static_switch

Meta-programmatically select a C++ type at compile-time based upon a compile-time constant or preprocessor #define.

I recently had the need to select a C++ type based upon the value of a preprocessor #define macro. The problem can be solved by using a class which is templated on the macro's value, as follows;

#define SOME_VALUE 0

template<int I> struct CPP_type{ typedef int type; /* default value */ };
template<> struct CPP_type<0>{ typedef char type; };
template<> struct CPP_type<1>{ typedef short type; };
template<> struct CPP_type<2>{ typedef long type; };

using some_type = CPP_type<SOME_VALUE>::type;

where additional cases are provided by specializing the template class CPP_type. But if one had many cases, or needed to do this more than once, this solution would rapidly become unwieldy. Each C++ type requires a new template class and each case requires a new specialization (including all the syntax which accompanies that) of said class. Further it might not be immediately obvious to another reader what this piece of code does.

Therefore I wrote a meta-function called static_switch which recreates this behaviour but syntactically looks much more like the usual switch statement. The use of static_switch maintains the readability of the code. Using static_switch the example above becomes

#define SOME_VALUE 0

using some_type = static_switch<SOME_VALUE
				,int // default case
				,static_case<0,char>
				,static_case<1,short>
				,static_case<2,long>
				>::type;