When is the copy constructor called in cpp?

Question

#include<iostream>
#include<vector>
#include<list>
#include<queue>
#include<map>
using namespace std;
class dog{
    public:
        string name;
        dog();
        dog(const dog & d);
        void barkname(){
            cout<<"bark "<<name<<endl;
        }
        virtual ~dog(){
            //cout<<"delete dog "<<name<<endl;
        }
};

dog::dog(){
    cout<<"blank dog"<<endl;
    this->name="blank";
}

dog::dog(const dog &d){
    cout<<"copy dog"<< " "+d.name<<endl;
    string temp=d.name;
    this->name=temp+" copied";
}


int main(){
    dog d;
    d.name="d";
    dog dd;
    dd.name="dd";
    dog ddd;
    ddd.name="ddd";
    vector<dog> doglist;
    doglist.push_back(d);
    doglist.push_back(dd);
    doglist.push_back(ddd);
    return 0;
}

Hi, I am new to cpp. I tried using the copy constructor in my cool dog. I injected three dogs into a vector using push_back three times. Therefore, I expected the copy constructor to be called three times. However, after executing the code, I found that the copy constructor is called six times with the following results:

blank dog
blank dog
blank dog
copy dog d
copy dog dd
copy dog d copied
copy dog ddd
copy dog d copied copied
copy dog dd copied

I am very confused about why the dog is copied so many times. I only ask push_back three times. Thank.

Thank you for pointing out a similar question: why copy-constructor is called twice when vector.push_back is executed

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Answer 1

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doglist.reserve(3);

Answer 2

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#include <iostream>
#include <string>
#include <vector>

using namespace std;

class dog{
    public:
        string name;
        dog();
        dog(const dog & d);
        void barkname(){
            cout<<"bark "<<name<<endl;
        }
        virtual ~dog(){
            //cout<<"delete dog "<<name<<endl;
        }
};

dog::dog(){
    cout<<"blank dog"<<endl;
    this->name="blank";
}

dog::dog(const dog &d){
    cout<<"copy dog"<< " "+d.name<<endl;
    string temp=d.name;
    this->name=temp+" copied";
}


int main()
{
    dog d;
    d.name="d";
    dog dd;
    dd.name="dd";
    dog ddd;
    ddd.name="ddd";
    vector<dog> doglist;

    cout << "\nInitial capacity: " << doglist.capacity() << endl;

    doglist.push_back(d);

    cout << "After adding the first dog capacity: " << doglist.capacity() << endl;

    doglist.push_back(dd);

    cout << "After adding the second dog capacity: " << doglist.capacity() << endl;

    doglist.push_back(ddd);

    cout << "After adding the second dog capacity: " << doglist.capacity() << endl;

    return 0;
}

blank dog
blank dog
blank dog

Initial capacity: 0
copy dog d
After adding the first dog capacity: 1
copy dog dd
copy dog d copied
After adding the second dog capacity: 2
copy dog ddd
copy dog d copied copied
copy dog dd copied
After adding the second dog capacity: 4

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#include <iostream>
#include <string>
#include <vector>

using namespace std;

class dog{
    public:
        string name;
        dog();
        dog(const dog & d);
        void barkname(){
            cout<<"bark "<<name<<endl;
        }
        virtual ~dog(){
            //cout<<"delete dog "<<name<<endl;
        }
};

dog::dog(){
    cout<<"blank dog"<<endl;
    this->name="blank";
}

dog::dog(const dog &d){
    cout<<"copy dog"<< " "+d.name<<endl;
    string temp=d.name;
    this->name=temp+" copied";
}


int main()
{
    dog d;
    d.name="d";
    dog dd;
    dd.name="dd";
    dog ddd;
    ddd.name="ddd";
    vector<dog> doglist;
    doglist.reserve( 3 );
    //^^^^^^^^^^^^^^^^^^^

    cout << "\nInitial capacity: " << doglist.capacity() << endl;

    doglist.push_back(d);

    cout << "After adding the first dog capacity: " << doglist.capacity() << endl;

    doglist.push_back(dd);

    cout << "After adding the second dog capacity: " << doglist.capacity() << endl;

    doglist.push_back(ddd);

    cout << "After adding the second dog capacity: " << doglist.capacity() << endl;

    return 0;
}

blank dog
blank dog
blank dog

Initial capacity: 3
copy dog d
After adding the first dog capacity: 3
copy dog dd
After adding the second dog capacity: 3
copy dog ddd
After adding the second dog capacity: 3

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