c++ - 迭代 std::string 元素

Question

将迭代器移动到以下字符串元素是否需要常数时间:

std::string str // string of size 100 MB
std::string::iterator iter = str.begin();
std::advance(iter, str.size()-1);

像按索引搜索一样需要固定时间吗？

char c = str[str.size()-1];

Answer 1

是的，没错。这是由 C++ 标准(§24.3，迭代器操作)保证的:

Since only random access iterators provide + and - operators, the library provides two function templates advance and distance. These function templates use + and - for random access iterators (and are, therefore, constant time for them);