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  1. #include <algorithm>
  2.     #include <functional>
  3.     #include <iostream>
  4.     #include <vector>
  5.  
  6.     template<typename InIt, typename OutIt, typename Fun>
  7.     class is_valid_transform
  8.     {
  9.     private:
  10.             class YES
  11.             { 
  12.                     char dummy[2]; // size > 1
  13.             };
  14.  
  15.             typedef char NO; // size == 1
  16.  
  17.             // std::transform has return type OutIt which will only convert to 
  18.             // InIt if OutIt does not have stronger const-ness properties.
  19.             static YES test(InIt);
  20.  
  21.             // SFINAE fallback: ... has lowest priority
  22.             static NO test(...);
  23.  
  24.     public: 
  25.             // equivalent to sizeof(test(OutIt()) != sizeof(NO) 
  26.             static const bool value = sizeof(test(std::transform(InIt(), InIt(), OutIt(), Fun()))) != sizeof(NO);
  27.     };
  28.  
  29. int main()
  30. {
  31.         typedef std::vector<int>::iterator iter1;
  32.         typedef std::vector<int>::const_iterator iter2;
  33.         typedef std::vector<std::string>::iterator iter3;
  34.         typedef std::negate<int> func;
  35.  
  36.         auto strinf = [](int)->std::string { return "hello"; };
  37.  
  38.         std::cout << "valid transform compiles: " << is_valid_transform<iter1,iter1,func>::value << std::endl;
  39.         std::cout << "invalid transform compiles: " << is_valid_transform<iter1,iter2,func>::value << std::endl;
  40.         std::cout << "valid transform compiles: " << is_valid_transform<iter2,iter1,func>::value << std::endl;
  41.         std::cout << "invalid transform compiles: " << is_valid_transform<iter1,iter3,decltype(&strinf)>::value << std::endl;
  42.         std::transform(iter1(), iter1(), iter3(), strinf);
  43.  
  44.         return 0;
  45. }
Success #stdin #stdout 0s 2828KB
comments (?)
stdin 
Standard input is empty
stdout 
valid transform compiles: 1
invalid transform compiles: 0
valid transform compiles: 1
invalid transform compiles: 0
https://ideone.com/12qfo
language:
C++14 (gcc 8.3)
created:
13 years ago
visibility:
public

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