字符串find_first_not_of（）与find_last_not_of（）中只有空格

Question

I want to check if a wstring only contains whitespaces (to be precise: " \\t\\r\\n"). I've found several solutions using the method find_last_not_of() .

I've got 2 questions regarding this method:

1. when i know that in the cases where the wstring will contain non-whitespace-characters, those characters will be at the beginning of the string, wouldn't it be better if I'd use find_first_not_of() since it returns as soon as it found something?

2. do both methods have a complexity of O(n) or am i wrong here?

I know there's a lot of information about these methods in the web, but i've found some contradictory statements on this topic.

Answer 1

The way std::basic_string::find_last_not_of is implemented is not part of the specification; thus we can't say if the characters are looked up in natural or reversed order.

Hence,

1. Implementation-dependent
2. Implementation-dependent

Let's have a look of the libstdc++'s implementation of std::basic_string::find_last_not_of (basic_string.tcc):

1317   template<typename _CharT, typename _Traits, typename _Alloc>
1318     typename basic_string<_CharT, _Traits, _Alloc>::size_type
1319     basic_string<_CharT, _Traits, _Alloc>::
1320     find_last_not_of(const _CharT* __s, size_type __pos, size_type __n) const
1321     {
1322       __glibcxx_requires_string_len(__s, __n);
1323       size_type __size = this->size();
1324       if (__size)
1325    {
1326      if (--__size > __pos)
1327        __size = __pos;
1328      do
1329        {
1330          if (!traits_type::find(__s, __n, _M_data()[__size]))
1331            return __size;
1332        }
1333      while (__size--);
1334    }
1335       return npos;
1336     }

As one could have guessed, the string is looked up backward. In your specific case, std::basic_string::find_first_not_of should be the way to go.