是否真的需要Delphi记录构造函数?
形势 我正在学习Nick Hodges的“Delphi中的更多编码”,他正在使用一个是否真的需要Delphi记录构造函数?,delphi,constructor,records,Delphi,Constructor,Records,形势 我正在学习Nick Hodges的“Delphi中的更多编码”,他正在使用一个TFraction记录来解释运算符重载。我自己写了这张唱片: type TFraction = record strict private aNumerator: integer; aDenominator: integer; function GCD(a, b: integer): integer; public constructor Create(aNumerat
TFraction
记录来解释运算符重载。我自己写了这张唱片:
type
TFraction = record
strict private
aNumerator: integer;
aDenominator: integer;
function GCD(a, b: integer): integer;
public
constructor Create(aNumerator: integer; aDenominator: integer);
procedure Reduce;
class operator Add(fraction1, fraction2: TFraction): TFraction;
class operator Subtract(fraction1, fraction2: TFraction): TFraction;
//... implicit, explicit, multiply...
property Numerator: integer read aNumerator;
property Denominator: integer read aDenominator;
end;
当然,我必须创建一个构造函数,因为在Q(有理数)中,我必须有一个不等于零的分母
constructor TFraction.Create(aNumerator, aDenominator: integer);
begin
if (aDenominator = 0) then
begin
raise Exception.Create('Denominator cannot be zero in rationals!');
end;
if ( (aNumerator < 0) or (aDenominator < 0) ) then
begin
Self.aNumerator := -aNumerator;
Self.aDenominator := -aDenominator;
end
else
begin
Self.aNumerator := aNumerator;
Self.aDenominator := aDenominator;
end;
end;
正如您在这里看到的,我正在创建一个从函数返回的tmp
当我读马可·坎图的书时,他使用了另一种方法:
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
begin
Result.aNumerator := (fraction1.Numerator*fraction2.Denominator+fraction1.Denominator*fraction2.Numerator);
Result.aDenominator := fraction1.Denominator*fraction2.Denominator;
end;
我做了一些测试,我发现两者都给出了正确的结果,但有些东西我不明白。在第一种方法中,我声明tmp,然后调用构造函数,以便返回t操作
。在第二种方法中,我没有创建任何内容,因为记录有一个自动构造函数。事实上,文件中说:
记录是使用默认的无参数自动构造的
构造函数,但类必须显式构造。因为
记录有一个默认的无参数构造函数,任何用户定义的
记录构造函数必须有一个或多个参数
这里我有一个用户定义的记录构造函数。因此:
tmp
上的构造函数调用是否不需要?如果我想调用Reduce
(这是一个过程),我需要创建一个变量。结果
是否只是返回一份tmp
的副本而不创建任何内容Result.aNumerator
和Result.aDenominator
是自动创建的构造函数的参数吗记录构造函数并不是什么神奇的东西。它和其他方法一样,只是一个实例方法。你写道:
tmp := TFraction.Create(...);
但你也可以这样写:
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
var
tmp: TFraction;
begin
//simple algorithm of the sum
tmp := TFraction.Create(fraction1.Numerator*fraction2.Denominator+fraction1.Denominator*fraction2.Numerator, fraction1.Denominator*fraction2.Denominator);
tmp.Reduce;
//return the result
Result := tmp;
end;
tmp.Create(...);
class function New(aNumerator, aDenominator: Integer): TFraction; static;
class function TFraction.New(aNumerator, aDenominator: Integer): TFraction;
begin
Result.aNumerator := ...;
Result.aDenominator := ...;
end;
frac := TFraction.New(num, denom);
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
begin
Result.Create(
fraction1.Numerator*fraction2.Denominator + fraction1.Denominator*fraction2.Numerator,
fraction1.Denominator*fraction2.Denominator
);
Result.Reduce;
end;
我个人认为这两种方法都不是特别有用,因为我习惯于为分配和默认初始化内存的类调用构造函数语义,然后调用构造函数方法
尤其是第二个变体让我很恼火,因为这看起来像是新手Delphi程序员在开始创建类实例时犯的典型错误。如果t牵引
是一个类,那么该代码将是不好的,但对于记录来说,它是好的
如果是我,我将放弃记录构造函数,而是使用一个静态类函数返回一个新创建的记录类型实例。我的习惯是将这些东西命名为新的。但这些都是个人偏好的问题
如果您这样做,它将被声明如下:
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
var
tmp: TFraction;
begin
//simple algorithm of the sum
tmp := TFraction.Create(fraction1.Numerator*fraction2.Denominator+fraction1.Denominator*fraction2.Numerator, fraction1.Denominator*fraction2.Denominator);
tmp.Reduce;
//return the result
Result := tmp;
end;
tmp.Create(...);
class function New(aNumerator, aDenominator: Integer): TFraction; static;
class function TFraction.New(aNumerator, aDenominator: Integer): TFraction;
begin
Result.aNumerator := ...;
Result.aDenominator := ...;
end;
frac := TFraction.New(num, denom);
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
begin
Result.Create(
fraction1.Numerator*fraction2.Denominator + fraction1.Denominator*fraction2.Numerator,
fraction1.Denominator*fraction2.Denominator
);
Result.Reduce;
end;
其实施方式如下:
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
var
tmp: TFraction;
begin
//simple algorithm of the sum
tmp := TFraction.Create(fraction1.Numerator*fraction2.Denominator+fraction1.Denominator*fraction2.Numerator, fraction1.Denominator*fraction2.Denominator);
tmp.Reduce;
//return the result
Result := tmp;
end;
tmp.Create(...);
class function New(aNumerator, aDenominator: Integer): TFraction; static;
class function TFraction.New(aNumerator, aDenominator: Integer): TFraction;
begin
Result.aNumerator := ...;
Result.aDenominator := ...;
end;
frac := TFraction.New(num, denom);
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
begin
Result.Create(
fraction1.Numerator*fraction2.Denominator + fraction1.Denominator*fraction2.Numerator,
fraction1.Denominator*fraction2.Denominator
);
Result.Reduce;
end;
然后你会这样称呼它:
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
var
tmp: TFraction;
begin
//simple algorithm of the sum
tmp := TFraction.Create(fraction1.Numerator*fraction2.Denominator+fraction1.Denominator*fraction2.Numerator, fraction1.Denominator*fraction2.Denominator);
tmp.Reduce;
//return the result
Result := tmp;
end;
tmp.Create(...);
class function New(aNumerator, aDenominator: Integer): TFraction; static;
class function TFraction.New(aNumerator, aDenominator: Integer): TFraction;
begin
Result.aNumerator := ...;
Result.aDenominator := ...;
end;
frac := TFraction.New(num, denom);
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
begin
Result.Create(
fraction1.Numerator*fraction2.Denominator + fraction1.Denominator*fraction2.Numerator,
fraction1.Denominator*fraction2.Denominator
);
Result.Reduce;
end;
但正如我所说,这是一个偏好的问题。如果您喜欢记录构造器,请随意使用它们
您询问是否可以跳过构造函数。就记录的分配而言,是的,您可以跳过它。就在构造函数中运行代码而言,只有您可以确定这一点。您希望该代码执行还是不执行 如果希望执行该代码,但不希望使用临时变量,则可以这样编写代码:
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
var
tmp: TFraction;
begin
//simple algorithm of the sum
tmp := TFraction.Create(fraction1.Numerator*fraction2.Denominator+fraction1.Denominator*fraction2.Numerator, fraction1.Denominator*fraction2.Denominator);
tmp.Reduce;
//return the result
Result := tmp;
end;
tmp.Create(...);
class function New(aNumerator, aDenominator: Integer): TFraction; static;
class function TFraction.New(aNumerator, aDenominator: Integer): TFraction;
begin
Result.aNumerator := ...;
Result.aDenominator := ...;
end;
frac := TFraction.New(num, denom);
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
begin
Result.Create(
fraction1.Numerator*fraction2.Denominator + fraction1.Denominator*fraction2.Numerator,
fraction1.Denominator*fraction2.Denominator
);
Result.Reduce;
end;
或者,如果您更喜欢静态类函数,它将是:
class operator TFraction.Add(fraction1, fraction2: TFraction): TFraction;
begin
Result := TFraction.New(
fraction1.Numerator*fraction2.Denominator + fraction1.Denominator*fraction2.Numerator,
fraction1.Denominator*fraction2.Denominator
);
Result.Reduce;
end;
我更喜欢构造函数方式,因为它对我来说更干净。Nick在做你建议我做的关于类函数的事情,所以我不知道结果。是否将其视为正态变量?我看你可以叫减价了!IMO建造商的记录具有误导性(一般而言)。没有办法确保它会被使用。记录是一个“值”,而不是一个对象。我在那一章中帮了尼克一点忙。似乎您知道结果是什么,因为在代码中使用它。当您分配给它时。它只是一个包含返回值的隐式var param。@delmo否。事实上,我回答中的代码演示了一个访问严格私有字段的类方法。在一个不相关的注释中,通常使用
F
作为类中私有字段的前缀,使用a
作为方法参数的前缀(您确实使用的)。在Delphi中,这很好,但显然,在Lazarus中,这不会编译。好的,谢谢你,我不知道,我正在使用a作为参数和变量。我会改变我的习惯!@Jerry你的意思是什么。FPC不执行命名约定规则。@David@Jerry我不确定我是否相信。我怀疑Remy的评论有正确的信息。