c++ - constructor of string from char* not implicitly called? -

i have following code.

void print_pair(const std::pair<const std::string&, const int&>& p) {     std::cout << p.first << "," << p.second << std::endl; } print_pair(std::pair<const std::string&, const int&>("test",1));//1 print_pair(std::pair<const std::string&, const int&>(std::string("test"),1));//2 

which produces following output:

,1 test,1 

shouldn't 2 lines produce same output, in first case constructor string char* should implicitly called? why const reference not seem prolong life of first pair argument in first case?

it compiled gcc4.9 -std=c++11 -o3.

the problem pair pair of references, not values. when have:

using pair_t = std::pair<const std::string&, const int&>; pair_t("test",1); 

the compiler needs find best candidate construction, best match in c++11 is:

template< class u1, class u2 > constexpr pair( u1&& x, u2&& y ); 

where types u1 = const char(&)[5] , u2 = int. internally, bind reference going create temporary string, when expression completes (and before returning constructor), temporary go away , left dangling reference.

the program has undefined behavior, , prints empty string in same way have crashed or print garbage.

in case do:

pair_t(std::string("test"), 1); 

the expression in temporary created same expression calls function, , lifetime of temporary last until function completes, second line correct , exhibits expected behavior.