面试PHP时经常碰到一种判断各种类型的空值是否为true的题:
<code>$a = <span>''</span>; $a = null $a = false; <span>if</span>($a){<span>...</span>} <span>if</span>(isset($a)){<span>...</span>} <span>if</span>(empty($a)){<span>...</span>} <span>...</span></code>
由下面的例子我们来简单看下zend引擎中对if是怎么处理的:
<code><span><span><?php </span><span>$a</span> = <span>''</span>; <span>//array();</span><span>if</span>(<span>$a</span>){ <span>echo</span><span>"Y"</span>; }</span></span></code>
这里例子比较简单,结果将什么也不输出。(文中涉及代码均为php-7.0.4版本)
之前的文章介绍过zend执行阶段的入口zend_execute函数,我们直接从这里开始,不熟悉的可以翻一下前面的文章。
编译生成的opcodes如下:
其中opcode=38是$a = ”的执行操作,opcode=43是if的操作,下面具体看这一步是如何执行的。
根据opcode及两个操作数类型可以找到对应的handler为:ZEND_JMPZ_SPEC_CV_HANDLER
<code><span>//zend_vm_execute.h #28307</span><span>static</span> ZEND_OPCODE_HANDLER_RET ZEND_FASTCALL ZEND_JMPZ_SPEC_CV_HANDLER(ZEND_OPCODE_HANDLER_ARGS) { USE_OPLINE zval *<span>val</span>; <span>val</span> = _get_zval_ptr_cv_undef(execute_data, opline->op1.var); <span>if</span> (Z_TYPE_INFO_P(<span>val</span>) == IS_TRUE) { ZEND_VM_SET_NEXT_OPCODE(opline + <span>1</span>); ZEND_VM_CONTINUE(); } <span>else</span><span>if</span> (EXPECTED(Z_TYPE_INFO_P(<span>val</span>) if (IS_CV == IS_CV && UNEXPECTED(Z_TYPE_INFO_P(<span>val</span>) == IS_UNDEF)) { SAVE_OPLINE(); GET_OP1_UNDEF_CV(<span>val</span>, BP_VAR_R); ZEND_VM_JMP(OP_JMP_ADDR(opline, opline->op2)); } <span>else</span> { ZEND_VM_SET_OPCODE(OP_JMP_ADDR(opline, opline->op2)); ZEND_VM_CONTINUE(); } } SAVE_OPLINE(); <span>if</span> (i_zend_is_true(<span>val</span>)) { opline++; } <span>else</span> { opline = OP_JMP_ADDR(opline, opline->op2); } <span>if</span> (UNEXPECTED(EG(<span>exception</span>) != NULL)) { HANDLE_EXCEPTION(); } ZEND_VM_JMP(opline); }</code>
从这个函数可以看出if的执行过程:如果条件为true的话则opline++,顺序执行下一条opcode(即if内语句),否则进行跳转,跳过if内语句直接执行if外语句。
i_zend_is_true这个函数就是用来判断各种类型的值是否为真,前面那部分是判断是否为bool型,是的话则直接处理。
<code><span>//</span>zend_operators.h <span>#283</span> static zend_always_inline int i_zend_is_true(zval *op) { int result = <span>0</span>; <span>again</span>: <span>switch</span> (Z_TYPE_P(op)) { <span>case</span><span>IS_TRUE</span>: result = <span>1</span>; <span>break</span>; <span>//</span>数值类型long、double直接判断即可,与c用法相同 <span>case</span><span>IS_LONG</span>: <span>if</span> (Z_LVAL_P(op)) { result = <span>1</span>; } <span>break</span>; <span>case</span><span>IS_DOUBLE</span>: <span>if</span> (Z_DVAL_P(op)) { result = <span>1</span>; } <span>break</span>; <span>//</span>字符串类型根据长度判断:长度><span>1</span>,或=<span>1</span>且不为<span>'0'</span>为<span>true</span>,所以上面那个例子<span>''</span><span> =></span><span>false</span><span>case</span><span>IS_STRING</span>: <span>if</span> (Z_STRLEN_P(op) > <span>1</span> || (Z_STRLEN_P(op) && Z_STRVAL_P(op)[<span>0</span>] != <span>'0'</span>)) { result = <span>1</span>; } <span>break</span>; <span>//</span>数组类型根据数组元素的个数判断:大于<span>0</span>即为真 <span>case</span><span>IS_ARRAY</span>: <span>if</span><span><span>(zend_hash_num_elements(Z_ARRVAL_P(op)))</span> { // <span>(Z_ARRVAL_P(op))</span>-></span>nNumOfElements result = <span>1</span>; } <span>break</span>; <span>case</span><span>IS_OBJECT</span>: result = zend_object_is_true(op); <span>break</span>; <span>//</span>资源类型实际就是整形(后续会专门介绍资源类型),所以直接判断即可 <span>case</span><span>IS_RESOURCE</span>: <span>if</span> (EXPECTED(Z_RES_HANDLE_P(op))) { result = <span>1</span>; } <span>break</span>; <span>//</span>引用类型则根据指向的值判断 <span>case</span><span>IS_REFERENCE</span>: op = Z_REFVAL_P(op); goto again; <span>break</span>; <span>default</span>: <span>break</span>; } <span>return</span> result; }</code>
isset、empty函数后续补充……
').addClass('pre-numbering').hide(); $(this).addClass('has-numbering').parent().append($numbering); for (i = 1; i ').text(i)); }; $numbering.fadeIn(1700); }); });以上就介绍了语法:IF判断的实现,包括了方面的内容,希望对PHP教程有兴趣的朋友有所帮助。