String front, back return code point/unit

[vit]

Hello,

I am implementing @nogc string struct similar to c++ 
std::basic_string (link 
https://code.dlang.org/packages/basic_string).

C++ string has methods front, back and pop_back 
returning/deleting code units.
D strings has functions (std.range) front, back and popBack 
returning/deleting code points (autodecoding?).

Which behavior is better?