c++ - 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] << '
'; // whole match
        std::cout << what[1] << '
'; // first capture
        std::cout << what[2] << '
'; // 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