delphi - 为什么const记录参数按值传递？

Question

如果我有记录，例如

type
  TMyRec = record
    x : string;
  end;

还有一个过程测试(const x : TMyRec);。为什么该参数按值传递(即传入副本)而不是按引用传递。从效率的角度来看，我认为它应该通过引用传递(比如整数)。

Answer 1

看起来这和记录的大小有关，对于

procedure test(const x : TMyRec);

类型

type
  TMyRec = record
    x : Array[1..4] of byte;
  end;

将按值传递，并且

type
  TMyRec = record
    x : Array[1..5] of byte
  end;

将在 32 位上通过引用传递，对于 64 位我们需要一个 9 字节的记录在东京(10.2.3)或更早版本上通过引用传递，里约热内卢(10.3)在 32 位和 64 位上的行为相同少量。感谢所有对我的问题发表评论并提供额外引用/建议的人。

参见东京文档 here .特别是

Value and constant (const) parameters are passed by value or by reference, depending on the type and size of the parameter:

...

Sets, records, and static arrays of 1, 2, or 4 bytes are passed as 8-bit, 16-bit, and 32bit values. Larger sets, records, and static arrays are passed as 32-bit pointers to the value. An exception to this rule is that records are always passed directly on the stack under the cdecl, stdcall, and safecall conventions; the size of a record passed this way is rounded upward to the nearest double-word boundary.

如果你想强制传递引用，你可以将你的参数声明为const [ref]，像这样:

procedure test(const [ref] x : TMyRec);