How to conditionally define a typedef to be one of two types

Question

I'm sure that boost has some functions for doing this, but I don't know the relevant libraries well enough. I have a template class, which is pretty basic, except for one twist where I need to define a conditional type. Here is the psuedo code for what I want

struct PlaceHolder {}; template <typename T> class C { typedef (T == PlaceHolder ? void : T) usefulType; };

How do I write that type conditional?

Interesting. Under what circumstance would this be useful? Can you provide an example. — Stephen
– Stephen, Commented Jun 9, 2010 at 17:53
Here was my example. For one the template arguments, say TYPE, having value PlaceHolder means "turn off some feature". There are a set of callbacks that have return type TYPE* that the natural meaning of turning off the feature is for the callbacks to have return type void. usefulType is the return value for the callbacks. — pythonic metaphor
– pythonic metaphor, Commented Jun 9, 2010 at 19:18

Jan Schultke · Accepted Answer · 2023-09-22 18:18:18Z

In modern C++ using the convenience templates std::conditional_t and std::is_same_v:

using usefulType = std::conditional_t<std::is_same_v<T, PlaceHolder>, void, T>;

In C++11, using only typedef and without the convenience aliases:

typedef typename std::conditional<std::is_same<T, PlaceHolder>::value, void, T>::type usefulType;

stijn · Accepted Answer · 2010-06-09 17:58:50Z

I think this is the principle you're after:

template< class T > struct DefineMyTpe { typedef T usefulType; }; template<> struct DefineMyType< PlaceHolder > { typedef void usefulType; }; template< class T > class C { typedef typename DefineMyType< T >::usefulType usefulType; };

Edward Strange · Accepted Answer · 2010-06-09 18:48:06Z

2

template < typename T > struct my_mfun : boost::mpl::if_ < boost::is_same<T,PlaceHolder> , void , T > {}; template < typename T > struct C { typedef typename my_mfun<T>::type usefulType; };

edited Jun 9, 2010 at 18:48

answered Jun 9, 2010 at 17:57

Edward Strange

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GManNickG Over a year ago

Code should be prefixed by 4 spaces. You can select text and click the 101010 button to do it in bulk.

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