To minimize the time you have to wait on the compiler, it's good to + only include the headers you really need. Many people simply include + <iostream> when they don't need to -- and that can penalize + your runtime as well. Here are some tips on which header to use + for which situations, starting with the simplest. +
<iosfwd> should be included whenever you simply + need the name of an I/O-related class, such as + "ofstream" or "basic_streambuf". Like the name + implies, these are forward declarations. (A word to all you fellow + old school programmers: trying to forward declare classes like + "class istream;" won't work. Look in the iosfwd header if + you'd like to know why.) For example, +
+ #include <iosfwd>
+
+ class MyClass
+ {
+ ....
+ std::ifstream& input_file;
+ };
+
+ extern std::ostream& operator<< (std::ostream&, MyClass&);
+ <ios> declares the base classes for the entire + I/O stream hierarchy, std::ios_base and std::basic_ios<charT>, the + counting types std::streamoff and std::streamsize, the file + positioning type std::fpos, and the various manipulators like + std::hex, std::fixed, std::noshowbase, and so forth. +
The ios_base class is what holds the format flags, the state flags, + and the functions which change them (setf(), width(), precision(), + etc). You can also store extra data and register callback functions + through ios_base, but that has been historically underused. Anything + which doesn't depend on the type of characters stored is consolidated + here. +
The template class basic_ios is the highest template class in the + hierarchy; it is the first one depending on the character type, and + holds all general state associated with that type: the pointer to the + polymorphic stream buffer, the facet information, etc. +
<streambuf> declares the template class + basic_streambuf, and two standard instantiations, streambuf and + wstreambuf. If you need to work with the vastly useful and capable + stream buffer classes, e.g., to create a new form of storage + transport, this header is the one to include. +
<istream>/<ostream> are + the headers to include when you are using the >>/<< + interface, or any of the other abstract stream formatting functions. + For example, +
+ #include <istream>
+
+ std::ostream& operator<< (std::ostream& os, MyClass& c)
+ {
+ return os << c.data1() << c.data2();
+ }
+ The std::istream and std::ostream classes are the abstract parents of + the various concrete implementations. If you are only using the + interfaces, then you only need to use the appropriate interface header. +
<iomanip> provides "extractors and inserters
+ that alter information maintained by class ios_base and its derived
+ classes," such as std::setprecision and std::setw. If you need
+ to write expressions like os << setw(3); or
+ is >> setbase(8);, you must include <iomanip>.
+
<sstream>/<fstream> + declare the six stringstream and fstream classes. As they are the + standard concrete descendants of istream and ostream, you will already + know about them. +
Finally, <iostream> provides the eight standard + global objects (cin, cout, etc). To do this correctly, this header + also provides the contents of the <istream> and <ostream> + headers, but nothing else. The contents of this header look like +
+ #include <ostream>
+ #include <istream>
+
+ namespace std
+ {
+ extern istream cin;
+ extern ostream cout;
+ ....
+
+ // this is explained below
+ static ios_base::Init __foo; // not its real name
+ }
+ Now, the runtime penalty mentioned previously: the global objects + must be initialized before any of your own code uses them; this is + guaranteed by the standard. Like any other global object, they must + be initialized once and only once. This is typically done with a + construct like the one above, and the nested class ios_base::Init is + specified in the standard for just this reason. +
How does it work? Because the header is included before any of your + code, the __foo object is constructed before any of + your objects. (Global objects are built in the order in which they + are declared, and destroyed in reverse order.) The first time the + constructor runs, the eight stream objects are set up. +
The static keyword means that each object file compiled
+ from a source file containing <iostream> will have its own
+ private copy of __foo. There is no specified order
+ of construction across object files (it's one of those pesky NP
+ problems that make life so interesting), so one copy in each object
+ file means that the stream objects are guaranteed to be set up before
+ any of your code which uses them could run, thereby meeting the
+ requirements of the standard.
+
The penalty, of course, is that after the first copy of + __foo is constructed, all the others are just wasted + processor time. The time spent is merely for an increment-and-test + inside a function call, but over several dozen or hundreds of object + files, that time can add up. (It's not in a tight loop, either.) +
The lesson? Only include <iostream> when you need to use one of + the standard objects in that source file; you'll pay less startup + time. Only include the header files you need to in general; your + compile times will go down when there's less parsing work to do. +