why const CString & host cannot use the GetBuffer() function?

code:
if(inet_addr( host.GetBuffer(0) )==INADDR_NONE)
{ .....}
error:
C2663: “ATL::CSimpleStringT<BaseType>::GetBuffer” : 2 个重载没有“this”指针的合法转换
with
[
BaseType=TCHAR
]
help me , thanks!