoid potential "Most Vexing Parse" issues 
>        std::string my_contents(
>        std::istreambuf_iterator<char>(
>           (std::ifstream("reference.cpp").rdbuf())),
>           (std::istreambuf_iterator<char>()));
> }


Can you please tell me in brief how it works. 2nd, you said "Avoid the 
copy", so your program does not copy ?

All I can see is its a template. What about error checking whether file 
was opened successfully or not, we are nto even closing the file after we 
finish ? I have Stroustrup 3/e in my hands, checking out what rdbuf() 
does from section 21.6.3, page 644.


-- 
-- arnuld
www.LispMachine.Wordpress.com

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#8578

On 8/8/2011 10:13 PM, arnuld wrote:
>> On Mon, 08 Aug 2011 13:23:49 -0700, red floyd wrote:
>
>> Avoid the copy.
>>
>>
>> #include<iostream>
>> #include<fstream>
>> #include<string>
>> #include<iterator>
>>
>> int main()
>> {
>>      // note: extra parens on the constructor args to
>>      // avoid potential "Most Vexing Parse" issues
>>         std::string my_contents(
>>         std::istreambuf_iterator<char>(
>>            (std::ifstream("reference.cpp").rdbuf())),
>>            (std::istreambuf_iterator<char>()));
>> }
>
>
> Can you please tell me in brief how it works. 2nd, you said "Avoid the
> copy", so your program does not copy ?

The example I was referring to had constructed an empty string, and then
copied the file into it.

My example creates the string with the contents of the file, by using
a constructor that takes two iterators.

In actual practice, there won't be much difference, but one-upmanship
sometimes comes into play on this newsgroup!  <grin>

> All I can see is its a template. What about error checking whether file
> was opened successfully or not, we are nto even closing the file after we
> finish ? I have Stroustrup 3/e in my hands, checking out what rdbuf()
> does from section 21.6.3, page 644.

It is "template" code.  Error checking, etc... is left as an exercise
for the reader.

rdbuf() returns the streambuf underlying the fstream.

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#8579

> On Mon, 08 Aug 2011 23:12:30 -0700, red floyd wrote:

> My example creates the string with the contents of the file, by using a
> constructor that takes two iterators.

Chapter Strings, Section 20.3.4 I don't see any constructir which takes 2 
arguments. If I go to section 16.3.4 then it does have two iterators in a 
constructor but that is std::vector not std::string
 

> It is "template" code.  Error checking, etc... is left as an exercise
> for the reader.

you mean this code will be real-life code based on the ideas you have 
given me:

#include <iostream>
#include <fstream>
#include <string>


int main()
{
  std::ifstream my_file("reference.cpp");
  if(!my_file)
    {
      std::cerr << "Error Opening file" << std::endl;
      exit(EXIT_FAILURE);
    }

  std::string my_contents(std::istreambuf_iterator<char>(my_file.rdbuf()),
			  (std::istreambuf_iterator<char>()));

  std::cout << "String contents are: "<< "\n"
	    << my_contents << std::endl;

  my_file.close();

  return 0;
}
 
as usual compiled with "gcc -ansi -pedantic -Wall -Wextra" and it 
compiles and runs fine.



-- 
-- arnuld
www.LispMachine.Wordpress.com

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#8593

On 8/8/2011 11:45 PM, arnuld wrote:
>> On Mon, 08 Aug 2011 23:12:30 -0700, red floyd wrote:
>
>> My example creates the string with the contents of the file, by using a
>> constructor that takes two iterators.
>
> Chapter Strings, Section 20.3.4 I don't see any constructir which takes 2
> arguments. If I go to section 16.3.4 then it does have two iterators in a
> constructor but that is std::vector not std::string
>

ISO/IEC 14882:2003, sections 21.3/6 [lib.basic.string] and
                              21.3.1/14 [lib.string.cons]

template<class InputIterator>
basic_string(InputIterator begin, InputIterator end,
              const Allocator& a = Allocator());

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#8615

> On Tue, 09 Aug 2011 09:26:58 -0700, red floyd wrote:

> ISO/IEC 14882:2003, sections 21.3/6 [lib.basic.string] and
>                               21.3.1/14 [lib.string.cons]

> template<class InputIterator>
> basic_string(InputIterator begin, InputIterator end,
>               const Allocator& a = Allocator());

I have n3242.pdf and in section 21.3 (page 624) I can't find this (may 
template syntax is too alien for me). 2nd, even if its there, we used 2 
arguments unlike 3 mentioned here. I assume it will have some default 
value.






-- 
-- arnuld
www.LispMachine.Wordpress.com

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#8629

arnuld wrote:
>> On Tue, 09 Aug 2011 09:26:58 -0700, red floyd wrote:
>
>> ISO/IEC 14882:2003, sections 21.3/6 [lib.basic.string] and
>>                               21.3.1/14 [lib.string.cons]
>
>> template<class InputIterator>
>> basic_string(InputIterator begin, InputIterator end,
>>               const Allocator& a = Allocator());
>
> I have n3242.pdf and in section 21.3 (page 624) I can't find this
> (may template syntax is too alien for me). 2nd, even if its there,
> we used 2 arguments unlike 3 mentioned here. I assume it will have
> some default value.

It is a member (constructor). See page 629 and bottom of page 634.



Bo Persson

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#8638

On 8/10/2011 3:58 AM, arnuld wrote:
>> On Tue, 09 Aug 2011 09:26:58 -0700, red floyd wrote:
>
>> ISO/IEC 14882:2003, sections 21.3/6 [lib.basic.string] and
>>                                21.3.1/14 [lib.string.cons]
>
>> template<class InputIterator>
>> basic_string(InputIterator begin, InputIterator end,
>>                const Allocator&  a = Allocator());
>
> I have n3242.pdf and in section 21.3 (page 624) I can't find this (may
> template syntax is too alien for me). 2nd, even if its there, we used 2
> arguments unlike 3 mentioned here. I assume it will have some default
> value.

The section numbering of n3242 (C++11 draft) differs from the official
2003 Standard.

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#8595

On 8/8/2011 11:45 PM, arnuld wrote:
> you mean this code will be real-life code based on the ideas you have
> given me:
>
> #include<iostream>
> #include<fstream>
> #include<string>
>
>
> int main()
> {
>    std::ifstream my_file("reference.cpp");
>    if(!my_file)
>      {
>        std::cerr<<  "Error Opening file"<<  std::endl;
>        exit(EXIT_FAILURE);
>      }
>
>    std::string my_contents(std::istreambuf_iterator<char>(my_file.rdbuf()),
> 			  (std::istreambuf_iterator<char>()));
>
>    std::cout<<  "String contents are:"<<  "\n"
> 	<<  my_contents<<  std::endl;
>
>    my_file.close();
>
>    return 0;
> }

Just an FYI, the my_file.close() is not required, as the ifstream
destructor will close it for you.

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#8596

On 8/8/2011 11:45 PM, arnuld wrote:

> you mean this code will be real-life code based on the ideas you have
> given me:
>
> #include<iostream>
> #include<fstream>
> #include<string>
>
>
> int main()
> {
>    std::ifstream my_file("reference.cpp");
>    if(!my_file)
>      {
>        std::cerr<<  "Error Opening file"<<  std::endl;
>        exit(EXIT_FAILURE);
>      }
>
>    std::string my_contents(std::istreambuf_iterator<char>(my_file.rdbuf()),
> 			  (std::istreambuf_iterator<char>()));
>
>    std::cout<<  "String contents are:"<<  "\n"
> 	<<  my_contents<<  std::endl;
>
>    my_file.close();
>
>    return 0;
> }
>
> as usual compiled with "gcc -ansi -pedantic -Wall -Wextra" and it
> compiles and runs fine.
Technically. you need to #include <cstdlib> to define EXIT_FAILURE.

In actual practice, obviously one of <iostream>, <fstream>, or <string>
is including it.

Also, I believe std::endl is defined in <ostream>.  In C++11 <iostream> 
covers that, but in C++03, <iostream> does not officially include
<ostream>, though as far as I know, all known C++ compilers do the 
nested include.

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#8618

> On Tue, 09 Aug 2011 09:33:41 -0700, red floyd wrote:


> Technically. you need to #include <cstdlib> to define EXIT_FAILURE.

I wanted to do it C++ way, how C++ defines exit for failure/success.

 
> In actual practice, obviously one of <iostream>, <fstream>, or <string>
> is including it.

 
> Also, I believe std::endl is defined in <ostream>.  In C++11 <iostream>
> covers that, but in C++03, <iostream> does not officially include
> <ostream>, though as far as I know, all known C++ compilers do the
> nested include.

That is strange. I thought both <istream> and <ostream> were children of 
parent class <iostream> and they will inherit std::endl from <iostream>.






-- 
-- arnuld
www.LispMachine.Wordpress.com

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#8659

On Wed, 10 Aug 2011 12:13:33 +0000, arnuld wrote:

> That is strange. I thought both <istream> and <ostream> were children of 
> parent class <iostream> and they will inherit std::endl from <iostream>.

<istream>, <ostream> and <iostream> are headers, not classes, although
classes with those names exist.

Also, std::endl() is a function, not a method.

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#8666

Nobody wrote:
> On Wed, 10 Aug 2011 12:13:33 +0000, arnuld wrote:
>
>> That is strange. I thought both <istream> and <ostream> were
>> children of parent class <iostream> and they will inherit
>> std::endl from <iostream>.
>
> <istream>, <ostream> and <iostream> are headers, not classes,
> although classes with those names exist.
>
> Also, std::endl() is a function, not a method.

And even better (or worse?) is that iostream inherits from both 
istream and ostream, not the other way round.


Bo Persson

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#8687

On Thu, 11 Aug 2011 12:51:10 +0200, Bo Persson wrote:

>>> That is strange. I thought both <istream> and <ostream> were
>>> children of parent class <iostream> and they will inherit
>>> std::endl from <iostream>.
>>
>> <istream>, <ostream> and <iostream> are headers, not classes,
>> although classes with those names exist.
>>
>> Also, std::endl() is a function, not a method.
> 
> And even better (or worse?) is that iostream inherits from both 
> istream and ostream, not the other way round.

Right. istream and ostream inherit from ios, the common base class for
all streams. iostream is the input-AND-output stream class (i.e.
read-write streams, analogous to fopen()'s "plus" modes: r+/w+/a+).

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#8802

Gee, thank you very much. This one liner magic works! I owe you one. Do you
accept bitcoins? Just a token of appreciation...

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#8803

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Nana J. Osei writes:

> Gee, thank you very much. This one liner magic works! I owe you one. Do you
> accept bitcoins? Just a token of appreciation...

If you're addressing me, judging by the References: header, thanks but I'm  
not into bitcoins.

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