PHP anonymous functions and closures-PHP Tutorial-php.cn

[iefreer] reprinted an article that explains the PHP closure syntax in depth, and will also repost an article later on how to apply these new syntaxes skillfully.

Anonymous functions appeared relatively early in programming languages. They first appeared in Lisp language. Later, many programming languages began to have this function.

Javascript and C# are currently widely used, and PHP did not start to be implemented until 5.3. Supports anonymous functions, and the new C++ standard C++0x has also begun to support it.

Anonymous function is a type of function or subroutine that can be called without specifying an identifier. Anonymous functions can be conveniently passed as parameters to other functions. The most common application is as a callback function.

Closure

When it comes to anonymous functions, we have to mention closures. Closures are short for lexical closures (Lexical Closure). They are functions that reference free variables. This free variable is applied Will exist with this function even if it leaves the environment in which it was created, so a closure can also be thought of as an entity composed of a function and its associated references. In some languages, when defining another function within a function, a closure may occur if the inner function references a variable of the outer function. When running an external function, a closure is formed. The word

is easily confused with anonymous functions. In fact, they are two different concepts. This may be because many languages allow closures to be formed when implementing anonymous functions.

Use create_function() to create an "anonymous" function

As mentioned earlier, anonymous functions were only officially supported in PHP5.3. At this point, careful readers may have opinions, because there is a function that can generate anonymous functions. : create_function function, you can find this function in PHP4.1 and PHP5 in the manual. This function can usually also be used as an anonymous callback function, for example, as follows:

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$array = array(1, 2, 3, 4 );
array_walk ($array, create_function('$value', 'echo $value'));

This code just outputs the values in the array in sequence. Of course more can be done. So why is this not a true anonymous function? Let’s first look at the return value of this function. This function returns a string. Usually we can call a function like this:

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function a() { echo
'function a'; }
$a
'a'; $a
We are implementing the callback function You can also use this method, for example:

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function
$callback) { //doing
# …
}
This can be implemented in the function After do_something() is executed, the function specified by $callback is called. Returning to the return value of the create_function function: the function returns a unique string function name, or FALSE if an error occurs. So this function only dynamically creates a function, and this function has a function name, which means that it is not actually anonymous. It just creates a globally unique function.
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2. $func = create_function('', 'echo "Function created dynamic";');
3. echo $func; // lambda_1
5. $func(); //Function created dynamic
7. $my_func = 'lambda_1';
8. $my_func(); //This function does not exist
9. lambda_1(); // This function does not exist
The front of the above code is easy to understand. This is how create_function is used. However, later calling it through the function name fails. This is a bit difficult to understand. How does PHP work? Ensure that this function is globally unique? lambda_1 seems to be a very common function name. What if we first define a function called lambda_1? The return string of the function here will be lambda_2, which will check whether this is the case when creating the function. Whether the function exists or not, we need to find the appropriate function name, but what if we define a function called lambda_1 after create_function? This will cause the problem of repeated definition of functions. This implementation may not be the best method. In fact, if If you really define a function named lambda_1, the problem I mentioned will not occur. What is going on? The last two lines of the above code also illustrate this problem. In fact, there is no function named lambda_1 defined.
That is to say, our lambda_1 and the lambda_1 returned by create_function are not the same!? How could this be the case? That only means that we have not seen the essence, only the surface. The surface is that we output lambda_1 when echoing. And our lambda_1 is typed in by ourselves. Let's use the debug_zval_dump function to take a look.
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2. $func = create_function('', 'echo "Hello";' );
4. $my_func_name = 'lambda_1'; // string(9) "lambda_1" refcount(2)
5. debug_zval_dump($my_func_name); // string(8) "lambda_1" refcount(2)
6. See it, theirs The length is actually different, the length Different means that it is not the same function, so of course the function we call does not exist. Let's just look at what the create_function function does. See the implementation: $PHP_SRC/Zend/zend_builtin_functions.c
1. #define LAMBDA_TEMP_FUNCNAME "__lambda_func"
3. ZEND_FUNCTION(create_function)
4. {
5. // ... 省去无关代码
6. function_name = (char *) emalloc(sizeof("0lambda_")+MAX_LENGTH_OF_LONG);
7. function_name[0] = ''; // <--- 这里
8. do {
9. function_name_length = 1 + sprintf(function_name + 1, "lambda_%d", ++EG(lambda_count));
10. } while (zend_hash_add(EG(function_table), function_name, function_name_length+1, &new_function, sizeof(zend_function), NULL)==FAILURE);
11. zend_hash_del(EG(function_table), LAMBDA_TEMP_FUNCNAME, sizeof(LAMBDA_TEMP_FUNCNAME));
12. RETURN_STRINGL(function_name, function_name_length, 0);
13. }
该函数在定义了一个函数之后，给函数起了个名字，它将函数名的第一个字符变为了''也就是空字符，然后在函数表中查找是否已经定义了这个函数，如果已经有了则生成新的函数名，第一个字符为空字符的定义方式比较特殊，因为在用户代码中无法定义出这样的函数，也就不存在命名冲突的问题了，这也算是种取巧(tricky)的做法，在了解到这个特殊的函数之后，我们其实还是可以调用到这个函数的，只要我们在函数名前加一个空字符就可以了， chr()函数可以帮我们生成这样的字符串，例如前面创建的函数可以通过如下的方式访问到:
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3. $my_func = chr(0) . "lambda_1";
4. $my_func(); // Hello
这种创建"匿名函数"的方式有一些缺点:
1. 函数的定义是通过字符串动态eval的，这就无法进行基本的语法检查;
2. 这类函数和普通函数没有本质区别，无法实现闭包的效果.
真正的匿名函数
在PHP5.3引入的众多功能中，除了匿名函数还有一个特性值得讲讲: 新引入的__invoke 魔幻方法。
__invoke魔幻方法
这个魔幻方法被调用的时机是: 当一个对象当做函数调用的时候，如果对象定义了__invoke魔幻方法则这个函数会被调用，这和C++中的操作符重载有些类似，例如可以像下面这样使用:
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2. class Callme {
3. public function __invoke($phone_num) {
4. echo "Hello: $phone_num";
5. }
6. }
8. $call = new Callme();
9. $call(13810688888); // "Hello: 13810688888
匿名函数的实现
前面介绍了将对象作为函数调用的方法，聪明的你可能想到在PHP实现匿名函数的方法了，PHP中的匿名函数就的确是通过这种方式实现的。我们先来验证一下:
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2. $func = function() {
3. "Hello, anonymous function";
6. echo gettype($func); // object
7. echoget_class($ func); //Closure
It turns out that the anonymous function is just an ordinary class. Students who are familiar with Javascript are very familiar with the use of anonymous functions. PHP also uses a syntax similar to Javascript to define. Anonymous functions can be assigned to a variable. Because the anonymous function is actually a class instance, it is easy to understand that it can be copied. , In Javascript, you can assign an anonymous function to the property of an object, for example:
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1. var a = {};
2. function() {alert("called");}
3. // alert called
This is very common in Javascript, but this is not possible in PHP. Copying the properties of an object cannot be called, so using it will Causes the class to search for methods defined in the class. In PHP, attribute names and defined method names can be repeated. This is determined by PHP's class model. Of course, PHP can be improved in this regard, and may be improved in subsequent versions. Allowing such calls makes it easier to flexibly implement some functions. At present, there is a way to achieve such an effect: use another magic method __call(). As for how to achieve it, I will leave it as an exercise to the readers.
The use of closures
PHP uses closures (Closure) to implement anonymous functions. The most powerful function of anonymous functions is some of the dynamic features and closure effects provided by anonymous functions. When defining anonymous functions, if If you need to use variables outside the scope, you need to use the following syntax:
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2. $name = 'TIPI Team';
3. $func = function() use($name) {
4. } $ func
5. (); // Hello TIPI Team
7. This use statement looks quite awkward, especially compared with Javascript, but this should also be used by PHP-Core after comprehensive consideration syntax, because it is different from the scope of Javascript, variables defined in PHP functions are local variables by default, while in Javascript it is the opposite. Except for explicit definitions, PHP cannot determine whether the variable is a local variable when mutating. Of course, there may be a way to determine whether local variables are still variables in the upper scope at compile time, but this will have a great impact on the efficiency and complexity of the language. This syntax is relatively straightforward. If you need to access variables in the upper scope, you need to use the use statement to declare it. This is also simple and easy to read. Speaking of which, you can actually use use to achieve effects similar to the global statement. The anonymous function can access the variables in the upper scope every time it is executed. These variables always save their own state before the anonymous function is destroyed, such as the following example:
2. function getCounter() {
3. $i = 0;
4. return function() use($i) { // If you use a reference to pass in variables: use(&$i)
5. ++ $i ; $counter
6. (); // 1
8. $counter
9. (); // 1
10. is different from Javascript. The two function calls here do not increment the $i variable. The default PHP The upper-layer variable is passed into the anonymous function by copying. If the value of the upper-layer variable needs to be changed, it needs to be passed by reference. So the above code does not output 1, 2 but 1, 1. Implementation of closure
11. For example, the following code: [php] view plaincopy