For this example: # include <istream> # include <istream> using namespace std; ifstream x; ifstream y(); int main(int argc, char** argv) { return 0; } current mainline g++ says: foo.cc:6: error: aggregate ‘std::ifstream x’ has incomplete type and cannot be defined This is accurate but confusing for the uninitiated. How about something like foo.cc:6: error: ‘std::ifstream’ was declared but not defined
>foo.cc:6: error: ‘std::ifstream’ was declared but not defined This is even more confusing to me. As incomplete types are understood easier than just being declared and not defined.
Yo have to add the library fstream at the beginning. Example: #include <fstream> That will fix the problem =)