C++ 到 Delphi：FOR 循环中的变量具有已分配的声明

Question

Delphi 中这个 C++ 代码中 for 循环 ChildWindowFromPoint() 的等价物是什么：

HWND  hWnd;
POINT point;

...

for (HWND currHwnd = hWnd;;)
{
    hWnd = currHwnd;
    ScreenToClient(currHwnd, &point);
    currHwnd = ChildWindowFromPoint(currHwnd, point);
    if (!currHwnd || currHwnd == hWnd)
        break;
}

我的尝试是这样的，但我不确定它是否正确：

var
  hWndWindow, currHwnd: HWND;
  MousePoint: TPoint;

...

while True do
begin
  currHwnd := hWndWindow;
  hWndWindow := currHwnd;
  ScreenToClient(currHwnd, MousePoint);
  currHwnd := ChildWindowFromPoint(currHwnd, MousePoint);

  if (currHwnd = 0) or (currHwnd = hWndWindow) then
    Break;
end;

Answer 1

您的翻译几乎是正确的，但您确实犯了一个错误。您需要将 currHwnd 的初始赋值移动到 while 循环之外：

var
  hWndWindow, currHwnd: HWND;
  MousePoint: TPoint;

...

currHwnd := hWndWindow; // <-- moved here!
while True do
begin
  hWndWindow := currHwnd;
  ScreenToClient(currHwnd, MousePoint);
  currHwnd := ChildWindowFromPoint(currHwnd, MousePoint);

  if (currHwnd = 0) or (currHwnd = hWndWindow) then
    Break;
end;

每 for loop cppreference.com：

formal syntax:

attr(optional) for ( init-statement condition(optional) ; iteration_expression(optional) ) statement

informal syntax:

attr(optional) for ( declaration-or-expression(optional) ; declaration-or-expression(optional) ; expression(optional) ) statement

attr(C++11) - any number of attributes

init-statement - either

an expression statement (which may be a null statement ";")

a simple declaration, typically a declaration of a loop counter variable with initializer, but it may declare arbitrary many variables

Note that any init-statement must end with a semicolon ;, which is why it is often described informally as an expression or a declaration followed by a semicolon.

condition - either

an expression which is contextually convertible to bool. This expression is evaluated before each iteration, and if it yields false, the loop is exited.

declaration of a single variable with a brace-or-equals initializer. the initializer is evaluated before each iteration, and if the value of the declared variable converts to false, the loop is exited.

iteration_expression - any expression, which is executed after every iteration of the loop and before re-evaluating condition. Typically, this is the expression that increments the loop counter

statement - any statement, typically a compound statement, which is the body of the loop

Explanation

The above syntax produces code equivalent to:
{
    init_statement 
    while ( condition ) { 
        statement 
        iteration_expression ; 
    }
}

也就是说，我会将 C++ 循环翻译成 Delphi repeat..until 循环（并编写 C++ 代码以使用 do..while 循环）：

HWND hWnd;
POINT point;

...

HWND currHwnd = hWnd;
do
{
    hWnd = currHwnd;
    ScreenToClient(currHwnd, &point);
    currHwnd = ChildWindowFromPoint(currHwnd, point);
}
while (currHwnd && currHwnd != hWnd);

var
  hWndWindow, currHwnd: HWND;
  MousePoint: TPoint;

...

currHwnd := hWndWindow;
repeat
  hWndWindow := currHwnd;
  ScreenToClient(currHwnd, MousePoint);
  currHwnd := ChildWindowFromPoint(currHwnd, MousePoint);
until (currHwnd = 0) or (currHwnd = hWndWindow);

C++ 到 Delphi：FOR 循环中的变量具有已分配的声明

C++ to Delphi: variable in FOR loop with assigned declaration

delphi

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