c++ - What is this weird colon-member (" : ") syntax in the constructor? -

recently i've seen example following:

#include <iostream>  class foo { public:   int bar;   foo(int num): bar(num) {}; };  int main(void) {   std::cout << foo(42).bar << std::endl;   return 0; } 

what strange : bar(num) mean? somehow seems initialize member variable i've never seen syntax before. looks function/constructor call int? makes no sense me. perhaps enlighten me. and, way, there other esoteric language features this, you'll never find in ordinary c++ book?

it's member initialization list. should find information in good c++ book.

you should, in cases, initialize member objects in member initialization list (however, note exceptions listed @ end of faq entry).

the takeaway point faq entry that,

all other things being equal, code run faster if use initialization lists rather assignment.