c++ - "C2593: operator = is ambiguous" when populating std::map -

c++ - "C2593: operator = is ambiguous" when populating std::map -

i have std::map i'm trying initialize initialization list. in 2 places, in 2 different ways. first 1 works, while other 1 causes error mentioned in title.

here's 1 works:

void foo() { static std::map<std::string, std::string> foomap = { { "first", "abc" }, { "second", "def" } }; }

while 1 not:

class bar { public: bar(); private: std::map<std::string, std::string> barmap; }; bar::bar() { barmap = { // <-- error line { "first", "abc" }, { "second", "def" } }; }

why error when trying initialize class member, while static map works? @ moment, can populate fellow member first creating local variable , swapping fellow member this:

bar::bar() { std::map<std::string, std::string> tmpmap = { { "first", "abc" }, { "second", "def" } }; barmap.swap(tmpmap); }

however, feels rather counter-intuitive compared populating fellow member directly.

edit: here's compiler output.

this bug in compilers overload resolution mechanism - or standard library implementation. overload resolution states in [over.ics.rank]/3 that

— list-initialization sequence l1 improve conversion sequence list-initialization sequence l2 if l1 converts std::initializer_list<x> x , l2 not.

here, x std::pair<std::string, std::string>. l1 converts list parameter of

map& operator=( std::initializer_list<value_type> ilist );

whilst l2 converts list 1 of next functions' parameters:

map& operator=( map&& other ); map& operator=( const map& other );

which aren't initializer_lists. seek use

barmap = decltype(barmap){ { "first", "abc" }, { "second", "def" } };

which should select move-assignment operator (demo vc++). temporary should optimized away according re-create elision.

c++ c++11