[英]copy constructor,destructor and temporaries
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:
#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:我期望注释中的行为:
#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-----------------那里似乎跳过了对象的创建(我想到了临时用例来初始化t2,因为它不会在之后立即销毁该行),因此计数相减一。
如果唯一的检测方式是通过跟踪构造函数和析构函数调用来检测临时对象的存在,则该标准具有特定的规定,允许(但不要求)实现消除临时对象(即省略临时对象的创建)。
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,并且编译器选择不创建那些临时对象。 请注意,更改临时人员的决定会因实现而异(并且通常会受到优化设置的影响)。
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