c++ - Template argument deduction fails on lambda using tuple -

i'm trying make query function lets me query stl container element characteristics, , return result set. it's syntactic sugar around normal stl operations (specifically, copy_if , back_inserter).

#include <string> #include <tuple> #include <functional> #include <vector>  // query function, compiles fine template<typename t, typename ... u> t query(t const& input, std::function<bool(std::tuple<u...> const& row)> pred) {     t result;     std::copy_if(std::begin(input), std::end(input), std::back_inserter(result), pred);     return result; }  // define row, each column having type using row = std::tuple<int, float, std::string>;  int main() {     // create sample vector of rows     std::vector<row> vec;     for(size_t = 0; < 100; ++i)         vec.emplace_back(i, 5.0f, "hello");      // how perform query      // **** line doesn't compile due template arg deduction failure ****     auto result = query(vec, [](row const& row) -> bool { return true; });     return 0; } 

and here compiler output (clang 3.3)

main.cpp:27:19: error: no matching function call 'query'     auto result = query(vec, [](row const& row) -> bool { return true; });                   ^~~~~ main.cpp:8:3: note: candidate template ignored: failed template argument deduction t query(t const& input, std::function<bool(std::tuple<u...> const& row)> pred)   ^ 1 error generated. 

a lambda not std::function, class type overloaded operator(), 2 distinct types. query function requires std::function<...> argument, , template argument deduction requires types exact matches (besides cv-qualifiers), conversion lambda expression std::function never deduced.

one solution, of course, construct std::function before calling query

auto result = query(vec,                      std::function<bool(row const&)>(                         [](row const&) { return true; })); 

another option change function itself

template<typename t, typename unarypredicate> t query(t const& input, unarypredicate&& pred) {     t result;     std::copy_if(std::begin(input), std::end(input),                   std::back_inserter(result), std::forward<unarypredicate>(pred));     return result; } 

now can called using original code.

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