C++ emplace_back

Table of contents

You should use push_back when:

• You already have a copy of the element that you want to insert into the
• The object you want to insert is cheap to copy.
• The object has a non-explicit copy constructor

You should use emplace_back when:

• You have the arguments to construct the object, but not the object
• The object is expensive to copy.
• The object has a non-explicit constructor that takes the same arguments

In general, use emplace back when building an element in place. When we need to build an object straight inside the container, we should use emplace back() since it will directly generate an object within the container, eliminating the need for a move/copy ().

struct Test {
  int a, b;
  Test(int x, int y) : a(x), b(y) {}
};

  std::vector<Test> v1;
  std::vector<Test> v2;
  auto start = std::chrono::high_resolution_clock::now();
  for (int i = 0; i < 1000000; ++i) {
    v1.push_back(Test(i, i));
  }
  auto end = std::chrono::high_resolution_clock::now();
  std::cout << "push_back took "
            << std::chrono::duration_cast<std::chrono::microseconds>(end -
                                                                     start)
                   .count()
            << " microseconds\n";

  start = std::chrono::high_resolution_clock::now();
  for (int i = 0; i < 1000000; ++i) {
    v2.emplace_back(i, i);
  }
  end = std::chrono::high_resolution_clock::now();
  std::cout << "emplace_back took "
            << std::chrono::duration_cast<std::chrono::microseconds>(end -
                                                                     start)
                   .count()
            << " microseconds\n";

  return 0;

Possible output

push_back took 67699 microseconds
emplace_back took 30729 microseconds

References

• [Modern C++ Series — vector push_back or