Question

I have a preprocessing code to choose which vector class is used as following:

#define USE_BOOST_VECTOR

#ifdef USE_BOOST_VECTOR
    #include <boost/container/vector.hpp>
    #define VECTOR boost::container::vector
#else
    #include <vector>
    #define VECTOR std::vector
#endif

I am not sure if this is a good way. And how about if I have more options to choose, what should I do?

Figured out if there are more options

#define USE_MY_VECTOR 1
#define USE_BOOST_VECTOR 2
#define USE_STD_VECTOR 3

#define CHOOSE_VECTOR USE_BOOST_VECTOR

#if CHOOSE_VECTOR == USE_MY_VECTOR
    #include "Vector.h"
    #define VECTOR Vector
#elif CHOOSE_VECTOR == USE_BOOST_VECTOR 
    #include <boost/container/vector.hpp>
    #define VECTOR boost::container::vector
#elif CHOOSE_VECTOR == USE_STD_VECTOR
    #include <vector>
    #define VECTOR std::vector
#endif

But I need to define 1, 2, 3, and more for more options. Just for a brain work, any better way?

Answer 1

Don’t use pre-processor macros unless you have no other choice – typedefs, constants and if really needed, templates usually can have the intended result, while being much safer.

Now, unless you have a really good reason to prefer Boost vectors over std::vector, go with std::vector. Why? Because if you don’t know what you want, what the std namespace provides should be more than enough.

EDIT: Like leemes says, you would need C++11 to make use of templated type aliases.

#define USE_BOOST_VECTOR

#ifdef USE_BOOST_VECTOR
    #include <boost/container/vector.hpp>
    template <typename T> using VECTOR = boost::container::vector<T>;
#else
    #include <vector>
    template <typename T> using VECTOR = std::vector<T>;
#endif

// ....
// You could then use it with
VECTOR<int> vec;

Finally, here is some more information on templated typedef aliases

If you don’t have a C++11 compiler, you could use namespace aliases:

#define USE_BOOST_VECTOR

#ifdef USE_BOOST_VECTOR
    #include <boost/container/vector.hpp>
    namespace Container = boost::container;
#else
    #include <vector>
    namespace Container = std;
#endif

//And use it with...
Container::vector<int> vec;

I also need to add here, that the above method can lead to some very confusing results. I highly recommend that you basically decide up front which vector implementation you’d like to go with and getting rid of your pre-processor macro once and for all. Your code will thank you over time 🙂