xxxInPlace or xxxCopy?

[bearophile]

Andrej Mitrovic:

> I think what might help out in D is if we had a way to mark some
> functions so the compiler guarantees that their return values *are
> not* to be discarded. For example, this code will compile:
> 
> import std.stdio;
> import std.string;
> void main()
> {
>     string s = "Mary has a lil lamb.";
>     replace(s, "lil", "li'l");  // returns a copy, but discards it
> }
> 
> If the replace function is marked with some kind of @nodiscard
> annotation, then his would be a compile error since it doesn't make
> sense to construct a new string, return it, and discard it.
> 
> But maybe that's going overboard. How often do these kinds of bugs creep in?

Such bugs are common enough. GNU C has the warn_unused_result attribute (that is like your @nodiscard if you use -Werror to turn warnings into errors):
http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html

Some C lints require a void cast where you don't want to use a function result:
cast(void)replace(s, "lil", "li'l");

In a language the default is different and where you don't want to use a function result you have to add an annotation:
unused replace(s, "lil", "li'l");

Something like @nodiscard is more useful in C than D because in C there are no true built-in exceptions, so error return values are common, and ignoring them is a mistake. In some cases like replace() or the C realloc() ignoring a result is always a programmer error. So something like @nodiscard is useful in D too.

Bye,
bearophile