c++ - 拷贝构造函数、析构函数和临时变量

Question

我写这个类是为了测试默认构造函数、复制构造函数、赋值运算符和析构函数的行为:

#include <iostream>

class Test {

  public:
    Test();

    Test(const Test&);

    ~Test();

    Test &operator=(const Test&);

  private:
    static int count;
    int label;

};

Test::Test() : label(count++) 
{
  std::cout<<"constructor of "<<label<<std::endl;
}

Test::Test(const Test &other) : label(count++)
{
  std::cout<<"copy-constructor of "<<label<<std::endl;
}

Test::~Test()
{
  std::cout<<"destructor of "<<label<<std::endl;
}

Test &Test::operator=(const Test &other)
{
  std::cout<<"copy assignment operator of "<<label<<std::endl;
}

int Test::count=0;

我在不同的上下文中使用了这个类来深入理解每个函数的调用方式和时间:我期望评论中的行为:

#include "Test.h"



// argument passed and returned by reference 
const Test &funct_by_ref(const Test &ref)
{
  return ref;
}

// argument passed and returned by value
// call copy constructor to initialize parameter
Test funct_by_val(Test val)
{
  // calls copy constructor to initialize temporary
  return val;
}  // exits local scope,val is destroyed,calls val destructor 



int main()
{
  // creates a temporary,calls 0 default constructor 
  Test();  // the temporary is destroyed at the end of the expression that created it
           // calls 0 destructor
  // creates a temporary (calls 1 default constructor) and calls 2 copy constructor 
  Test t2=Test(); // same as Test t2((Test()));
  std::cout<<"--------------------------------"<<std::endl;
  // calls 3 copy constructor
  Test t3=t2;
  // calls 4 default constructor
  Test t4;
  {
    // calls 5 copy constructor
    Test t5(t4);
  } // local scope,t5 is destroyed,calls 5 destructor
  // calls 4 assignment operator 
  t4=t2;
  std::cout<<"-------------------------------"<<std::endl;
  // nothing happens here
  funct_by_ref(t4);
  std::cout<<"-------------------------------"<<std::endl;
  // calls copy constructor twice 6,7
  funct_by_val(t4);
  // temporary is destroyed at the end of the expression,calls destructor
}

但我得到以下输出:

constructor of 0
destructor of 0
constructor of 1
------------------------
copy-constructor of 2
constructor of 3
copy-constructor of 4
destructor of 4
copy assignment operator of 3
--------------------------
---------------------------
copy-constructor of 5
copy-constructor of 6
destructor of 6
destructor of 5
destructor of 3
destructor of 2
destructor of 1

一切都很好，直到第一个 ----------------- 它似乎跳过了对象的创建(我想到了初始化 t2 的临时用途，因为它不是在那行之后立即销毁)所以计数减一..

Answer 1

Test t2=Test(); // same as Test t2((Test()));

编译器在这里(并且可以)eliding the copy并且将具有与执行相同的效果:

Test t2;