Both STL algorithms and the range-based for loop assume that the begin and end positions of the iteration are known in advance. In some situations, however, it is hardly possible to know the end position before reaching it by iteration.

A very simple example for this is iterating over plain C-Style strings, the length of which is not known before runtime. The code which iterates over such strings usually looks like this:

for (const char *c_ponter = some_c_string; *c_pointer != ''; ++c_pointer) {
    const char c = *c_pointer;
    // do something with c
}

The only way to put this into a range-based for loop seems to be wrapping it into an std::string, which has begin() and end() functions:

for (char c : std::string(some_c_string)) { /* do something with c */ }

However, the constructor of std::string will iterate over the whole string before our for loop can iterate over it. Since C++17, we also have std::string_view, but its constructor will also iterate through the string once. This is not worth the real hassle for short strings, but this is also only an example for a problem class, which can be worth the hassle in other situations. The std::istream_iterator also has to deal with this when it captures input from std::cin, as its end iterator cannot realistically point to the end of the user input while the user is still typing keys.

C++17 comes with the great news that it does not constrain begin and end iterators to be of the same type. This section demonstrates how to put this little rule change to great use.