I was reading the answers for this question and found that there is actually a method called length() for std::string (I always used size()). Is there any specific reason for having this method in std::string class? I read both MSDN and CppRefernce, and they seem to indicate that there is no difference between size() and length(). If that is so, isn't it making more confusing for the user of the class?
As per the documentation, these are just synonyms. size() is there to be consistent with other STL containers (like vector, map, etc.) and length() is to be consistent with most peoples' intuitive notion of character strings. People usually talk about a word, sentence or paragraph's length, not its size, so length() is there to make things more readable.
Ruby's just the same, btw, offering both #length and #size as synonyms for the number of items in arrays and hashes (C++ only does it for strings).
Minimalists and people who believe "there ought to be one, and ideally only one, obvious way to do it" (as the Zen of Python recites) will, I guess, mostly agree with your doubts, @Naveen, while fans of Perl's "There's more than one way to do it" (or SQL's syntax with a bazillion optional "noise words" giving umpteen identically equivalent syntactic forms to express one concept) will no doubt be complaining that Ruby, and especially C++, just don't go far enough in offering such synonymical redundancy;-).
length of string ==how many bits that string having, size==size of those bits, In strings both are same if the editor allocates size of character is 1 byte
来源：https://stackoverflow.com/questions/905479/stdstring-length-and-size-member-functions