持久变量(GNU Octave(版本9.4.0))

7.2持久变量¶

详见关键字:persistent

已语句的变量持久的在一个函数内,将在对同一函数的后续调用之间将其内容保留在内存中。持久变量和全局变量之间的区别在于,持久变量在特定函数的作用域中是局部的,在其他地方不可见。

下面的示例使用一个持久变量来创建一个函数,该函数打印它被调用的次数。

function count_calls ()
  persistent calls = 0;
  printf ("'count_calls' has been called %d times\n",
          ++calls);
endfunction

for i = 1:3
  count_calls ();
endfor

-| 'count_calls' has been called 1 times
-| 'count_calls' has been called 2 times
-| 'count_calls' has been called 3 times

如示例所示,可以使用persistent语句语句。以下语句都是持久性语句。

persistent a
persistent a b
persistent c = 2
persistent d = 3 e f = 5

持久变量的行为等价于C中静态变量的行为。

持久变量的一个限制是,函数的输入或输出参数都不能是持久的:

function y = foo ()
  persistent y = 0;  # Not allowed!
endfunction

foo ()
-| error: can't make function parameter y persistent

与全局变量一样,持久变量只能初始化一次。例如,在执行以下代码之后

persistent pvar = 1
persistent pvar = 2

持久变量的值pvar是1,而不是2。

如果一个持久变量被语句但没有初始化为特定值,它将包含一个空矩阵。因此,也可以通过检查它是否为空来初始化持久变量,如下例所示。

function count_calls ()
  persistent calls;
  if (isempty (calls))
    calls = 0;
  endif
  printf ("'count_calls' has been called %d times\n",
          ++calls);
endfunction

此实现的行为方式与的上一个实现完全相同count_calls.

持久变量的值一直保存在内存中,直到它被明确清除为止。假设的支持count_calls保存在磁盘上,我们会得到以下行为。

for i = 1:2
  count_calls ();
endfor
-| 'count_calls' has been called 1 times
-| 'count_calls' has been called 2 times

clear
for i = 1:2
  count_calls ();
endfor
-| 'count_calls' has been called 3 times
-| 'count_calls' has been called 4 times

clear all
for i = 1:2
  count_calls ();
endfor
-| 'count_calls' has been called 1 times
-| 'count_calls' has been called 2 times

clear count_calls
for i = 1:2
  count_calls ();
endfor
-| 'count_calls' has been called 1 times
-| 'count_calls' has been called 2 times

也就是说,只有当包含变量的函数被删除时,持久变量才会从内存中删除。请注意,如果函数定义直接输入到Octave提示符中,则persistentvariable将通过简单的clear命令作为entirefunction定义将从内存中删除。如果即使函数已清除,也不希望从内存中删除持久变量,则应使用mlock函数(详见函数锁定).