How does the constructors and destructors are called when using vectors in c++

Question

I was trying out a simple code below

#include <iostream>
#include <string>
#include <vector>
using namespace std;
class employee
{
    private:
        int emp_id;
    public:
        void getEmpid(){cout<<emp_id<<endl;}
        void setEmpid(){ cin>>emp_id;}
        employee():emp_id(10){cout<<"construct 1 "<<"employee id "<<emp_id<< endl;}
        employee(int empid):emp_id(empid){cout<<"construct 2 "<<"employee id "<<emp_id<<endl;}
        employee(const employee &emp):emp_id(emp.emp_id){cout<<"copy construct 3  "<<"employee id "<<emp_id<<endl;}
        employee(employee&& other) : emp_id(other.emp_id) {cout<<"move construct 4 "<<"employee id "<<emp_id<<endl;}
        ~employee(){cout<<"destructor"<<endl;}
};

int main()
{
    vector<employee>a;
    employee s[8]={1,2,3,4,5}; 

    for(int i=0;i<sizeof(s)/sizeof(s[0]);i++)
        a.push_back(s[i]);
    a.push_back(20);
    a.push_back(30);
    a.push_back(40);
    a.push_back(50);
    a.push_back(60);
    for(int i=0;i<a.size();i++)
        a[i].getEmpid();

    return 0;
}

*I get the below output. Not very clear how the constructors and destructors are called and in which order. could some one please throw some light? *

Output:
construct 2 employee id 1
construct 2 employee id 2
construct 2 employee id 3
construct 2 employee id 4
construct 2 employee id 5
construct 1 employee id 10
construct 1 employee id 10
construct 1 employee id 10
copy construct 3 employee id 1
copy construct 3 employee id 2
copy construct 3 employee id 1
destructor
copy construct 3 employee id 3
copy construct 3 employee id 1
copy construct 3 employee id 2
destructor
destructor
copy construct 3 employee id 4
copy construct 3 employee id 5
copy construct 3 employee id 1
copy construct 3 employee id 2
copy construct 3 employee id 3
copy construct 3 employee id 4
destructor
destructor
destructor
destructor
copy construct 3 employee id 10
copy construct 3 employee id 10
copy construct 3 employee id 10
construct 2 employee id 20
move construct 4 employee id 20
copy construct 3 employee id 1
copy construct 3 employee id 2
copy construct 3 employee id 3
copy construct 3 employee id 4
copy construct 3 employee id 5
copy construct 3 employee id 10
copy construct 3 employee id 10
copy construct 3 employee id 10
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
construct 2 employee id 30
move construct 4 employee id 30
destructor
construct 2 employee id 40
move construct 4 employee id 40
destructor
construct 2 employee id 50
move construct 4 employee id 50
destructor
construct 2 employee id 60
move construct 4 employee id 60
destructor
1
2
3
4
5
10
10
10
20
30
40
50
60
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor
destructor

Answer 1

Making a little modification to your code will make the output easily understandable. Specifically, change the destructor:

   ~employee(){cout<<"destructor " << emp_id <<endl;} // shows emp_id when destructing

And change your main to give a smaller, yet similar output. I added a few cout s to separate things and make it more understandable when viewing the output:

int main()
{
    vector<employee>a;
    
    std::cout << "\ncreating array of employee[3]...\n" << std::endl;
    
    employee s[3]={1,2};
    
    std::cout << "\nstarting loop, copy into vec a\n" << std::endl;

    for(int i=0;i<sizeof(s)/sizeof(s[0]);i++) {
        cout << "Vec size now: " << a.size() << " Capacity: " << a.capacity() << endl;
        a.push_back(s[i]);
    }

    cout << "Outside, Vec size now: " << a.size() << " Capacity: " << a.capacity() << endl;
    
    std::cout << "\ndoing push back outside loop\n" << std::endl;
        
    a.push_back(20);
    a.push_back(30);
    
    std::cout << "\nAll done exiting...\n" << std::endl;

     //removed this, since we are only talking about ctors / dtors
     //reduces noise
    // for(int i=0;i<a.size();i++)
    //     a[i].getEmpid();

    return 0;
}

The output that I got was following, I will split it and explain:

Some definitions regarding vector:

• Capacity : the number of elements that can be held in currently allocated storage
• Size : the number of elements in vector

Code: employee s[3]={1,2};

Output:

creating array of employee[3]...

construct 2 employee id 1
construct 2 employee id 2
construct 1 employee id 10

Three constructors are called, 2 employee(int) and 1 default. The first two elements of array call the employee(int) . The 3rd element is default constructed.

Code:

    for(int i=0;i<sizeof(s)/sizeof(s[0]);i++) {
        cout << "Vec size now: " << a.size() << " Capacity: " << a.capacity() << endl;
        a.push_back(s[i]);
    }

Output:

starting loop, copy into vec a

//iteration 1
Vec size now: 0 Capacity: 0 //vec is zero initially
copy construct 3  employee id 1 //id 1 is copy constructed and pushed back

//iteration 2
Vec size now: 1  Capacity: 1  //vec is now size: 1, with 1 element
//we are doing a push back, but the size needs to grow. Vector reallocates and size becomes 2. The previous memory and the elements in that memory are deallocated/destructed.

copy construct 3  employee id 2 //push back(), copy contruct into vector
copy construct 3  employee id 1 // since we reallocated, the previous memory is gone,
                                //we need to add the previous element as well
destructor 1                    //previous element, id: 1, now being destructed.

//iteration 3
//follows the same logic as in iteration 2. size by the end will be 3 (3 elements). Capacity will be 4. That means we can do one more push back without destroying everything and reallocating.

Vec size now: 2  Capacity: 2
copy construct 3  employee id 10
copy construct 3  employee id 1
copy construct 3  employee id 2
destructor 1
destructor 2

Outside, Vec size now: 3 Capacity: 4                                                                                                    

Code:

    a.push_back(20);
    a.push_back(30);

Output:

doing push back outside loop
//remember we have capacity 4

construct 2 employee id 20     //construct id 20
move construct 4 employee id 20 //move into push_back() (because 20 is temporary, rvalue)
destructor 20                   //destroy what we constructed

//capacity: 4, size: 4

construct 2 employee id 30      // construct id 30
move construct 4 employee id 30 //move into push_back()
//uh oh.. capacity was full, deallocations, destructions, reconstructions:
copy construct 3  employee id 1
copy construct 3  employee id 2
copy construct 3  employee id 10
copy construct 3  employee id 20
destructor 1
destructor 2
destructor 10
destructor 20
destructor 30 //destroy our temporary id: 30

All done, now just the destructors for all the elements will be called one by one.

All done exiting...

destructor 10
destructor 2
destructor 1
destructor 1
destructor 2
destructor 10
destructor 20
destructor 30

Lesson: Use reserve() whenever you can.

Moreover, look into vector::clear() , vector::shrink_to_fit , see what it does. Read the docs for additional info.

• SO link on vector growth: About Vectors growth
• std::vector docs at cppreference