Collapse expression

Let's take a look at an example , There are three summation functions in this example sum1,sum2,sum3.

auto sum1()
{
    
	return 0;
}

template<typename T, typename...U>
auto sum1(T a, U... rest)
{
    	
	return a + sum1(rest...);
}


template<typename T,typename...U>
auto sum2(T a, U... rest)
{
    
	if constexpr(sizeof...(rest)>0)
	{
    
		return a + sum2(rest...);
	}
	else
	{
    
		return a;
	}
}


template<typename...U>
auto sum3(U... rest)
{
    
	return (... + rest);
}

int main()
{
    
	cout << sum1(1, 2, 3, 5.5, 6) << endl;
	cout<<sum2(1, 2, 3, 5.5, 6) << endl;
	cout << sum3(1, 2, 3, 5.5, 6) << endl;
	system("pause");
	return 0;
}

result ：

         In the above code sum3 You use the collapse expression , You can see that the code is very concise , But it also realizes the same function .
The collapse expression is c++17 Standard introduced , The main purpose of introducing folding expression is to calculate a certain value , What makes this value special is ： It relates to all variable parameters , It is not related to a single parameter .
         Folding expressions generally have 4 format （ Note that each format is enclosed in parentheses ）. Left fold means that parameters are calculated from the left , Turning right means that parameters are calculated from the right .

One yuan is off to the left

Format ：(… Operator A package of parameters )
Calculation method ：（(（ Parameters 1 Operator Parameters 2） Operator Parameters 3））… Operator Parameters N)
Example ：

template<typename ...U>
auto sub_left(U...paras)
{
    
	return (... - paras);
}

int main()
{
    
	cout << sub_left(10, 20, 30, 40) << endl;
	system("pause");
	return 0;
}

result ：-80

One dollar right discount

Format ：（ A package of parameters Operator …）
Calculation method ： Parameters 1 Operator （ Parameters 2…（ Parameters N-1 Operator Parameters N)）
Example ：

template<typename ...U>
auto sub_right(U...paras)
{
    
	return (paras - ...);
}

int main()
{
    
	cout << sub_right(10, 20, 30, 40) << endl;
	system("pause");
	return 0;
}

result ：-20

Binary left fold

Format ：（init Operator … Operator A package of parameters ）
Calculation method ：（(（init Operator Parameters 1） Operator Parameters 2）… Operator Parameters N)
Example ：

template<typename ...U>
auto sub_left(U...paras)
{
    
	return (220 - ... - paras);
}

int main()
{
    
	cout << sub_left(10, 20, 30, 40) << endl;
	system("pause");
	return 0;
}

result ：120
（（（（220-10）-20）-30）-40）

Two yuan right turn

Format ：（ A package of parameters Operator … Operator init）
Calculation method ：( Parameters 1 Operator （…( Parameters N Operator init）)
Example ：

template<typename ...U>
auto sub_right(U...paras)
{
    
	return (paras - ... - 220);
}

int main()
{
    
	cout << sub_right(10, 20, 30, 40) << endl;
	system("pause");
	return 0;
}

result ：200
10-(20-(30-(40-220)))=200