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Showing posts from 2010

Generic Transpose using boost tuples

Recently, while working on LEESA I faced an interesting problem of creating a transpose using generic programming. The problem is pretty straight forward. Suppose you are given a tuple of 2 vectors i.e. tuple<vector<int>, vector<long> >. Assuming both the vectors are of the same size, how would create a vector <tuple<int, long> > containing the data from the earlier two vectors? Of course, the size of the tuple may vary.

Lets call the function transpose. A brute force solution would be to overload transpose function N times such that each one takes different size of tuple. Here's one that takes a three parameter tuple.

template <class C1, class C2, class C3> // Assuming all Cs are STL containers
std::vector<tuple<C1::value_type, C2::value_type, C3::value_type> >
transpose (tuple<C1, C2, C3> const &) { ... }

The implementation is fairly straight forward for those who have seen tuples in the past. We basically need a loop with t…

Faster meta-programs using gcc 4.5 and C++0x

One of the practical issues with C++ meta-programming is its speed. C++ programs that use heavy meta-programming can be notoriously slow to compile on contemporary compilers. Things are changing, however. Check the following comparison of gcc 4.5 against gcc 4.4.3.
The first graph is obtained from a program that creates a binary tree of template instantiations. The x-axis shows the number of instantiations when value of N goes from 8 to 17. I could not build up patience for gcc 4.4.3 beyond 16363 instantiations (N=13). On the other hand, gcc 4.5 does pretty good and its increase in compilation time is indeed linear as mentioned here. Here is the program that creates a binary tree of template instantiations. template <int Depth, int A, typename B> struct Binary { enum { value = 1 + Binary<depth-1, 0, Binary>::value + Binary<depth-1, 1, Binary>::value }; }; template<int a, typename B> struct Binary<0, A, B> { enum { value = 1 }; };…