c++ - std::functions and lambda function passing -

i have class takes std::function parameter assign lambda function. works in constructor stops working after that. debugger says f "empty" after running first line. why?

#include <iostream> #include <string> #include <functional>  typedef std::function<void(std::string)> const& fn;  class testclass { public:     testclass(fn _f) : f(_f) { f(); }     void f() { f("hello"); };  private:     fn f; };   int main() {     testclass t([](std::string str) {std::cout << str << std::endl; });      t.f();      return 0; } 

calling t.f() causes fault. why?

i can solve changing following:

int main() {     fn __f = [](std::string str) {std::cout << str << std::endl; };     testclass t(__f);      t.f();      return 0; } 

but again, not work when change fn auto!

int main() {     auto __f = [](std::string str) {std::cout << str << std::endl; };     testclass t(__f);      t.f();      return 0; } 

what explanation of why happening?

note (1) fn defined reference (to const); (2) lambda , std::function not same type; (3) can't bind reference object different type directly.

for 1st case,

testclass t([](std::string str) {std::cout << str << std::endl; }); t.f(); 

a temporary lambda created , converted std::function temporary too. temporary std::function bound parameter _f of constructor , bound member f. temporary destroyed after statement, f becomes dangled, when t.f(); fails.

for 2nd case,

fn __f = [](std::string str) {std::cout << str << std::endl; }; testclass t(__f); t.f(); 

a temporary lambda created , bound reference (to const). lifetime extended lifetime of reference __f, code fine.

for 3rd case,

auto __f = [](std::string str) {std::cout << str << std::endl; }; testclass t(__f); t.f(); 

lambda created , converted std::function temporary. temporary std::function bound parameter _f of constructor , bound member f. temporary destroyed after statement, f becomes dangled, when t.f(); fails.

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^{(1) declare fn non-reference typedef std::function<void(std::string)> fn;, std::function copied , every case work well.
(2) don't use names begin double underscore, they're reserved in c++.}