Question

我正在尝试创建仅FnOnce的函数。

以下代码段不起作用：

// Attempt 1
//  Error doesn't have a size known at compile-time
let f: FnOnce() -> () = || println!("Hello");

// Attempt 2
fn g<T: FnOnce() -> ()>(c: T) -> (FnOnce() -> ()) {
    c
}
// Error: doesn't have a size known at compile-time
let f = g(|| println!("Hello"));

// Attempt 3
// Error: cast to unsized type
let f = (|| println!("Hello")) as (FnOnce() -> ());

Answer 1

'VBA Open excel to copy TC to master list Dir Sub Copy_Paste__To_New_Sheet() 'Variable Declaration Dim sFilePath As String Dim sFileName As String Dim wb As Excel.Workbook Dim rngCopy As Range, acell As Range, bcell As Range Dim strSearch As String Dim strFile As Variant Dim wb2 As Excel.Workbook 'Specify File Path sFilePath = "C:\temp\new" 'Check for back slash If Right(sFilePath, 1) <> "\" Then sFilePath = sFilePath & "\" End If sFileName = Dir(sFilePath) Do While Len(sFileName) > 0 Set rngCopy = Nothing Application.DisplayAlerts = False Set wb = Workbooks.Open(Filename:=sFilePath & sFileName) Sheets("TestCases").Activate ' Range("E:E").Insert 'Display file name in immediate window ' Debug.Print sFileName strSearch = "TC1" Set WS = Worksheets("TestCases") With WS Set acell = .Columns(1).Find(What:=strSearch, LookIn:=xlValues, _ LookAt:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, _ MatchCase:=False, SearchFormat:=False) If Not acell Is Nothing Then Set bcell = acell If rngCopy Is Nothing Then Set rngCopy = WS.Range(WS.Cells(acell.Row + 1, 1), WS.Cells(acell.Row + 2, 4)) Else Set rngCopy = Application.Union(rngCopy, .Rows((acell.Row + 1) & ":" & (acell.Row + 2))) End If Do Set acell = .Columns(1).FindNext(After:=acell) If Not acell Is Nothing Then If acell.Address = bcell.Address Then Exit Do If rngCopy Is Nothing Then Set rngCopy = WS.Range(WS.Cells(acell.Row + 1, 1), WS.Cells(acell.Row + 2, 4)) Else Set rngCopy = Application.Union(rngCopy, .Rows((acell.Row + 1) & ":" & (acell.Row + 2))) End If Else Exit Do End If Loop Else MsgBox SearchString & " not Found" End If '~~> I am pasting to Output sheet. Change as applicable Set wb2 = Workbooks.Open("C:\temp\output\outputtest.xlsx") If Not rngCopy Is Nothing Then Sheets("Output").Cells(Rows.Count, 1).End(xlUp).Offset(1, 0).Resize(rngCopy.Rows.Count, 4).Value = rngCopy.Value ' If Not rngCopy Is Nothing Then Sheets("Output").Cells(Rows.Count, 1).End(xlUp).Offset(1, 0).Resize(1, 4).Value = rngCopy.Value ' If Not rngCopy Is Nothing Then rngCopy.Copy Sheets("Output").Cells(1, 1).Resize(rngCopy.Rows.Count, rngCopy.Columns.Count) ' .End (xlDown) + 1 ' Sheets("Output").Rows(1) Application.DisplayAlerts = False wb2.Close savechanges = False End With是闭包的特征，闭包只能被调用一次，通常是因为捕获的值被移入闭包，并且在调用期间被消耗。例如，我们可以捕获一个值并将其作为返回值移出：

FnOnce

请注意，fn delay<A: 'static>(a: A) -> Box<dyn FnOnce() -> A> { Box::new(move || a) } fn main() { let a = "hello".to_string(); let f = delay(a); println!("{}", f()); }关键字不是严格必需的。编译器发现move需要移到闭包中才能作为值返回并移出。

如何创建`FnOnce`函数

1 个答案: