Promotion Traits

PromotionTraits is a technique which enables the return type of a function to be determined by the function arguments. This is described in CppTemplatesTheCompleteGuide, where working code is given. -- JohnFletcher

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Example of function definition

  template <typename T1, typename T2>
  typename Promotion<T1, T2>::ResultT operator+ ( T1 const &, T2 const &);

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A word of warning. Use of this can have side effects. I have had a problem where code to do with standard vectors would not work correctly and this turned out to be due to picking up an incorrect operator+. If V is a vector then V[n] is implemented for GnuCpp using *(V.begin()+n) which involves addition to the iterator. -- JohnFletcher

Does it touch such issues as subclasses of a common super-class (and finding most-derived common superclass)? That gets to be pretty damn difficult, though doable if you design specializations that indicate all superclasses of a class.

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Related to OperatorOverloading

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April 2012 Now that CeePlusPlusEleven is available I am planning to phase out use of this as soon as possible in favour of auto and decltype which can do a better job as there is no need to set up macros for the cases needed. The only case I cannot solve so far is one involving VariadicTemplates. -- JohnFletcher

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(For those interested, an example of the above using C++11:)

  template <class T1, class T2>
  auto operator+(const T1& l, const T2& r) -> decltype(l + r);

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CategoryCppTemplates CategoryCpp