Re: How to load a text file into a char **?

From:
"Daniel T." <daniel_t@earthlink.net>
Newsgroups:
comp.lang.c++
Date:
Sat, 19 Jul 2008 08:43:33 -0400
Message-ID:
<daniel_t-FB8677.08433319072008@earthlink.vsrv-sjc.supernews.net>
James Kanze <james.kanze@gmail.com> wrote:

"Daniel T." <danie...@earthlink.net> wrote:

ampheta...@gmail.com wrote:

Is there a safe (unlikely to cause overflows or segfaults)
way to load a text file into a char ** array?


A char** array? Are you sure you got that right?


Maybe he needs to interface with some legacy C code.
(Otherwise, of course, std::vector< std::string > is the obvious
choice.)


A vector<string> would equate to a char* array, not a char** array.

char[] char_array = "hello world";
char*[] char_ptr_array = { "a", "b", "c" };
char**[] char_ptr_ptr_array = ?

Maybe the OP just messed up a bit in his terminology?

I thought of using getline, but it needs a fixed-length
string, and I don't know how many lines or characters per
line the file has.


The below is the same as what I had before, except the last step of
turning the vector<vector<char> > into a char** is added.

   template < typename T >
struct PtrToFirst : unary_function< vector< T >, T* > {
   T* operator()( vector< char >& vec ) const {
      return &vec[0];
   }
};

   typedef vector< vector< char > > temp_type;
   temp_type tmp;
   string str;
   while ( getline( file, str ) ) {
      tmp.push_back( vector<char>( str.begin(), str.end() ) );
      tmp.back().push_back( 0 ); // null terminate each line?
   }
   vector< char* > result;
   transform( tmp.begin(), tmp.end(), back_inserter( result ),
         PtrToFirst<char>() );
   // or the above line could be done with:
   //for ( temp_type::iterator it = tmp.begin(); it != tmp.end(); ++it )
   // result.push_back( &it->at( 0 ) );
   result.push_back( 0 );

   char** wow = &vector[0]

I think he wants one string per line. But I'd still use a
vector of string for the reading, only converting into char**
once the file had been read, e.g.:

    std::vector< std::string > tmp ;
    std::string line ;
    while ( std::getline( file, line ) ) {
        tmp.push_back( line ) ;
    }
    std::vector< char const* > result ;
    for ( std::vector< std::string >::const_iterator
                                iter = tmp.begin() ;
            iter != tmp.end() ;
            ++ iter ) {
        result.push_back( iter->c_str() ) ;
    }
    result.push_back( NULL ) ; // if needed.
    // use &result[0].

(In fact, I just did exactly this yesterday, to interface with
openldap.)

Note that in this case, you cannot simply return &result[0], and
expect it to work. For obvious reasons, you must use &result[0]
before either tmp or result go out of scope. In a larger
application, the solution, I think would be to create a class
which contained these two members, contructed the above in its
constructor, and had a function to return the char**. (It the
needed type really is char**, as was the case with openldap,
you'll have to const_cast.)


I agree, but I say avoid the (multiple) const_casts by using a vector<
vector< char > > instead of a vector< string >.

Note that our solutions are remarkably similar. The only difference is
that I don't have to go through the extra step to remove the const.

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