oop - Lazarus/Delphi-Create vs TSomeClass.Create内部构造函数-为什么这会引起麻烦？

Question

突出

此构造函数：

constructor TCoords.Create(const AX, AY: Integer);
begin
    TCoords.Create(Point(AX, AY));
end;

已确认Linux Lazarus 2和Windows Delphi XE6均出现故障。

这可能是错误吗？



我是Lazarus / Delphi中OOP的新手，请原谅新手错误。谢谢。

我似乎无法理解为什么下面的Lazarus /（类似于Delphi）自定义序列非常基本，一个对象将无法工作。我已经尝试调试了几个小时，从那时起我发现：

什么有效：

调用一个没有参数的构造函数，并直接使用TPoint参数调用一个构造函数。

什么不是：

称之为：

constructor Create(const AX, AY: Integer);

但是，我发现这是可行的-但前提是在构造函数中不带其类名的情况下调用它。为什么会引起麻烦？



声明书

// WORKS - this creates instance of TCoords initialized to PointOutOfReach
constructor Create; reintroduce;
// WORKS - this creates instance of TCoords initialized to user coordinates
constructor Create(const ACoords: TPoint);
// DOES NOT WORK, strangely returns Point(0, 0), if called with class name
// WORKS only if called without class name - confusing or error on my side?
constructor Create(const AX, AY: Integer);

来电

// OK - WORKING
NewCoords := TCoords.Create;
NewCoords.X:=12;
NewCoords.Y:=120;
ShowMessage(NewCoords.X.ToString + ' : ' + NewCoords.Y.ToString);
NewCoords.Free;

// OK - WORKING
NewCoords := TCoords.Create(Point(12, 120));
ShowMessage(NewCoords.X.ToString + ' : ' + NewCoords.Y.ToString);
NewCoords.Free;

// NOT WORKING as expected
NewCoords := TCoords.Create(12, 120);
ShowMessage(NewCoords.X.ToString + ' : ' + NewCoords.Y.ToString);
NewCoords.Free;

具有TCoords对象定义的Coords单元

unit Coords;

{$mode objfpc}{$H+}

interface

uses
    Classes;

type
    // Flexible X,Y coordinates object.
    TCoords = class(TObject)

    // these declarations are accessible within this unit only
    private
        // this is the variable we are working with
        FCoords: TPoint;
        // property for this function is unnecessary, but I like it as it is
        function IsInitialized: Boolean;

    // these declarations are accessible to all
    public
        // this creates instance of TCoords initialized to PointOutOfReach
        constructor Create; reintroduce;
        // this creates instance of TCoords initialized to user coordinates
        constructor Create(const ACoords: TPoint);

        // THIS ONE DOES NOT WORK, strangely returns Point(0, 0)
        constructor Create(const AX, AY: Integer);

        // this indicates if instance was initialized or not by the user
        property Initialized: Boolean read IsInitialized;
        // this works directly with private FCoords variable storing coordinates
        property P: TPoint read FCoords write FCoords;
        // these two are shortcuts for X,Y coordinates' direct access
        property X: Integer read FCoords.X write FCoords.X;
        property Y: Integer read FCoords.Y write FCoords.Y;

    end;

implementation

var
  // this gets initialized when loading this unit
  PointOutOfReach: TPoint;

constructor TCoords.Create;
begin
    // this is the same as `inherited`, but I like to be explicit
    inherited Create;
    // since called without argument, we have to ensure, there is some nonsense
    FCoords := PointOutOfReach;
end;

constructor TCoords.Create(const ACoords: TPoint);
begin
    // this is the same as `Create`, but I like to be explicit
    TCoords.Create;
    // in the previous mandatory call we have initialized FCoords already
    // but to PointOutOfReach; here we overwrite it with user coordinates
    FCoords := ACoords;
end;

constructor TCoords.Create(const AX, AY: Integer);
begin
    // this is the same as `Create(TPoint)`, but I like to be explicit
//    TCoords.Create(Point(AX, AY));

    // Why can't I call the above, shouldn't it be the very same?
    Create(Point(AX, AY));
end;

function TCoords.IsInitialized: Boolean;
begin
    // this returns True in case FCoords has been initialized
    // initialized means here for the FCoords point to be different from PointOutOfReach
    // achieved either by calling `Create(APoint)`, or later overwriting PointOutOfReach
    Result := FCoords <> PointOutOfReach;
end;

initialization

    // initialize PointOutOfReach to "impossible" coordinates when loading unit
    PointOutOfReach := Point(MAXINT, MAXINT);

end.

在此先感谢您，我自己似乎看不出两​​者之间的区别。



已纠正主要错误-不变

构造函数声明中缺少 overload;的问题已得到纠正。遗憾的是，仍然从最后一个构造函数获得0,0坐标。

Answer 1

David Heffernan在评论中指出了我的方法不起作用的原因，请允许我引用：


TCoords.Create(Point(AX, AY))创建一个新实例。在构造函数中执行此操作时，现在有两个实例。将其替换为Create(Point(AX, AY))。


谢谢您的解释！



如果甚至解决了，我认为更好的方法将是不链接那些constructor。



应用此规则可以正常工作，使用未链接的构造函数的代码段：

constructor TCoords.Create;
begin
    inherited Create;
    // since called without argument, we have to ensure,
    // there are some corner-case coordinates, so that we can
    // differentiate between a [0:0] and uninitialized state
    FCoords := PointOutOfReach;
end;

constructor TCoords.Create(const ACoords: TPoint);
begin
    inherited Create;
    FCoords := ACoords;
end;

constructor TCoords.Create(const AX, AY: Integer);
begin
    inherited Create;
    FCoords := Point(AX, AY);
end;

PS：为了使代码正常工作，到目前为止，我认为没有必要像 Nasreddine Galfout的 answer中那样应用“ setter”。