我正在尝试将返回的基础对象强制转换为它的特定泛型类型.我认为下面的代码应该工作,但会产生内部编译器错误,还有另一种方法吗？
type
TPersistGeneric<T> = class
private
type
TPointer = ^T;
public
class function Init : T;
end;
class function TPersistGeneric<T>.Init : T;
var
o : TXPersistent; // root class
begin
case PTypeInfo(TypeInfo(T))^.Kind of
tkClass : begin
// xpcreate returns txpersistent,a root class of T
o := XPCreate(GetTypeName(TypeInfo(T))); // has a listed of registered classes
result := TPointer(pointer(@o))^;
end;
else
result := Default(T);
end;
end;
解决方法
我正在使用一个类型转换辅助类来执行类型转换,并检查这两个类是否兼容.
class function TPersistGeneric<T>.Init: T;
var
o : TXPersistent; // root class
begin
case PTypeInfo(TypeInfo(T))^.Kind of
tkClass : begin
// xpcreate returns txpersistent,a root class of T
o := XPCreate(GetTypeName(TypeInfo(T))); // has a listed of registered classes
Result := TTypeCast.DynamicCast<TXPersistent,T>(o);
end;
else
result := Default(T);
end;
这是班级：
type
TTypeCast = class
public
// ReinterpretCast does a hard type cast
class function ReinterpretCast<ReturnT>(const Value): ReturnT;
// StaticCast does a hard type cast but requires an input type
class function StaticCast<T,ReturnT>(const Value: T): ReturnT;
// DynamicCast is like the as-operator. It checks if the object can be typecasted
class function DynamicCast<T,ReturnT>(const Value: T): ReturnT;
end;
class function TTypeCast.ReinterpretCast<ReturnT>(const Value): ReturnT;
begin
Result := ReturnT(Value);
end;
class function TTypeCast.StaticCast<T,ReturnT>(const Value: T): ReturnT;
begin
Result := ReinterpretCast<ReturnT>(Value);
end;
class function TTypeCast.DynamicCast<T,ReturnT>(const Value: T): ReturnT;
var
TypeT,TypeReturnT: PTypeInfo;
Obj: TObject;
LClass: TClass;
ClassNameReturnT,ClassNameT: string;
FoundReturnT,FoundT: Boolean;
begin
TypeT := TypeInfo(T);
TypeReturnT := TypeInfo(ReturnT);
if (TypeT = nil) or (TypeReturnT = nil) then
raise Exception.Create('Missing Typeinformation');
if TypeT.Kind <> tkClass then
raise Exception.Create('Source type is not a class');
if TypeReturnT.Kind <> tkClass then
raise Exception.Create('Destination type is not a class');
Obj := TObject(Pointer(@Value)^);
if Obj = nil then
Result := Default(ReturnT)
else
begin
ClassNameReturnT := UTF8ToString(TypeReturnT.Name);
ClassNameT := UTF8ToString(TypeT.Name);
LClass := Obj.ClassType;
FoundReturnT := False;
FoundT := False;
while (LClass <> nil) and not (FoundT and FoundReturnT) do
begin
if not FoundReturnT and (LClass.ClassName = ClassNameReturnT) then
FoundReturnT := True;
if not FoundT and (LClass.ClassName = ClassNameT) then
FoundT := True;
LClass := LClass.ClassParent;
end;
//if LClass <> nil then << TObject doesn't work with this line
if FoundT and FoundReturnT then
Result := ReinterpretCast<ReturnT>(Obj)
else
if not FoundReturnT then
raise Exception.CreateFmt('Cannot cast class %s to %s',[Obj.ClassName,ClassNameReturnT])
else
raise Exception.CreateFmt('Object (%s) is not of class %s',ClassNameT]);
end;
end;