Differences in Vector Allocation / Deallocation between C++ Versions

c++ output

[cswiercz@clavius vectors] clang++ ./construction.cpp && ./a.out

=========================
===== Experiment #1 =====
=========================
Resizing and new element construction

-- v.resize(3)
-- resizing members
[#0] constructed
[#1] constructed
[#2] constructed
-- v.capacity() = 3
-- v.resize(4)
[#3] copy constructed (from element [#2])
[#4] copy constructed (from element [#1])
[#5] copy constructed (from element [#0])
[#2] destroyed
[#1] destroyed
[#0] destroyed

===== END =====
[#3] destroyed
[#4] destroyed
[#5] destroyed

=========================
===== Experiment #2 =====
=========================
Using push_back.

-- v.resize(3)
-- pushing elements back
[#0] constructed
[#1] copy constructed (from element [#0])
[#0] destroyed
[#2] constructed
[#3] copy constructed (from element [#2])
[#2] destroyed
[#4] constructed
[#5] copy constructed (from element [#4])
[#4] destroyed
-- v.capacity() = 3
-- v.resize(4)
[#6] copy constructed (from element [#5])
[#7] copy constructed (from element [#3])
[#8] copy constructed (from element [#1])
[#5] destroyed
[#3] destroyed
[#1] destroyed

===== END =====
[#6] destroyed
[#7] destroyed
[#8] destroyed

c++11 output

[cswiercz@clavius vectors] clang++ --std=c++11 ./construction.cpp && ./a.out

=========================
===== Experiment #1 =====
=========================
Resizing and new element construction

-- v.resize(3)
-- resizing members
[#0] constructed
[#1] constructed
[#2] constructed
-- v.capacity() = 3
-- v.resize(4)

===== END =====
[#2] destroyed
[#1] destroyed
[#0] destroyed

=========================
===== Experiment #2 =====
=========================
Using push_back.

-- v.resize(3)
-- pushing elements back
[#0] constructed
[#1] moved (from element [#0])
[#0] destroyed
[#2] constructed
[#3] moved (from element [#2])
[#2] destroyed
[#4] constructed
[#5] moved (from element [#4])
[#4] destroyed
-- v.capacity() = 3
-- v.resize(4)

===== END =====
[#5] destroyed
[#3] destroyed
[#1] destroyed

=========================
===== Experiment #3 =====
=========================
Using emplace_back.

-- v.resize(4)
-- emplacing elements back
[#0] constructed
[#1] constructed
[#2] constructed
-- v.capacity() = 3
-- v.resize(4)

===== END =====
[#2] destroyed
[#1] destroyed
[#0] destroyed

vector.cpp

#include <vector>
#include <iostream>

int NUM_INSTANCES = 0;

class InstanceCounter
{
  int _instance_number;
public:
  InstanceCounter()
  {
    _instance_number = NUM_INSTANCES++;
    std::cout << "[#" << _instance_number << "] constructed\n";
  }

  InstanceCounter(const InstanceCounter& rhs)
  {
    _instance_number = NUM_INSTANCES++;
    std::cout << "[#" << _instance_number << "] copy constructed ";
    std::cout << "(from element [#" << rhs._instance_number << "])\n";
  }

// move constructors not available before C++11
#if __cplusplus > 199711L
  InstanceCounter(InstanceCounter&& rhs)
  {
    _instance_number = NUM_INSTANCES++;
    std::cout << "[#" << _instance_number << "] moved ";
    std::cout << "(from element [#" << rhs._instance_number << "])\n";
  }
#endif

  ~InstanceCounter()
  {
    std::cout << "[#" << _instance_number << "] destroyed\n";
  }
};


int experiment1(void)
{
  NUM_INSTANCES = 0;
  std::cout << "\n=========================\n";
  std::cout << "===== Experiment #1 =====\n";
  std::cout << "=========================\n";
  std::cout << "Resizing and new element construction\n\n";

  std::vector<std::vector<InstanceCounter> > v;
  std::cout << "-- v.resize(3)\n";
  v.resize(3);
  std::cout << "-- resizing members\n";
  v[0].resize(1);
  v[1].resize(1);
  v[2].resize(1);
  std::cout << "-- v.capacity() = " << v.capacity() << "\n-- v.resize(4)\n";
  v.resize(4);
  std::cout << "\n===== END =====\n";

  return 0;
}


int experiment2(void)
{
  NUM_INSTANCES = 0;
  std::cout << "\n=========================\n";
  std::cout << "===== Experiment #2 =====\n";
  std::cout << "=========================\n";
  std::cout << "Using push_back.\n\n";

  std::vector<std::vector<InstanceCounter> > v;
  std::cout << "-- v.resize(3)\n";
  v.resize(3);
  std::cout << "-- pushing elements back\n";
  v[0].push_back(InstanceCounter());
  v[1].push_back(InstanceCounter());
  v[2].push_back(InstanceCounter());
  std::cout << "-- v.capacity() = " << v.capacity() << "\n-- v.resize(4)\n";
  v.resize(4);
  std::cout << "\n===== END =====\n";

  return 0;
}


int experiment3(void)
{
// vector.emplace_back not availabe before C++11
#if __cplusplus > 199711L
  NUM_INSTANCES = 0;
  std::cout << "\n=========================\n";
  std::cout << "===== Experiment #3 =====\n";
  std::cout << "=========================\n";
  std::cout << "Using emplace_back.\n\n";

  std::vector<std::vector<InstanceCounter> > v;
  std::cout << "-- v.resize(4)\n";
  v.resize(3);
  std::cout << "-- emplacing elements back\n";
  v[0].emplace_back();
  v[1].emplace_back();
  v[2].emplace_back();
  std::cout << "-- v.capacity() = " << v.capacity() << "\n-- v.resize(4)\n";
  v.resize(4);
  std::cout << "\n===== END =====\n";
#endif
  return 0;
}


int main(int argc, char* argv[])
{
  experiment1();
  experiment2();
  experiment3();
  return 0;
}