什么是函数句柄，它如何有用？ [英] What is a function handle and how is it useful?

问题描述

有人可以向我解释 @ （函数句柄）运算符的含义，以及为什么要使用它？

解决方案

The MATLAB中的函数句柄运算符的作用基本上就像指向特定函数实例的指针。其他一些答案已经讨论了它的一些用途，但是我会在这里添加另一个我经常用到的用法：保持对不在范围内的函数的访问。

例如，下面的函数初始化一个值 count ，然后返回一个嵌套函数 increment $ b $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $'
fHandle = @increment;

函数增量
count = count + 1;
disp（count）;
end

end

由于函数 increment 是一个嵌套函数 a>，它只能在 start_counting 函数中使用（即 start_counting 的工作空间是它的范围） 。但是，通过返回函数 increment 的句柄，我仍然可以在 start_counting 之外使用它，它仍然保留访问 start_counting 工作区中的变量！这允许我这样做：

 >> fh = start_counting（3）; ％初始化计数为3并返回句柄
 3 
 
>> FH（）; ％使用其句柄调用增量函数
 4 
 
>> FH（）; 
 5 
  

请注意，即使我们不在函数 start_counting 。但是，通过再次调用 start_counting 并使用另一个数字并将函数句柄存储在另一个变量中，您可以做更有趣的事情：

 >> fh2 = start_counting（-4）; 
 -4 
 
>> FH2（）; 
 -3 
 
>> FH2（）; 
 -2 
 
>> FH（）; ％调用第一个句柄以增加
 6 
 
>> FH2（）; ％调用第二个句柄以增加
 -1 
  

请注意，这两个不同的计数器独立运行。函数处理 fh 和 fh2 指向函数 increment ，其中包含 count 的唯一值的不同工作空间。除了上述内容，还可以使用函数句柄与嵌套函数的结合也可以帮助简化GUI设计，正如我在这篇其他SO贴子中所说明的那样。

Can somebody explain to me the meaning of the @ (function handle) operator and why to use it?

解决方案

The function handle operator in MATLAB acts essentially like a pointer to a specific instance of a function. Some of the other answers have discussed a few of its uses, but I'll add another use here that I often have for it: maintaining access to functions that are no longer "in scope".

For example, the following function initializes a value count, and then returns a function handle to a nested function increment:

function fHandle = start_counting(count)

  disp(count);
  fHandle = @increment;

  function increment
    count = count+1;
    disp(count);
  end

end

Since the function increment is a nested function, it can only be used within the function start_counting (i.e. the workspace of start_counting is its "scope"). However, by returning a handle to the function increment, I can still use it outside of start_counting, and it still retains access to the variables in the workspace of start_counting! That allows me to do this:

>> fh = start_counting(3);  % Initialize count to 3 and return handle
     3

>> fh();  % Invoke increment function using its handle
     4

>> fh();
     5

Notice how we can keep incrementing count even though we are outside of the function start_counting. But you can do something even more interesting by calling start_counting again with a different number and storing the function handle in another variable:

>> fh2 = start_counting(-4);
    -4

>> fh2();
    -3

>> fh2();
    -2

>> fh();  % Invoke the first handle to increment
     6

>> fh2();  % Invoke the second handle to increment
    -1

Notice that these two different counters operate independently. The function handles fh and fh2 point to different instances of the function increment with different workspaces containing unique values for count.

In addition to the above, using function handles in conjunction with nested functions can also help streamline GUI design, as I illustrate in this other SO post.

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