Output range with custom string type

[Cecil Ward via Digitalmars-d-learn]

On Monday, 28 August 2017 at 14:27:19 UTC, Jacob Carlborg wrote:
> I'm working on some code that sanitizes and converts values of 
> different types to strings. I thought it would be a good idea 
> to wrap the sanitized string in a struct to have some type 
> safety. Ideally it should not be possible to create this type 
> without going through the sanitizing functions.
>
> The problem I have is that I would like these functions to push 
> up the allocation decision to the caller. Internally these 
> functions use formattedWrite. I thought the natural design 
> would be that the sanitize functions take an output range and 
> pass that to formattedWrite.
>
> Here's a really simple example:
>
> import std.stdio : writeln;
>
> struct Range
> {
>     void put(char c)
>     {
>         writeln(c);
>     }
> }
>
> void sanitize(OutputRange)(string value, OutputRange range)
> {
>     import std.format : formattedWrite;
>     range.formattedWrite!"'%s'"(value);
> }
>
> void main()
> {
>     Range range;
>     sanitize("foo", range);
> }
>
> The problem now is that the data is passed one char at the time 
> to the range. Meaning that if the user implements a custom 
> output range, the user is in full control of the data. It will 
> now be very easy for the user to make a mistake or manipulate 
> the data on purpose. Making the whole idea of the sanitized 
> type pointless.
>
> Any suggestions how to fix this or a better idea?

Q is it an option to let the caller provide all the storage in an 
oversized fixed-length buffer? You could add a second helper 
function to compute and return a suitable safely pessimistic ott 
max value for the length reqd which could be called once 
beforehand to establish the reqd buffer size (or check it). This 
is the technique I am using right now. My sizing function is 
ridiculously fast as I am lucky in the particular use-case.