variable assignment - output from move constructor class -

class test {     char *str;     public:         test(char *p_str) /**default constructor**/         {             cout<<"default\n";             int l = strlen(p_str) + 1;             str = (l ? new char[l] : 0);             memcpy(str,p_str,l);         }         test(const test& ob) /**copy**/         {             cout<<"copy\n";             int l = strlen(ob.str) + 1;             str = (l ? new char[l] : 0);             memcpy(str,ob.str,l);         }         test(test&& ob) /**move constructor **/         {             cout<<"move constructor\n";             str = ob.str;             ob.str = nullptr;         }         void my_swap(test &ob1 , test &ob2) /**swap function**/         {             swap(ob1.str , ob2.str);         }         test& operator=(test ob) /**copy called because of pass value **/         {             cout<<"copy assignment operator\n";             my_swap(*this,ob);             return *this;             /**copy destroyed object passed value destroyed after function returns;**/         }         ~test()         {             cout<<"destructor\n";             delete[] str;         }         void print_str()         {             cout<<str<<"\n";         }  }; 

the above class contains simple implementation of rule of 5 in c++.now when vector of class created following.

int main() {     vector<test> vec1;     vec1.push_back(test("hi!there"));     return 0; } 

i following output

default

move constructor

destructor

destructor

and when 1 more object pushed vector this

int main() {     vector<test> vec1;     vec1.push_back(test("hi!there"));     vec1.push_back(test("hello! there"));     return 0; } 

the output

default

move constructor

destructor

default

move constructor

copy

destructor

destructor

destructor

destructor

now question why copy constructor called in second case , not first.

thanku

since have 5 destructor calls instead of 4 assume vector had reallocate it's internal storage when tried push second item. means vector had move first pushed item newly allocated (larger) memory area. when vector reallocates storage , has transfer items old memory block the new one, may happen item transferred copy instead of move. reason: how enforce move semantics when vector grows?

in case correct solution using vec1.reserve(2) (before pushing items) avoid unnecessary reallocation if reallocation happens move. frequent reallocation target of memory usage optimization.