NRVO

nrvo.cc

#include <iostream>

class A {
 public:
  A() {}
  ~A() { std::cout << "destructed" << std::endl; }

  A(const A&) {
    std::cout << "copy" << std::endl;
  }
  A(A&&) {
    std::cout << "move" << std::endl;
  }

  A& operator=(const A&) {
    std::cout << "copy assignment" << std::endl;
    return *this;
  }

  static A create() {
    return A();
  }

  static A create_named() {
    A a;
    return a;
  }
};

class B {
 public:
  B(A a) : a_(a) {}
  ~B() {
    std::cout << "B: destructed" << std::endl;
  }

 private:
  A a_;
};

int main() {
  {
    const auto a = A::create();
  }
  {
    const auto a = A::create_named();
  }
  return 0;
}

nrvoc02.cc

#include <utility>
#include <iostream>
#include <string>

class A {
 public:
  A() : name_("hello") {}
  ~A() { std::cout << "destructed" << std::endl; }

  A(const A& o) {
    std::cout << "copy" << std::endl;
    name_ = o.name_;
  }

  A(A&& o) {
    std::cout << "move" << std::endl;
    name_ = std::move(o.name_);
  }

  A& operator=(const A& o) {
    std::cout << "copy assignment" << std::endl;
    name_ = o.name_;
    return *this;
  }

  static A create() { return A(); }

  static A create_named() {
    A a;
    return a;
  }

  std::string name() const { return name_; }

 private:
  std::string name_;
};

class B {
 public:
  B(A a) : a_(std::move(a)) {}

  static B create() { return B(A::create()); }

  // NRVO?
  static B create_named() {
    auto a = A::create();
    // return B(a); // copy will occur
    return B(std::move(a));  // fix
  }

  std::string name() const { return a_.name(); }

 private:
  A a_;
};

int main() {
  {
    const auto b = B::create();
    std::cout << b.name() << std::endl;
  }
  {
    const auto b = B::create_named();
    std::cout << b.name() << std::endl;
  }
  return 0;
}