复制构造函数，析构函数和临时函数 [英] copy constructor,destructor and temporaries

问题描述

我编写了此类，以测试默认构造函数，副本构造函数，赋值运算符和析构函数的行为:

I wrote this class to test the behaviour of the default constructor,the copy constructor, the assignment operator and the destructor:

#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此类，以深刻理解每个函数的调用方式和时间:我希望注释中的行为:

I used this class in diferent contextes to deeply understand how and when each function is called: I expect the behaviour in the comments:

#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
}

但是我得到以下输出:

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-----------------似乎跳过了对象的创建(我想到了用来初始化t2的临时用例，因为它不是在该行之后立即销毁)，因此计数减一.

all is fine until the first ----------------- where it seems to skip the creation of an object (I think of the temporary uset to initialize t2, because it's not destroyed right after that line) and so the count is off by one..

推荐答案

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

在这里，编译器是(并且可以)删除副本，其效果与在做:

Here the compiler is(and can) eliding the copy and will have the same effect as doing:

Test t2;

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