c++ - “Overload” function template based on function object operator() signature in C++98 -

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i want make template function takes function , vector , uses function map vector vector returned function template.

if function taken argument free function, may have 1 of 2 signatures.

// t parameter of function template t sig1(const t x); t sig2(const t x, const std::vector<t>& v);

it may function object in operator() behave free functions. use of function template of 4 possibilities should transparent.

std::vector<int> v; // ... fill v somehow ...  // foo either free function or function object instance const std::vector<int> = map_vec(foo, v);

i asked how c++11 , got great answer 0x499602d2.

"overload" function template based on function object operator() signature

0x499602d2's answer makes use of fact these 2 distinct template signatures in c++11:

template<typename f, typename t> auto map_vec(f&& fnc, const std::vector<t>& source)     -> decltype(void(fnc(std::declval<t>())), std::vector<t>{});  template<typename f, typename t> auto map_vec(f&& fnc, const std::vector<t>& source)     -> decltype(void(fnc(std::declval<t>(), source)), std::vector<t>{});

i know how solve in c++98.

here effort far. have sfinae struct can determine if function objects takes 2 args. don't know how working both function objects , free functions. either need change sfinae struct work on both function objects , free functions or need use overloading route function objects , free functions separately.

http://coliru.stacked-crooked.com/a/1471088cbc3b8544

here's approach:

template <std::size_t, typename t = void> struct ignore_value {typedef t type;};  template <typename t> t& declval();  template<typename f, typename t> typename ignore_value<sizeof(declval<f>()(declval<t const>())),        std::vector<t> >::type map_vec(f fnc, const std::vector<t>& source);  template<typename f, typename t> typename ignore_value<sizeof(declval<f>()                          (declval<t const>(), declval<const std::vector<t> >())),        std::vector<t> >::type map_vec(f fnc, const std::vector<t>& source);

it works same demo 0x499602d2 used, both gcc , clang in c++98 mode.


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