-w 的意思是关闭编译时的警告
就是说编译后不会显示任何warning.
- Wall 启用大部分警告
同时也会启用以下警告标志
-Waddress
-Warray-bounds=1 (only with -O2)
-Wbool-compare
-Wbool-operation
-Wc++11-compat
-Wc++14-compat
-Wcatch-value (C++ and Objective-C++ only)
-Wchar-subscripts
-Wcomment
-Wduplicate-decl-specifier (C and Objective-C only)
-Wenum-compare (in C/ObjC; this is on by default in C++)
-Wformat
-Wint-in-bool-context
-Wimplicit (C and Objective-C only)
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C++)
-Wlogical-not-parentheses
-Wmain (only for C/ObjC and unless -ffreestanding)
-Wmaybe-uninitialized
-Wmemset-elt-size
-Wmemset-transposed-args
-Wmisleading-indentation (only for C/C++)
-Wmissing-attributes
-Wmissing-braces (only for C/ObjC)
-Wmultistatement-macros
-Wnarrowing (only for C++)
-Wnonnull
-Wnonnull-compare
-Wopenmp-simd
-Wparentheses
-Wpointer-sign
-Wreorder
-Wrestrict
-Wreturn-type
-Wsequence-point
-Wsign-compare (only in C++)
-Wsizeof-pointer-div
-Wsizeof-pointer-memaccess
-Wstrict-aliasing
-Wstrict-overflow=1
-Wstringop-truncation
-Wswitch
-Wtautological-compare
-Wtrigraphs
-Wuninitialized
-Wunknown-pragmas
-Wunused-function
-Wunused-label
-Wunused-value
-Wunused-variable
-Wvolatile-register-var
要注意的是， -Wall 并不含有所有的警告标志。不包含的警告通常不需要注意，但并不代表不会出问题。
-W 标志，现在被 -Wextra 所代替
他将会启用 -Wall 选项没有启用的警告标志，其中包括
-Wclobbered
-Wcast-function-type
-Wempty-body
-Wignored-qualifiers
-Wimplicit-fallthrough=3
-Wmissing-field-initializers
-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only)
-Woverride-init
-Wsign-compare (C only)
-Wtype-limits
-Wuninitialized
-Wshift-negative-value (in C++03 and in C99 and newer)
-Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)
选项-Wextra选项还会在以下情况下显示警告消息：
- 指针与带有“ <”，“ <=”，“>”或“> =”的整数零进行比较。
- (仅C ++)枚举数和非枚举数都出现在条件表达式中。
- (仅C ++)模糊的虚拟基础。
- (仅C ++)对已声明为“寄存器”的数组进行下标。
- (仅C ++)采用已声明为“寄存器”的变量的地址。
- (仅C ++)未在派生类的副本构造函数中初始化基类。