Meaning of "scope" in D (for a parameter)
What does scope
in
void foo(scope void* p) { }
mean?
(I'm not talking about scope(exit)
or scope int x = 5;
, but about scope
as used i开发者_StackOverflow社区nside a parameter list.)
There are 3 uses for scope
in D.
scope
statements. This is when you usescope(success)
,scope(failure)
, orscope(exit)
. The statements in the block that follows are run when exiting the scope that thescope
statement is in if no exception is thrown, if an exception is thrown, or regardless of whether an exception is thrown for success, failure, and exit respectively. This use ofscope
is staying in the language.scope
on a local variable. This puts the variable on the stack - even if it's a class. The object is destroyed when it leaves scope. This use ofscope
is unsafe and will eventually be removed from the language (though std.typecons.scoped replaces it for those who want to live life dangerously).scope
on a function parameter (which is the use case that you're asking about). When placed on a parameter that is a delegate, it means that references to that parameter cannot be escaped (i.e. assigned to a global variable). And when the compiler sees this on delegates, it will avoid allocating a closure when taking the address of a local function. This is essential inopApply
loops (reference post on newsgroup). Currently,scope
has no effect on any function parameters other than delegates and is ignored for all other types, though it may or may not at some point in the future be expanded to affect types like pointers to prevent them from escaping the function.
When used on a function parameter, the in
keyword is an alias for const scope
, which is frequently how scope
on function parameters gets inadvertently used.
Searching on the digital mars newsgroup, I found two semi-related post about scope in that context: here and here.
From reading those two post, function parameter scope doesn't seem to do anything useful and it's there for backwards compatibility. It even sounds like later versions after D2 might have that qualifier removed altogether.
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