拷贝构造函数、析构函数和临时变量

Question

我写这个class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这个 class 来深入理解每个函数的调用方式和时间：我希望评论中的行为：

#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，因为它并没有在该行之后立即被销毁）所以计数减一..

Answer 1

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

这里的编译器是（并且可以）eliding the copy 并且将具有与以下操作相同的效果：

Test t2;

Answer 2

如果检测临时对象存在的唯一方法是跟踪构造函数和析构函数调用，则该标准有特定条款允许 - 但不要求 - 实现省略（即省略创建）临时对象。

您看到的差异是因为您预期的行为是基于临时对象的创建和销毁，而编译器选择不创建这些临时对象。请注意，消除临时变量的决定取决于实现（并且通常受优化设置影响）。

拷贝构造函数、析构函数和临时变量

copy constructor,destructor and temporaries

c++

destructor

copy-constructor

assignment-operator

temporaries