Precompiled headers with GCC

Question

Anyone had any success getting precompiled headers working with GCC? I have had no luck in my attempts and I haven't seen many good examples for how to set it up. I've tried on cygwin gcc 3.4.4 and using 4.0 on Ubuntu.

Answer 1

I have definitely had success. First, I used the following code:

 #include <boost/xpressive/xpressive.hpp> #include <iostream>  using namespace std; using namespace boost::xpressive;  //A simple regex test int main() {     std::string hello( "hello world!" );      sregex rex = sregex::compile( "(\\w+) (\\w+)!" );     smatch what;      if( regex_match( hello, what, rex ) )     {         std::cout << what[0] << '\n'; // whole match         std::cout << what[1] << '\n'; // first capture         std::cout << what[2] << '\n'; // second capture     }     return 0; } 

This was just a hello world from Boost Xpressive (see below for link). First, I compiled with the -H option in gcc. It showed an enormous list of headers that it used. Then, I took a look at the compile flags my IDE (code::blocks) was producing and saw something like this:

g++ -Wall -fexceptions -g -c main.cpp -o obj/Debug/main.o

So I wrote a command to compile the Xpressive.hpp file with the exact same flags:

sudo g++ -Wall -fexceptions -g /usr/local/include/boost/xpressive/xpressive.hpp

I compiled the original code again with the -H and got this output:

 g++ -Wall -fexceptions -H  -g     -c main.cpp -o obj/Debug/main.o ! /usr/local/include/boost/xpressive/xpressive.hpp.gch main.cpp . /usr/include/c++/4.4/iostream .. /usr/include/c++/4.4/x86_64-linux-gnu/bits/c++config.h .. /usr/include/c++/4.4/ostream .. /usr/include/c++/4.4/istream main.cpp 

The ! means that the compiler was able to use the precompiled header. An x means it was not able to use it. Using the appropriate compiler flags is crucial. I took off the -H and ran some speed tests. The precompiled header had an improvement from 14 seconds to 11 seconds. Not bad but not great.

Note: Here's the link to the example: http://www.boost.org/doc/libs/1_43_0/doc/html/xpressive/user_s_guide.html#boost_xpressive.user_s_guide.examples I couldn't get it to work in the post.

BTW: I'm using the following g++

g++ (Ubuntu 4.4.3-4ubuntu5) 4.4.3

Answer 2

Firstly, see the documentation here.

You compile headers just like any other file but you put the output inside a file with a suffix of .gch.

So for example if you precompile stdafx.h you will have a precompiled header that will be automatically searched for called stdafx.h.gch anytime you include stdafx.h

Example:

stdafx.h:

#include <string> #include <stdio.h> 

a.cpp:

#include "stdafx.h" int main(int argc, char**argv) {   std::string s = "Hi";   return 0; } 

Then compile as:

> g++ -c stdafx.h -o stdafx.h.gch
> g++ a.cpp
> ./a.out

Your compilation will work even if you remove stdafx.h after step 1.