c++ - boost optional and user-defined conversion -

i unable write correct user defined conversion type item. i've tried:

#include <iostream> #include <boost/optional.hpp> struct {     int x; };  struct item {     boost::optional<int> x_;      item(){}     item(const a& s)     : x_(s.x)     {     }      operator boost::optional<a>() const {         boost::optional<a> s;         if (x_) {             s->x = *x_;         }         return s;     } };  std::vector<a> geta(const std::vector<item> &items) {     std::vector<a> a;     (const auto &i : items) {         if (i.x_) {             a.push_back(*static_cast<boost::optional<a>>(i));  // <- line causes error         }     }     return a; } 

that how use it:

int main() {     a;     a.x = 3;     item i(a);     auto v = geta({i});      return 0; } 

g++ -std=c++11 says:

in file included /usr/include/boost/optional.hpp:15:0,                  test.cpp:2: /usr/include/boost/optional/optional.hpp: in instantiation of ‘void boost::optional_detail::optional_base<t>::construct(const expr&, const void*) [with expr = item; t = a]’: /usr/include/boost/optional/optional.hpp:262:25:   required ‘boost::optional_detail::optional_base<t>::optional_base(const expr&, const expr*) [with expr = item; t = a]’ /usr/include/boost/optional/optional.hpp:559:78:   required ‘boost::optional<t>::optional(const expr&) [with expr = item; t = a]’ test.cpp:30:55:   required here /usr/include/boost/optional/optional.hpp:392:8: error: no matching function call ‘a::a(const item&)’         new (m_storage.address()) internal_type(expr) ;         ^ /usr/include/boost/optional/optional.hpp:392:8: note: candidates are: test.cpp:3:8: note: a::a()  struct         ^ test.cpp:3:8: note:   candidate expects 0 arguments, 1 provided test.cpp:3:8: note: constexpr a::a(const a&) test.cpp:3:8: note:   no known conversion argument 1 ‘const item’ ‘const a&’ test.cpp:3:8: note: constexpr a::a(a&&) test.cpp:3:8: note:   no known conversion argument 1 ‘const item’ ‘a&&’ 

why try find a struct constructor instead of use user defined conversion operator?

you may point me directly position of user-defined conversion page because unable find reason this. example,

user-defined conversion function invoked on second stage of implicit conversion, consists of 0 or 1 converting constructor or 0 or 1 user-defined conversion function.

in opinion directly says if no conversion constructor defined user-defined conversion function used. wrong? , if yes, how can implement user-defined conversion without defining conversion cunstructor in struct a ?

you have 2 issues code. optional operator never initializes boost::optional. if don't that, accessing members undefined behavior. have is:

operator boost::optional<a>() const {     boost::optional<a> s;     if (x_) {         s = a{*x_};     }     return s; } 

the second issue when do:

static_cast<boost::optional<a>>(i); 

that equivalent to:

boost::optional<a> __tmp(i); 

but turns out boost::optional has explicit template constructor. preferred conversion function. error you're seeing compiling going down path of factory constructor, item not such factory.

you use boost::optional<a> directly:

std::vector<a> geta(const std::vector<item> &items) {     std::vector<a> a;     (boost::optional<a> opt : items) {         if (opt) {             a.push_back(*opt);         }     }     return a; } 

or, since constructor template explicit, use conversion operator in non-explicit context:

boost::optional<a> opt = i; a.push_back(*opt); 

this has added benefit of being easier read.