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Showing posts from September, 2008

Int-to-type idiom and infinite regress

A new writeup on Int-to-type idiom has been posted on More C++ Idioms wikibook. It is used to achieve static dispatching based on constant integral values. I'll finish the writeup of the idiom here with special attention to how int-to-type idiom can lead to infinite series of type instantiations and why. The idiomatic form of the Int-to-type idiom is given below.

template <int I>
struct Int2Type
enum { value = I };
typedef Int2Type<I> type;
typedef Int2Type<I+1> next;

The type typedef defined inside the template is the type itself. I.e, Int2Type<10>::type is same as Int2Type<10>. The next typedef gives the following type in order. However, compiler is not required to instantiate the next type unless and until two things happen: (1) an instance of the type is created or (2) an associated type is accessed at compile-time. For example, Int2Type<10> will be instantiated if one the following two things are written.

Int2Type<10> a; // a varia…

copy elision and copy-and-swap idiom

An updated writeup of the copy-and-swap idiom is now available on the More C++ Idioms wikibook. A comparison of two different styles of assignment operator is shown. First version accepts the parameter as pass-by-const-reference whereas the second version accepts it as pass-by-value. For some classes pass-by-value turns out to be more efficient as a copy of the object is elided when the right hand side is a rvalue.