Abolish include-files in C++

Question

Suppose i have the following code (literally) in a C++ source file:

// #include <iostream> // superfluous, commented-out using std::cout; using std::endl;  int main() {     cout << "Hello World" << endl;     return 0; } 

I can compile this code even though #include <iostream> is commented-out:

g++ -include my_cpp_std_lib_hack source.cpp 

Where my_cpp_std_lib_hack is a file in some central location that includes all the files of the C++ Standard Library:

#include <ciso646> #include <climits> #include <clocale> ... #include <valarray> #include <vector> 

Of course, i can use proper compilation options for all compilers i care about (that being MS Visual Studio and maybe a few others), and i also use precompiled headers.

Using such a hack gives me the following advantages:

• Fast compilation (because all of the Standard Library is precompiled)
• No need to add #includes when all i want is adding some debugging output
• No need to remember or look up all the time where the heck std::max is declared
• A feeling that the STL is magically built-in to the language

So i wonder: am i doing something very wrong here?

Will this hack break down when writing large projects?

Maybe everyone else already uses this, and no one told me?

Answer 1

So i wonder: am i doing something very wrong here?

Yes. Sure, your headers are precompiled, but the compiler still has to do things like name lookups on the entire included mass of stuff which slows down compilation.

Will this hack break down when writing large projects?

Yes, that's pretty much the problem. Plus, if anyone else looks at that code, they're going to be wondering where std::cout (well, assume that's a user defined type) came from. Without the #includes they're going to have no idea whatsoever.

Not to mention, now you have to link against a ton of standard library features that you may have (probably could have) avoided linking against in the first place.

If you want to use precompilation that's fine, but someone should be able to build each and every implementation file even when precompilation is disabled.

Answer 2

The only thing "wrong" is that you are relying upon a compiler-specific command-line flag to make the files compilable. You'd need to do something different if not using GCC. Most compilers probably do provide an equivalent feature, but it is best to write portable source code rather than to unnecessarily rely on features of your specific build environment.

Other programmers shouldn't have to puzzle over your Makefiles (or Ant files, or Eclipse workspaces, or whatever) to figure out how things are working.

This may also cause problems for users of IDE's. If the IDE doesn't know what files are being included, it may not be able to provide automatic completion, source browsing, refactoring, and other such features.

(FWIW, I do think it is a good idea to have one header file that includes all of the Standard Library headers that you are using in your project. It makes precompilation easier, makes it easier to port to a non-standard environment, and also helps deal with those issues that sometimes arise when headers are included in different orders in different source files. But that header file should be explicitly included by each source file; there should be no magic.)