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  1. #include <iostream>
  2. #include <boost/utility/enable_if.hpp>
  3. #include <boost/static_assert.hpp>
  4.  
  5. // Has_hello<T>::value is true if T has a hello function.
  6. template<typename T>
  7. struct has_hello {
  8.   typedef char yes[1];
  9.   typedef char no [2];
  10.   template <typename U> struct type_check;
  11.   template <typename U> static yes &chk(type_check<char[sizeof(&U::hello)]> *);
  12.   template <typename  > static no  &chk(...);
  13.   static const bool value = sizeof(chk<T>(0)) == sizeof(yes);
  14. };
  15.  
  16. template<typename T>
  17. void doSomething(T const& t,
  18.                  typename boost::enable_if_c<has_hello<T>::value>::type* = 0
  19.                  ) {
  20.   return t.hello();
  21. }
  22.  
  23. // Would need another doSomething` for types that don't have hello().
  24.  
  25. struct Foo {
  26.   void hello() const {
  27.     std::cout << "hello" << std::endl;
  28.   }
  29. };
  30.  
  31. // This check is ok:
  32. BOOST_STATIC_ASSERT(has_hello<Foo>::value);
  33.  
  34. int main() {
  35.   Foo foo;
  36.   doSomething<Foo>(foo);
  37. }
  38.  
Success #stdin #stdout 0.01s 2680KB
comments (?)
stdin 
Standard input is empty
stdout 
hello
https://ideone.com/HAclh
language:
C++ (gcc 8.3)
created:
13 years ago
visibility:
public

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