c++ - Default constructor is getting called on a const reference member despite non default constructor arguments -

here basic c++ outline of code:

#include <cstdlib> #include <iostream> #include <thread>  using namespace std;  class m { public:      m() = default;     ~m() {          cout << "called ~m" << endl;      } };  class { public:     a(int z) : _z(z) {         cout << "called a(int z)" << endl;         this_thread::sleep_for(chrono::milliseconds(1000));     }      a() {         cout << "called a()" << endl;         this_thread::sleep_for(chrono::milliseconds(1000));     }     a(const a& a) {         cout << "called a(const a& a)" << endl;         _z = a._z;     }     a(const a&& a) {         cout << "called a(const a&& a)" << endl;         _z = a._z;     }     a& operator=(const a& a) {         cout << "called a& operator=(const a& a)" << endl;         if (&a != this) {             cout << "executed a& operator=(const a& a)" << endl;         }     }     virtual ~a() {          cout << "called ~a" << endl;      }     int poll() const { return _z;  }  private:     m _m;     int _z = 300; };  class b : public { public:     // _a(10)     b() : _a(std::move(a(10))) {         cout << "called b()" << endl;     }     virtual ~b() {          cout << "called ~b" << endl;      } private:     const a& _a; };  int main(int argc, char** argv) {     b b;         a* aptr = &b;     a& aref = (*aptr);     cout << aref.poll() << endl;     return 0; } 

from setup above following output:

called a() called a(int z) called ~a called ~m called b() 300 called ~b called ~a called ~m 

my issue first line of output (all others make sense given first). initializing member _a in b() : _a(std::move(a(10))), forced _a const reference member. , ctor int argument gets called well, why default ctor called on a? why no move ctor? therefore temporary object seems constructed , destroyed, no real move happening (as can seen 300 output later on).

now issue not seem related move per se behaviour around const reference member. because if change initialization list to: b(): _a(10) same issue: somehow default object assigned const reference member , arguments in initialization list ignored. b(): _a(10) get:

called a() called a(int z) called b() 300 called ~b called ~a called ~m 

basically why first line default constructor? , how alter code 10 initialization appears instead of 300 default?

each object of type b has two subobjects of type a. 1 base-class subobject, , other _a member subobject. call constructor member, base-class subobject default-initialized since haven't explicitly called constructor in initialization list.

you by, example, following:

b() : a(arguments) //<--initialize base-class subobject     , _a(std::move(a(10))) {         cout << "called b()" << endl;     }