C++ stringstream read in fixed length string to char array

Question

Given data format as "int,int,...,int,string,int", is it possible to use stringstream (only) to properly decode the fields?

[Code]

int main(int c, char** v)
{
    std::string line = "0,1,2,3,4,5,CT_O,6";
    char delimiter[7];
    int id, ag, lid, cid, fid, did, j = -12345;
    char dcontact[4]; // <- The size of <string-field> is known and fixed
    std::stringstream ssline(line);
    ssline >> id >> delimiter[0]
    >> ag >> delimiter[1]
    >> lid >> delimiter[2]
    >> cid >> delimiter[3]
    >> fid >> delimiter[4]
    >> did >> delimiter[5]  // <- should I do something here?
    >> dcontact >> delimiter[6]
    >> j;
    std::cout << id << ":" << ag << ":" << lid << ":" << cid << ":" << fid << ":" << did << ":";
    std::cout << dcontact << "\n";
}

[Output] 0:1:2:3:4:5: :-45689 , the bolded part shows the stringstream failed to read 4 char only to dcontact. :-45689 ，粗体部分显示0:1:2:3:4:5: :-45689无法读取4个字符仅用于dcontact。 dcontact actually hold more than 4 chars, leaving j with garbage data.

Answer 1

Yes, there is no specific overload of operator >> (istream&, char[N]) for N and there is for char* so it sees that as the best match. The overload for char* reads to the next whitespace character so it doesn't stop at the comma.

You could wrap your dcontact in a struct and have a specific overload to read into your struct. Else you could use read, albeit it breaks your lovely chain of >> operators.

ssline.read( dcontact, 4 );

will work at that point.

To read up to a delimiter, incidentally, you can use getline . ( get will also work but getline free-function writing to a std::string will mean you don't have to guess the length).

(Note that other people have specified to use get rather than read , but this will fail in your case as you do not have an extra byte at the end of your dcontact array for a null terminator. IF you want dcontact to be null-terminated then make it 5 characters and use 'get` and the null will be appended for you).

Answer 2

Slightly more robust (handles the ',' delimiter correctly):

template <char D>
std::istream& delim(std::istream& in)
{
  char c;
  if (in >> c && c != D) in.setstate(std::ios_base::failbit);
  return in;
}

int main()
{
  std::string line = "0,1,2,3,4,5,CT_O,6";
  int id, ag, lid, cid, fid, did, j = -12345;
  char dcontact[5]; // <- The size of <string-field> is known and fixed
  std::stringstream ssline(line);
  (ssline >> id >> delim<','>
          >> ag >> delim<','>
          >> lid >> delim<','>
          >> cid >> delim<','>
          >> fid >> delim<','>
          >> did >> delim<','> >> std::ws
          ).get(dcontact, 5, ',') >> delim<','>
          >> j;
  std::cout << id << ":" << ag << ":" << lid << ":"
            << cid << ":" << fid << ":" << did << ":";
            << dcontact << "\n";
}

Answer 3

try this

  int main(int c, char** v) {
    string line = "0,1,2,3,4,5,CT_O,6";
    char delimiter[7];
    int id, ag, lid, cid, fid, did, j = -12345;
    char dcontact[5]; // <- The size of <string-field> is known and fixed

    stringstream ssline(line);

    ssline >> id >> delimiter[0]
            >> ag >> delimiter[1]
            >> lid >> delimiter[2]
            >> cid >> delimiter[3]
            >> fid >> delimiter[4]
            >> did >> delimiter[5];

    ssline.get(dcontact, 5);

    ssline >> delimiter[6]
            >> j;
    std::cout << id << ":" << ag << ":" << lid << ":" << cid << ":" << fid << ":" << did << ":";
    std::cout << dcontact << "\n" << j;
    }

Answer 4

The problem is that the >> operator for a string ( std::string or a C style string) actually implements the semantics for a word, with a particular definition of word. The decision is arbitrary (I would have made it a line), but since a string can represent many different things, they had to choose something.

The solution, in general, is not to use >> on a string, ever. Define the class you want (here, probably something like Symbol ), and define an operator >> for it which respects its semantics. You're code will be a lot clearer for it, and you can add various invarant controls as appropriate. If you know that the field is always exactly four characters, you can do something simple like:

class DContactSymbol
{
    char myName[ 4 ];
public:
    //  ...
    friend std::istream&
    operator>>( std::istream& source, DContactSymbol& dest );
    //  ...
};

std::istream&
operator>>( std::istream& source, DContactSymbol& dest )
{
    std::sentry guard( source );
    if ( source ) {
        std::string tmp;
        std::streambuf* sb = source.rdbuf();
        int ch = sb->sgetc();
        while ( source && (isalnum( ch ) || ch == '_') ) {
            tmp += static_cast< char >( ch );
            if ( tmp.size() > sizeof( dest.myName ) ) {
                source.setstate( std::ios_base::failbit );
            }
        }
        if ( ch == source::traits_type::eof() ) {
            source.setstate( std::ios_base::eofbit );
        }
        if ( tmp.size() != sizeof( dest.myName ) ) {
            source.setstate( std::ios_base::failbit );
        }
        if ( source ) {
            tmp.copy( dest.myName, sizeof( dest.myName ) );
        }
    }
    return source;
}

(Note that unlike some of the other suggestions, for example using std::istream::read , this one maintains all of the usual conventions, like skipping leading white space dependent on the skipws flag.)

Of course, if you can't guarantee 100% that the symbol will always be 4 characters, you should use std::string for it, and modify the >> operator accordingly.

And BTW, you seem to want to read four characters into dcontact , although it's only large enough for three (since >> will insert a terminating '\\0' ). If you read any more than three into it, you have undefined behavior.

Answer 5

由于字符串的长度已知，因此您可以使用std::setw(4) ，如

ssline >> std::setw(4) >> dcontact >> delimiter[6];