Question

Can someone explain why the top piece of code will not compile and the bottom one will?

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

using namespace std;

int main(int argc, char *argv[])
{
    ifstream testFile;
    testFile.open("opengl_functions", ios::in);
    set<string> myset(istreambuf_iterator<string>(testFile), istreambuf_iterator<string>());

    set<string>::iterator it;

    for (it = myset.begin(); it != myset.end(); ++it ) {
    }
}

//using namespace std;
//
//int main ()
//{
//  int myints[] = {75,23,65,42,13};
//  set<int> myset (myints,myints+5);

//  set<int>::iterator it;

//  cout << "myset contains:";
//  for ( it=myset.begin() ; it != myset.end(); it++ )
//    cout << " " << *it;

//  cout << endl;

//  return 0;
//}

—

[mehoggan@hogganz400 opengl_parser]$ make
g++ -o parser -Wall ./parser.cpp
./parser.cpp: In function ‘int main(int, char**)’:
./parser.cpp:17: error: request for member ‘begin’ in ‘myset’, which is of non-class type ‘std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >(std::istreambuf_iterator<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::char_traits<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::istreambuf_iterator<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::char_traits<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > (*)())’
./parser.cpp:17: error: request for member ‘end’ in ‘myset’, which is of non-class type ‘std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >(std::istreambuf_iterator<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::char_traits<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::istreambuf_iterator<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::char_traits<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > (*)())’
make: *** [parser] Error 1

Answer 1

Your compiler believes that myset is a function declaration. Look up “the most vexing parse”. I always run into it with istream iterators, so I always declare them beforehand. As a side benefit, I find it much easier to read:

std::istreambuf_iterator<string> begin(testFile), end;
std::set<std::string> myset(begin, end);

However, I don’t believe that will compile either, but for a different reason. istreambuf_iterator can only be templated on character types. You’ll want to use an istream_iterator instead.

std::istream_iterator<string> begin(testFile), end;
std::set<std::string> myset(begin, end);