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DCL50-CPP-EX2: As stated in the normative text, C-style variadic functions that are declared but never defined are permitted. For example, when a function call expression appears in an unevaluated context, such as the argument in a sizeof expression, overload resolution is performed to determine the result type of the call but does not require a function definition. Some template metaprogramming techniques that employ SFINAE use variadic function declarations to implement compile-time type queries, as in the following example:.

template <typename Ty>
class has_foo_function {
  typedef char yes[1];
  typedef char no[2];

  template <typename Inner>
  static yes& test(Inner *I, decltype(I->foo()) * = nullptr); // Function is never defined.

  template <typename>
  static no& test(...); // Function is never defined.

public:
  static const bool value = sizeof(test<Ty>(nullptr)) == sizeof(yes);
};

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Incorrectly using a variadic function can result in abnormal program termination, unintended information disclosure, or execution of arbitrary code.

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