An lvalue is something that can be assigned to / changed. The terminology comes from assignment statements: in “foo = bar”, “foo” is a left-value (lvalue) and “bar" is a right-value (rvalue). The difference is that the lvalue is a named location that can take on a new value and an rvalue is just a plain value.
See: https://en.wikipedia.org/wiki/Value_(computer_science)#lrvalue <https://en.wikipedia.org/wiki/Value_(computer_science)#lrvalue> In Swift’s AST, you’ll see @lvalue as the type coming from a declaration reference (declref) for var’s and for inout parameters inside a method implementation. You’ll see function application require @lvalue types for inout parameters (most of which will be the implicit first parameters of ‘mutating’ functions - the thing to be mutated). A load converts an expression from an lvalue to being an rvalue, and a store takes an lvalue and a new rvalue to be put there. Hope this helps! - Greg > On May 24, 2016, at 9:48 AM, Tim Bodeit via swift-dev <swift-dev@swift.org> > wrote: > > I couldn’t find any documentation about lvalues in Swift. Can anybody give me > a short explanation of what exactly an lvalue is? > > In C++ all variables, including those marked with the const modifier seem to > be lvalues. It seems to me, that this is not the case for let constants in > Swift. > > I compared the output of swiftc -dump-ast for > let i = 42 > let j = i+1 > and > var i = 42 > let j = i+1 > > Where i was respectively accessed as: > > (declref_expr type='Int' location=lvaluetest.swift:2:9 > range=[lvaluetest.swift:2:9 - line:2:9] > decl=lvaluetest.(file).i@lvaluetest.swift <mailto:i@lvaluetest.swift>:1:5 > direct_to_storage specialized=no) > > (load_expr implicit type='Int' location=lvaluetest.swift:2:9 > range=[lvaluetest.swift:2:9 - line:2:9] > (declref_expr type='@lvalue Int' accessKind=read > location=lvaluetest.swift:2:9 range=[lvaluetest.swift:2:9 - line:2:9] > decl=lvaluetest.(file).i@lvaluetest.swift <mailto:i@lvaluetest.swift>:1:5 > direct_to_storage specialized=no)) > > Is my assumption correct, that let constants are not lvalues? > > Cheers, > > Tim > _______________________________________________ > swift-dev mailing list > swift-dev@swift.org > https://lists.swift.org/mailman/listinfo/swift-dev
_______________________________________________ swift-dev mailing list swift-dev@swift.org https://lists.swift.org/mailman/listinfo/swift-dev
