std :: bind2nd和std :: bind与二维数组和结构数组 [英] std::bind2nd and std::bind with bidimensional arrays and arrays of structs

问题描述

我知道C ++具有lambda和std :: bind1st，std :: bind2nd和std :: bind已被弃用.

I know that C++ have lambdas and std::bind1st, std::bind2nd and std::bind are deprecated.

但是，从C ++的基础开始，我们可以了解更好的新功能.

However, start with the foundations of C++, we can understand better new features.

所以，我从这个非常简单的代码开始，使用一个 int数组:

So, I start with this very simple code, using an array of ints:

第一个示例:使用 std :: bind2nd

int array1[] = { 10, 20, 30, 40, 50, 60, 40 };
int c1, c2, c3;

c1 = count_if(array1, array1 + 7, bind2nd(greater<int>(), 40));
c2 = count_if(array1, array1 + 7, bind2nd(less<int>(), 40));
c3 = count_if(array1, array1 + 7, bind2nd(equal_to<int>(), 40));
cout << "There are " << c1 << " elements that are greater than 40." << endl;
cout << "There are " << c2 << " elements that are lesser than 40." << endl;
cout << "There are " << c3 << " elements that are equal to 40." << endl;

第二个示例:使用 std :: bind

greater<int> big;
less<int> small;
equal_to<int> equal;

c1 = count_if(array1, array1 + 7, bind(big, _1, 40));
c2 = count_if(array1, array1 + 7, bind(small, _1, 40));
c3 = count_if(array1, array1 + 7, bind(equal, _1, 40));
cout << "There are " << c1 << " elements that are greater than 40." << endl;
cout << "There are " << c2 << " elements that are lesser than 40." << endl;
cout << "There are " << c3 << " elements that are equal to 40." << endl;

在两种情况下，输出均为:

In both cases the output is:

There are 2 elements that are greater than 40.
There are 3 elements that are lesser than 40.
There are 2 elements that are equal to 40.

如何对如下所示的双向数组进行相同操作:
(我想对第二个坐标进行相同的操作)

How can I do the same with bidimentional arrays like below:
(I want to made the same operations with the 2nd coordinate)

int array2[7][2] = { { 1, 10 }, { 2, 20 }, { 3, 30 }, 
                     { 4, 40 }, { 5, 50 }, { 6, 60 }, { 4, 40 } };

并带有如下所示的结构体数组:

And with arrays of structs like this:

struct st
{
    char c;
    int i;
};

st array3[] = { { 'a', 10 }, { 'b', 20 }, { 'c', 30 }, 
                { 'd', 40 }, { 'e', 50 }, { 'f', 60 }, { 'd', 40 } };

在这种情况下，我想对结构数组中的字段"int"执行相同的操作.

In this case, I want to do the same operations with the field 'int' in the array of structs.

有人可以帮助我吗?

谢谢

推荐答案

bind1st，bind2nd及其弟兄在C ++ 11中已弃用，并在C ++ 17中被彻底删除.以防万一你不知道这一点.

bind1st, bind2nd and their brethren are deprecated in C++11 and outright removed in C++17. Just in case you didn't know this.

使用bind，解决方案非常简单，您可以使用以下事实:bind表达式是可组合的，并且可以使用bind提取数据成员(为简便起见，省略了placeholders):

With bind, the solution is fairly straightforward, you can use the fact that bind expressions are composable and that you can use bind to extract a data member (placeholders omitted for brevity):

auto gr = count_if(array3, array3 + 7, bind(greater<>{}, bind(&st::i, _1), 40));
auto ls = count_if(array3, array3 + 7, bind(less<>{}, bind(&st::i, _1), 40));
auto eq = count_if(array3, array3 + 7, bind(equal_to<>{}, bind(&st::i, _1), 40));

使用bind2nd并不是那么容易.您需要声明一个具有多个typedef的函数对象(不能使用函数).您可以使用binary_function缓解此问题:

With bind2nd it's not that easy. You need to declare a function object (can't use a function) that has several typedefs. You can use binary_function to ease this:

struct my_greater : binary_function<st, int, bool>
{
    bool operator()(st const& l, int r) const {
        return greater<>{}(l.i, r);
    }
};

然后您可以致电

auto old = count_if(array3, array3 + 7, bind2nd(my_greater{}, 40));

在C ++ 11中，您可以使用lambda:

In C++11 you cam employ lambdas:

auto XI = count_if(array3, array3 + 7, [](st const& l){ return l.i > 40});

所有人的演示

如果您具有C ++ 11或更高版本，则使用lambda几乎总是更好的选择.这不仅是一个很好的默认设置"，您还必须真正扭曲这种情况，以使bind成为更好的解决方案.

If you have C++11 or newer available, it's almost always the better choice to use lambdas. It's not just a "good default", you'd have to really contort the situation for bind to be a better solution.

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