Chained conversion between classes without public inheritances
Question
I have a series of ~10 template classes A, B, C, D, ...
I want to enable conversions from a class to previous classes in the series
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You can achieve this by constraining a templated constructor (which will be used in conversion) using
std::enable_ifand some template metaprogramming:template typename BaseTemplate, typename From, typename To, typename Enable = void> struct may_convert : public std::false_type {}; template typename BaseTemplate, typename T> struct may_convert, void> : public std::true_type {}; template typename BaseTemplate, typename T, typename U> struct may_convert may_convertwill walk up the templates of theFromtemplate parameter until it is equal toTo(in which case it inherits fromstd::true_type, i.e.may_convert<...>::valueistrue), or until the templates run out (in which casemay_convert<...>::valueisfalse).Now, all that remains is constraining your constructor appropriately:
templateclass A { public: A() {} template >::value>::type> A(const T&) {} }; This way, the constructor exists only if
may_convert<...>::valueistrue. Otherwise, the conversion will fail.
Examples
Here is an example of how
may_convertworks in your example (converting fromD = AtoB = A):The constructor only exists if
may_convertistruemay_convertmatches the last specialization (becauseD = AandB = A, the parameters are deduced asT = CandU = int) and inherits frommay_convertmay_convertagain matches the last specialization (T = B,U = int) and inherits frommay_convertThis time, the two types are equal, so the first specialization matches, and the entire thing inherits from
std::true_type, enabling the constructor.
On the other hand, imagine a
using E = Athat should not convert toB:The constructor will only be enabled if
may_convertistruemay_convertmatches the last specialization, and inherits frommay_convertBecause
doubleis not anA<...>, none of the specializations match, so we fall back to the default case, which inherits fromstd::false_type.Therefore,
may_convertisfalse, and the conversion fails.
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