前言：warnings是编码中很重要的一个环节，编译器给出合理的warning能帮助开发者找到自己代码的问题，防止很多bu*生。
默认用XCode创建一个工程，会自动开启一些重要的warnings，但是更多的时候，我们需要编译器更完整的提醒。
iOS开发采用Clang编译器。
默认的Warning可以在Build Settings里找到
在search里搜索Warnings，就可以看到如图,这是为所有语言开启的warnings
当然，也可以为不同语言开启warning，也在Build Settings里
但是，这样一个个的开启关闭定制化很好，有时候我们只需要开启全部或者开启全部重要的warnings即可
这时候,进入
可以添加一些build flag来启用警告，主要的就是三个
1.-Wall Clang认为自己能够准确报出的警告
2. -Wextra额外的苛刻的警告，这些警告不一定会造成错误。例如如果使用这个flag，把singned 赋值给unsigned就会触发警告，而大多数时候这样赋值是没问题的。
3.-Weverything 所有警告
一般的项目都是开启-Wall和-Wextra两个警告来保证没有严重错误，当然，如果有些明显的不会出错，可以用关闭某个或者某些警告。
<code class="hljs lasso has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-attribute" style="box-sizing: border-box;">-Wall</span> <span class="hljs-attribute" style="box-sizing: border-box;">-Wno</span><span class="hljs-attribute" style="box-sizing: border-box;">-unused</span><span class="hljs-attribute" style="box-sizing: border-box;">-variable</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//启用Wall但是剔除unused-variable</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
用语句强制开启或者关闭某个警告
强制开启一个警告
<code class="hljs cs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">warning</span> "This method can not be used"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
强制开启一个错误
<code class="hljs cs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">error</span> "You must add this key,or you will fail"</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
强制关闭一个警告
例如
这里会出现警告test这个selector没有实现
<code class="hljs ruby has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">[<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span> <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">performSelector:</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">@selector</span>(test) <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">withObject:</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">nil</span>];
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>
强制关闭这个警告
<code class="hljs cs has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">pragma</span> clang diagnostic push</span>
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">pragma</span> clang diagnostic ignored "-Wundeclared-selector"</span>
[self performSelector:@selector(test) withObject:nil];
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">pragma</span> clang diagnostic pop</span>
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>
常用的CLang Warning
- Wall
- Wbad-function-cast
- Wcast-align
- Wconversion
- Wdeclaration-after-statement
- Wdeprecated-implementations
- Wextra
- Wfloat-equal
- Wformat=2
- Wformat-nonliteral
- Wfour-char-constants
- Wimplicit-atomic-properties
- Wmissing-braces
- Wmissing-declarations
- Wmissing-field-initializers
- Wmissing-format-attribute
- Wmissing-noreturn
- Wmissing-prototypes
- Wnested-externs
- Wnewline-eof
- Wold-style-definition
- Woverlength-strings
- Wparentheses
- Wpointer-arith
- Wredundant-decls
- Wreturn-type
- Wsequence-point
- Wshadow
- Wshorten-64-to-32
- Wsign-compare
- Wsign-conversion
- Wstrict-prototypes
- Wstrict-selector-match
- Wswitch
- Wswitch-default
- Wswitch-enum
- Wundeclared-selector
- Wuninitialized
- Wunknown-pragmas
- Wunreachable-code
- Wunused-function
- Wunused-label
- Wunused-parameter
- Wunused-value
- Wunused-variable
- Wwrite-strings
困难模式
所谓的困难模式就是开启所有警告，并且把警告当作error。这意味着，只要还有一个警告存在，那么程序将不能编译运行。
参考链接
http://onevcat.com/2013/05/talk-about-warning/
http://programmers.stackexchange.com/questions/122608/clang-warning-flags-for-objective-c-development
http://amattn.com/p/better_apps_clang_weverything_or_wall_is_a_lie.html
http://nshipster.cn/clang-diagnostics/
原文地址：http://blog.csdn.net/hello_hwc/article/details/46425503