Question

我编写了这个类来测试默认构造函数，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类来深入理解每个函数的调用方式和时间：我期待评论中的行为：

#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-----------------它似乎跳过了一个对象的创建（我想到临时用来初始化t2，因为它＆＃ 39;在那条线之后没有被销毁）因此计数被一个人关闭..

Answer 1

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

这里编译器是（并且可以）eliding the copy并且具有与执行相同的效果：

Test t2;

Answer 2

该标准具有特定条款允许 - 但不要求 - 实现以消除（即省略创建）临时文件，如果检测其存在的唯一方法是通过跟踪构造函数和析构函数调用。

您看到的差异是因为您预期的行为是基于临时工的创建和破坏，编译器选择不创建那些临时工。请注意，消除临时性的决定因实施而异（并且通常受优化设置的影响）。

复制构造函数，析构函数和临时函数

2 个答案: