c++11 - std::unique 定义了删除哪个元素？

Question

基于 example code here 我写了这个小例子 ( ideone Link ):

#include <iostream>
#include <algorithm>
#include <string>

int main() 
{
    std::string s = "foo123bar456wibble";
    auto end = std::unique(s.begin(), s.end(), [](char l, char r){
        return std::isdigit(l) && std::isdigit(r);
    });
    // What does s hold?
    std::cout << std::string(s.begin(), end) << '\n';
}

我的输出是:

foo1bar4wibble

标准是否保证这种行为，或者这是否也可以接受？

foo2bar6wibble

链接的 cppreference 页面说:

Removing is done by shifting the elements in the range in such a way that elements to be erased are overwritten.

但那是规范性文本还是只是建议的实现？

此外，cplusplus.com says:

Removes all but the first element from every consecutive group of equivalent elements in the range [first,last).

但这又是规范的吗？

Answer 1

25.3.9 [alg.unique]/1

Effects: For a nonempty range, eliminates all but the first element from every consecutive group of equivalent elements referred to by the iterator i in the range [first + 1,last) for which the following conditions hold: *(i - 1) == *i or pred(*(i - 1), *i) != false.