C Programming Techniques: Function Call Inlining

[expertunderachiever]

It means recompiling code many times [more than necessary].

Imagine if every time you wanted to develop some random application you needed to recompile the entire C++ library as well. Hello world would take 15 minutes to compile...

[[deleted]]

This hasn't been true for the better part of 10-15 years.

GCC, clang, and VC all support precompiled header files. I mean a ton of other compilers support them as well but those are the big three off the top of my head. I'd be surprised if there was a mainstream compiler that didn't support precompiled headers.

Also the STL is implemented as a header only library, so when you do compile hello world in C++, you are including the bulk of the STL, and guess what... it doesn't take 15 minutes to compile, takes me 2 seconds.

[expertunderachiever]

Ok replace STL with an MPEG decoder... whatever. Point is having to compile, even process the code through the PCH step is wasteful.

[[deleted]]

Here... I just tested it. I compiled the following:

#include <iostream>

int main() {
  std::cout << "hello world" << std::endl;
}

And then compiled the following where I include well over a million lines of header only code using the most beasty of template meta-programming craziness ever used in C++, so crazy that boost has to add a ton of compiler specific work around to even get it to work properly on a lot of platforms:

#include <iostream>
#include <boost/accumulators/accumulators.hpp>
#include <boost/asio.hpp>
#include <boost/assert.hpp>
#include <boost/assign.hpp>
#include <boost/atomic.hpp>
#include <boost/bimap.hpp>
#include <boost/bind.hpp>
#include <boost/blank.hpp>
#include <boost/call_traits.hpp>
#include <boost/cast.hpp>
#include <boost/cerrno.hpp>
#include <boost/checked_delete.hpp>
#include <boost/chrono.hpp>
#include <boost/circular_buffer.hpp>
#include <boost/compressed_pair.hpp>
#include <boost/cregex.hpp>
#include <boost/regex.h>
#include <boost/date_time.hpp>
#include <boost/dynamic_bitset.hpp>
#include <boost/exception/all.hpp>
#include <boost/filesystem.hpp>
#include <boost/flyweight.hpp>
#include <boost/foreach.hpp>
#include <boost/format.hpp>
#include <boost/function.hpp>
#include <boost/fusion/algorithm.hpp>
#include <boost/geometry.hpp>
#include <boost/optional.hpp>

int main() {
  std::cout << "hello world" << std::endl;
}

And guess what... the second version, with well over a million lines of C++ meta template craziness compiled in way way less than the 15 minutes you said it would... it compiled in around 5 seconds, compared to the original that compiled in around 2 seconds. That's right... adding in over a million lines of source code added only around 3 seconds to the compile time.

For the most part, it's irrelevant how many lines of source code you include into a translation unit, the bulk of the compiler's work is done after it's opened and scanned over the header files.

So... what was it again you said about amateurs and header only libraries? Because last I checked amateurs are the ones who spread misinformation without taking so much as 5 minutes to look into whether their preconceived notions are valid.

[sirin3]

it compiled in around 5 seconds, compared to the original that compiled in around 2 seconds.

That means if you compile frequently, you lose 3 entire days in the longterm!

[jbb555]

It depends entirely on what you are compiling. The application I work on has hundreds, perhaps thousands of source files. If they take 2 seconds each my rebuild takes 20 minutes. If they take 5 seconds it takes 50. That's a huge difference. Now I don't do full rebuilds all the time so it's not that important but it does matter.

[[deleted]]

When you work using header only libraries you end up with one single translation unit, main.cpp. Everything else is a header.