c ++入门练习13.13，关于构造函数

Question

i did the exercise 13.13 of c++ primer, and here is my code

#include <iostream>
#include <vector>

using namespace std;

struct X{
X(){cout<<"X()"<<endl;}
X(const X&){cout<<"X(const X&)"<<endl;}
X& operator=(const X&){
    cout<<"X& operator=(const X&)"<<endl;
    return *this;
}
~X(){cout<<"~X()"<<endl;}
};

void foo(const X& cx, X bx){
vector<X> v;
v.push_back(cx);
v.push_back(bx);
}

int main(){
X* x1=new X;
X x2;
x2=*x1;
foo(*x1, x2);
delete x1;
return 0;
}

and I got the output which has four times of "X(const X&)", I guess the first three comes from passing x2, push_back(cx) and push_back(bx). but why there is the fourth "X(const X&)?

Answer 1

Possibly when the second element is added to the vector (the v.push_back(bx); ) a resize of the vector's underlying storage is done and the single element already in the vector is copied to the new storage.

Add the following around the second push_back() call:

cout << "about to do push_back(bx)" << endl;
v.push_back(bx);
cout << "done push_back(bx)" << endl;

Answer 2

It has something to do with the way push_back is implemented.

One of the copies is due to passing bx to foo by value. Make that parameter a reference, and the number of copies reduces by one.

The rest appear to all be due to push_back . I tried your code at http://www.tutorialspoint.com/compile_cpp_online.php with extra printouts before and after every push_back . I also pushed a total of 10 elements onto the vector. The second call to push_back caused the copy constructor to be called twice, the third call caused it to be called three times, the fifth call caused five times, and the ninth call caused nine times. All other calls to push_back caused the copy constructor to be called only once.

But when I did v.reserve(20) before the first call to push_back , every call to push_back resulted in exactly one call to the copy constructor. When I changed the 20 to 8, the ninth push_back resulted in nine calls to the copy constructor, all others resulted in just one call.

I conclude that the extra copy constructor calls are related to the maintenance the vector has to do (copying existing elements) when it grows without having "reserved" the space it requires.

So Michael Burr's hypothesis is correct, and you should be able to confirm this too by experimenting with the code.