When implementing new CVs and methods you may at time find it useful to make use functionality that has been implemented in some external library (eg boost ). This is strongly encouraged and good practise - you introduce fewer bugs if you write less code. Having said that we would prefer it if any functionality that is reliant on external libraries is not enabled by default. The reason for this is that we would like to ensure that the code remains easy to compile. We would rather not have users struggling to resolve lots of dependencies on external libraries just so that they can compile PLUMED to run metadynamics with an distance and a torsion.

The first step in ensuring that the code will compile even if your fancy library is not available is to thus put all the calls to the functions in these libraries inside an ifdef block. So for example here is a block of code that uses the boost graph library:

#ifdef __PLUMED_HAS_BOOST_GRAPH
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/connected_components.hpp>
#include <boost/graph/graph_utility.hpp>
#endif

Here the #ifdef __PLUMED_HAS_BOOST_GRAPH ensures that this part of the code is only compiled when the -D__PLUMED_HAS_BOOST_GRAPH flag is set at compile time.
This example is a bit of a special case as the particular PLMD::Action I took this example from both when the library is available and when it is not. Obviously, if your PLMD::Action does not work without the library then you should place everything in the file inside the file (the definition of the class, the defintions of the methods and the PLUMED_REGISTER_ACTION command) an ifdef block like the one shown above.

Once you have written the code and surrounded all the calls to the library by an ifdef block like the one above you then need to adjust the configure scripts so that the user is provided with the option to compile the code with your new functionality and the link to the external library available. To do this you need to edit the file configure.ac in the plumed2 directory. The first edit you need to make to this file should read as follows:

PLUMED_CONFIG_ENABLE([library_name],[search for library_name],[no])

Add this in the part of the file where there are similar names. This command allows users to enable the package using --enable-library_name when they run the configure command. Obviously, library_name here should be replaced with the name of the particular library you are using. A shell variable named library_name will be set to true if the library has been requested. The last argument says that, by default, the library is not requested. If you replace it with a [yes], then you will be able to use --disable--library_name to disable the library.

The second edit you need to make to the configure.ac file is as follows:

if test $library_name == true ; then
  PLUMED_CHECK_PACKAGE([header_file.hpp],[function],[__PLUMED_HAS_LIBRARY_NAME],[library_name])
fi

This command checks if the library is available on the machine that PLUMED is being compiled on. If it is the Makefiles are generated with the appropriate precompiler directives and lists of libraries to link. If it is not available a warning is issued to the user that tells him/her that PLUMED will not be compiled with the requested features. The PLUMED_CHECK_PACKAGE command that we use to do this check here takes in four arguments. The first is the name of one of the header files from the library that you are using. You specify the location of this header file in the same way as you specified its location within the code. So for example if you had #include <boost/graph/adjacency_list.hpp> in your cpp code you would replace header_file.hpp in the above with boost/graph/adjacency_list.hpp. The next argument to PLUMED_CHECK_PACKAGE is one of the functions that is within the library that you are trying to link. If in doubt you can use the exit funciton here, which should work even if you are using template functions. The third argument is the precompiler directive that appears around your library calling code and that we discussed earlier. The last argument meanwhile is the name of the library - the part that appears after the -l. In the example above the code would thus try and link -llibrary_name.

Notice that in the for C++ libraries sometimes one has to check something more complicated than the presence of a single function. In this case you can use a more advanced version of the command which allows you to write a short test. Look at this example:

if test $boost_serialization == true ; then
  PLUMED_CHECK_CXX_PACKAGE([boost serialization],[
#include <fstream>
#include <boost/archive/text_oarchive.hpp>
int main() {
    std::ofstream ofs("filename");
    boost::archive::text_oarchive oa(ofs);
    return 0;
}
  ], [__PLUMED_HAS_BOOST_SERIALIZATION],[boost_serialization boost_serialization-mt])
fi

The first argument here ([boost serialization]) is just used in the configure log. The second argument is a complete C++ program that should compile and link. The last arguments are similar to those of the PLUMED_CHECK_PACKAGE macro.

Notice that for both macros (PLUMED_CHECK_PACKAGE and PLUMED_CHECK_CXX_PACKAGE) you can omit the final library, which means that the function should be found without adding any extra -l option. In addition, you can put multiple libraries. In this case, autoconf will scan them to find the appropriate one.

Once you have made these edits issue the command:

autoconf

and the necessary changes will be made to the configure script. You should never edit the configure script directly