Why is the destructor called when initializing and

问题:

Something occurred in my program, and I can't find if it's supposed to happen or not. And if it is, I don't see why..

Here's the code :

#include <iostream>
#include <vector>

using namespace std;

class A{
  public:
    A();
    ~A();
};

A::A(){
  cout << "creating" << endl;
}

A::~A(){
  cout << "deleting" << endl;
}

int main(void){
  vector<vector<A > > vec;

  vec.resize(5);
  for(int i = 0; i < 5; ++i){
    vec[i].resize(5);
  }

  cout << "END" << endl;

  return 0;
}

And here's the output :

creating
deleting
creating
deleting
creating
deleting
creating
deleting
creating
deleting
END
deleting
deleting
[..more deleting here]

I understand why the destructor is called after the "END" message, but before, I don't. I thought that when the vector resize, the constructor of the class is called, but why the destructor?

回答1:

In C++03 vector<A>::resize() has a default parameter, with default value A(). It's this temporary that's destroyed. The elements of the vectors are copy constructed from it.

This is "fixed" in C++11, where there are two overloads of resize. One has a single parameter, and value-initializes any additional elements. The other has two parameters, and copy-initializes each additional element from the value supplied. In C++11 this program has this behavior:

creating
creating
creating
<repeated for a total of 25 "creating"s>
creating
creating
creating
END
deleting
deleting
deleting
<repeated for a total of 25 "deleting"s>
deleting
deleting
deleting

In C++03, if an instance of A is so shockingly expensive to construct that it's worth minimizing the number, then you could shave it from 5 no-args- + 25 copy-constructions down to 1 no-arg- and 25 copy-constructions with something like this:

A default_value;
for (int i = 0; i < 5; ++i) {
   // same end result as vec[i].resize(5)
   if (vec[i].size() >= 5) {
       vec[i].erase(vec.begin() + 5, vec.end());
   } else while(vec[i].size() < 5) {
       vec[i].push_back(default_value);
   }
}

You can probably write it slightly differently, and obviously for your example code you don't need the "if" case. But I don't get many opportunities to say "else while", so I'm taking it.