Editing D (programming language) (section)

====Function parameter lifetime annotations within @safe code====
When applied to function parameter which are either of pointer type or references, the keywords ''return'' and ''scope'' constrain the lifetime and use of that parameter.

The language standard dictates the following behaviour:<ref>{{Cite web|url=https://dlang.org/spec/function.html#param-storage|title=D Language Specification: Functions - Function Parameter Storage Classes}}</ref>
{| class="wikitable"
|+
!Storage Class
!Behaviour (and constraints to) of a parameter with the storage class
|-
|''scope''
|References in the parameter cannot be escaped. Ignored for parameters with no references
|-
|''return''
|Parameter may be returned or copied to the first parameter, but otherwise does not escape from the function. Such copies are required not to outlive the argument(s) they were derived from. Ignored for parameters with no references
|}
An annotated example is given below.<syntaxhighlight lang="D">
@safe:

int* gp;
void thorin(scope int*);
void gloin(int*);
int* balin(return scope int* p, scope int* q, int* r) {
     gp = p; // Error, p escapes to global variable gp.
     gp = q; // Error, q escapes to global variable gp.
     gp = r; // OK.

     thorin(p); // OK, p does not escape thorin().
     thorin(q); // OK.
     thorin(r); // OK.

     gloin(p); // Error, p escapes gloin().
     gloin(q); // Error, q escapes gloin().
     gloin(r); // OK that r escapes gloin().

     return p; // OK.
     return q; // Error, cannot return 'scope' q.
     return r; // OK.
}
</syntaxhighlight>