vector constructor / destructor calls

Question

I have the following code:

#include <iostream>
#include <vector>

using std::cout;
using std::endl;
using std::vector;

class A {
public:
  int i;
  A(int i=0):i(i) {
    cout << "A::A() called" << endl;
  }
  ~A() {
    cout << "A::~A() called" << endl;
  }

};

int main() {
  vector<A> *a = new vector<A>(3);
  delete a;
}

The program prints:

A::A() called
A::~A() called
A::~A() called
A::~A() called
A::~A() called

Why do I see one constructor and four destructors called ?

I am using g++ 4.8.4.

That output [is wrong](http://coliru.stacked-crooked.com/a/4dcfb98936905392) for the above snippet you listed — WhiZTiM, Sep 18 '16 at 20:52
@WhiZTiM This is not true: http://coliru.stacked-crooked.com/a/1ecb4852b136545d That output is expected to c++03 — Amadeus, Sep 18 '16 at 21:01
@Amadeus, You are right. :-). I was wrong, I assumed latest `C++` version by `C++` tag. — WhiZTiM, Sep 18 '16 at 21:03

Adam Trhon · Accepted Answer · 2016-09-18T21:08:01.583

1

When you construct vector of three instances of A, it is filled with default value. Default value is taken from vector constructor argument. You didn't provide one, so it is constructed either by constructor without arguments or by constructor with default argument values. You provided the latter, so the first constructor call.

All three elements are copy-constructed, so no call to your constructor.

Then the default value instance is destroyed, so the first destructor.

Then you delete the vector, which deletes all three instances, calling destructor three times.

Try printing value of this in the functions to try for yourself or try this sample: http://coliru.stacked-crooked.com/a/903fa70484f5c3bc

edited Sep 18 '16 at 21:08

answered Sep 18 '16 at 20:49

Adam Trhon

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Adding `A(const A& o) { cout << "copy-constructor on" << this << endl; }` confirms this answer. – Marcus Junius Brutus Sep 18 '16 at 21:07
Note: C++11 outlawed the practice of copy-constructing vector entries as seen in OP code, now it is required that each entry be default-constructed when the caller didn't supply initializers. – M.M Sep 18 '16 at 21:17

vector constructor / destructor calls

1 Answers1