To declare an empty slice, I know that you should prefer
var t []string
over
t := []string{}
as it doesn't allocate unecessary memory (https://github.com/golang/go/wiki/CodeReviewComments#declaring-empty-slices). Does this still apply if I have
type example struct {
s []string
}
e := &example{}
i.e. would it be better to use
e.s = []string{}
or
var s []string
e.s = s
example.s is already declared, so there's nothing you need to do.
e := &example{}
e.s = append(e.s, "val")
fmt.Println(e.s)
Related
I have a C DLL with a number of functions I'm calling from Delphi. One of the functions (say Func1) returns a pointer to a struct - this all works fine. The structs created by calling Func1 are stored in a global pool within the DLL. Using a second function (Func2) I get a pointer to a block of memory containing an array of pointers, and I can access the array elements using an offset.
I need to be able copy the address in the returned pointer for a struct (from Func1) to any of the memory locations in the array (from Func2). The idea is that I can build arrays of pointers to pre-defined structs and access the elements directly from Delphi using pointer offsets.
I tried using:
CopyMemory(Pointer(NativeUInt(DataPointer) + offset), PStruct, DataSize);
where DataPointer is the start of my array and PStruct is returned from Func1, but that doesn't copy the address I need.
In .NET it works using Marshal.WriteIntPtr and looking at the underlying code for this using Reflector I think I need something trickier than CopyMemory. Anyone got any ideas for doing this in Delphi?
Edit: This is part of a wrapper around vector structures returned from the R language DLL. I have a base vector class from which I derive specific vector types. I've got the wrapper for the numeric vector working, so my base class looks fine and this is where I get DataPointer:
function TRVector<T>.GetDataPointer: PSEXPREC;
var
offset: integer;
h: PSEXPREC;
begin
// TVECTOR_SEXPREC is the vector header, with the actual data behind it.
offset := SizeOf(TVECTOR_SEXPREC);
h := Handle;
result := PSEXPREC(NativeUInt(h) + offset);
end;
Setting a value in a numeric vector is easy (ignoring error handling):
procedure TNumericVector.SetValue(ix: integer; value: double);
var
PData: PDouble;
offset: integer;
begin
offset := GetOffset(ix); // -- Offset from DataPointer
PData := PDouble(NativeUInt(DataPointer) + offset);
PData^ := value;
end;
For a string vector I need to (i) create a base vector of pointers with a pre-specified length as for the numeric vector (ii) convert each string in my input array to an R internal character string (CHARSXP) using the R mkChar function (iii) assign the address of the character string struct to the appropriate element in the base vector. The string array gets passed into the constructor of my vector class (TCharacterVector) and I then call SetValue (see below) for each string in the array.
I should have thought of PPointer as suggested by Remy but neither that or the array approach seem to work either. Below is the code using the array approach from Remy and with some pointer vars for checking addresses. I'm just using old-fashioned pointer arithmetic and have shown addresses displayed for a run when debugging:
procedure TCharacterVector.SetValue(ix: integer; value: string);
var
PData: PSEXPREC;
offset: integer;
offset2: integer;
PTest: PSEXPREC;
PPtr: Pointer;
PPtr2: Pointer;
begin
offset := GetOffset(ix);
PPtr := PPointer(NativeUInt(DataPointer) + offset); // $89483D8
PData := mkChar(value); // $8850258
// -- Use the following code to check that mkChar is working.
offset2 := SizeOf(TVECTOR_SEXPREC);
PTest := PSEXPREC(NativeUInt(PData) + offset);
FTestString := FTestString + AnsiString(PAnsiChar(PTest));
//PPointerList(DataPointer)^[ix] := PData;
//PPtr2 := PPointer(NativeUInt(DataPointer) + offset); // Wrong!
PPointerArray(DataPointer)^[ix] := PData;
PPtr2 := PPointerArray(DataPointer)^[ix]; // $8850258 - correct
end;
I'd have thought the address in PData ($8850258) would now be in PPtr2 but I've been staring at this so long I'm sure I'm missing something obvious.
Edit2: The code for SetValue used in R.NET is as follows (ignoring test for null string):
private void SetValue(int index, string value)
{
int offset = GetOffset(index);
IntPtr stringPointer = mkChar(value);
Marshal.WriteIntPtr(DataPointer, offset, stringPointer);
}
From reflector, Marshal.WriteIntPtr uses the following C:
public static unsafe void WriteInt32(IntPtr ptr, int ofs, int val)
{
try
{
byte* numPtr = (byte*) (((void*) ptr) + ofs);
if ((((int) numPtr) & 3) == 0)
{
*((int*) numPtr) = val;
}
else
{
byte* numPtr2 = (byte*) &val;
numPtr[0] = numPtr2[0];
numPtr[1] = numPtr2[1];
numPtr[2] = numPtr2[2];
numPtr[3] = numPtr2[3];
}
}
catch (NullReferenceException)
{
throw new AccessViolationException();
}
}
You say you want to copy the struct pointer itself into the array, but the code you have shown is trying to copy the struct data that the pointer is pointing at. If you really want to copy just the pointer itself, don't use CopyMemory() at all. Just assign the pointer as-is:
const
MaxPointerList = 255; // whatever max array count that Func2() allocates
type
TPointerList = array[0..MaxPointerList-1] of Pointer;
PPointerList = ^TPointerList;
PPointerList(DataPointer)^[index] := PStruct;
Your use of NativeUInt reveals that you are using a version of Delphi that likely supports the {$POINTERMATH} directive, so you can take advantage of that instead, eg:
{$POINTERMATH ON}
PPointer(DataPointer)[index] := PStruct;
Or, use the pre-existing PPointerArray type in the System unit:
{$POINTERMATH ON}
PPointerArray(DataPointer)[index] := PStruct;
I have the following:
TDirection = (dirNorth, dirEast, dirSouth, dirWest);
TDirections = set of TDirection;
In a seperate class I have it declared as a property:
property Directions: TDirections read FDirections write FDirections;
What I want is to be able to treat them as if they were Booleans, so for example if dirNorth was True then it would be 1, if False it would be 0.
I am trying to imagine it as (1,0,0,0)
I think to check if a direction is True, I could use:
var
IsTrue: Boolean;
begin
IsTrue := (DirNorth in Directions);
Not sure if the above is correct or not, but then my other problem is how to change one of the directions to True or False?
I have now reached one of my confusion states :(
This is the last thing I tried to set the value but I am getting Illegal Expression (in Lazarus).
Directions(TDirection(DirNorth)) := True;
Directions is a set of elements of type TDirection.
To see if it contains dirNorth, do dirNorth in Directions. The result of using the in operator is a boolean; dirNorth in Directions is true iff the set Directions contains the element dirNorth.
To make sure dirNorth is included in Directions, do Directions := Directions + [dirNorth].
To make sure dirNorth is not included in Directions, do Directions := Directions - [dirNorth].
To set Directions to a particular value, simply assign: Directions := [dirNorth, dirSouth].
Formally, + computes the union of two sets; - computes the set difference of two sets. * computes the intersection of the two operands.
You also have the nice Include and Exclude functions: Include(Directions, dirNorth) does the same thing as Directions := Directions + [dirNorth]; Exclude(Directions, dirNorth) does the same thing as Directions := Directions - [dirNorth].
For example, if
type
TAnimal = (aDog, aCat, aRat, aRabbit);
TAnimalSet = set of TAnimal;
const
MyAnimals = [aDog, aRat, aRabbit];
YourAnimals = [aDog, aCat];
then
aDog in MyAnimals = true;
aCat in MyAnimals = false;
aRat in YourAnimals = false;
aCat in YourAnimals = true;
MyAnimals + YourAnimals = [aDog, aRat, aRabbit, aCat];
MyAnimals - YourAnimals = [aRat, aRabbit];
MyAnimals * YourAnimals = [aDog];
Implicit in my answer is the fact that the Delphi set type is modelled after the mathematical set. For more information about the Delphi set type, please refer to the official documentation.
You may add an item to a set by doing like this:
Include(Directions, dirNorth);
To remove it from the set:
Exclude(Diretions, dirNorth);
The help states that the result is the same as using the plus operator, but the code is more efficient.
Based on this helper, which doesn't work for properties, I created this one (requires XE6) - it can be used for variables and properties:
TGridOptionsHelper = record helper for TGridOptions
public
/// <summary>Sets a set element based on a Boolean value</summary>
/// <example>
/// with MyGrid do Options:= Options.SetOption(goEditing, False);
/// MyVariable.SetOption(goEditing, True);
/// </example>
function SetOption(GridOption: TGridOption; const Value: Boolean): TGridOptions;
end;
function TGridOptionsHelper.SetOption(
GridOption: TGridOption; const Value: Boolean): TGridOptions;
begin
if Value then Include(Self, GridOption) else Exclude(Self, GridOption);
Result:= Self;
end;
I am working on a COM Object library with function that returns a VARIANT with a SAFEARRAY of BSTRs. How can I display the values from this VARIANT instance and save it inside a TStringList? I tried searching the net with no clear answer.
I tried the following with no success:
Variant V;
String mystr;
VarClear(V);
TVarData(V).VType = varOleStr;
V = ComFunction->GetValues(); //<<<<----- V is empty
mystr = (wchar_t *)(TVarData(V).VString);
Memo1->Lines->Add(mystr);
VarClear(V);
You can use TWideStringDynArray and let Delphi do the conversion:
procedure LoadStringsFromVariant(const Values: TWideStringDynArray; Strings: TStrings);
var
I: Integer;
begin
Strings.BeginUpdate;
try
for I := Low(Values) to High(Values) do
Strings.Add(Values[I]);
finally
Strings.EndUpdate;
end;
end;
When you call this with your Variant safearray of BSTRs it will be converted to TWideStringDynArray automatically. An incompatible Variant will cause the runtime error EVariantInvalidArgError.
To check if a Variant holds a safe array of BSTR you can do this:
IsOK := VarIsArray(V) and (VarArrayDimCount(V) = 1) and (VarType(V) and varTypeMask = varOleStr);
uses ActiveX;
var
VSafeArray: PSafeArray;
LBound, UBound, I: LongInt;
W: WideString;
begin
VSafeArray := ComFunction.GetValues();
SafeArrayGetLBound(VSafeArray, 1, LBound);
SafeArrayGetUBound(VSafeArray, 1, UBound);
for I := LBound to UBound do
begin
SafeArrayGetElement(VSafeArray, I, W);
Memo1.Lines.Add(W);
end;
SafeArrayDestroy(VSafeArray); // cleanup PSafeArray
if you are creating ComFunction via late binding (CreateOleObject) you should use:
var
v: Variant;
v := ComFunction.GetValues;
for i := VarArrayLowBound(v, 1) to VarArrayHighBound(v, 1) do
begin
W := VarArrayGet(v, [i]);
Memo1.Lines.Add (W);
end;
How can I display the values from this VARIANT instance and save it inside a TStringList?
The COM VARIANT struct has parray and pparray data members that are pointers to a SAFEARRAY, eg:
VARIANT V;
LPSAFEARRAY sa = V_ISBYREF(&V) ? V_ARRAYREF(&V) : V_ARRAY(&V);
The VCL Variant class, on the other hand, has an LPSAFEARRAY conversion operator defined, so you can assign it directly (but only if the Variant.VType field that not have the varByRef flag present, that is), eg:
Variant V;
LPSAFEARRAY sa = V;
Either way, once you have the SAFEARRAY pointer, use the SafeArray API to access the BSTR values, eg:
bool __fastcall VariantToStrings(const Variant &V, TStrings *List)
{
// make sure the Variant is holding an array
if (!V_ISARRAY(&V)) return false;
// get the array pointer
LPSAFEARRAY sa = V_ISBYREF(&V) ? V_ARRAYREF(&V) : V_ARRAY(&V);
// make sure the array is holding BSTR values
VARTYPE vt;
if (FAILED(SafeArrayGetVartype(sa, &vt))) return false;
if (vt != VT_BSTR) return false;
// make sure the array has only 1 dimension
if (SafeArrayGetDim(sa) != 1) return false;
// get the bounds of the array's sole dimension
LONG lBound = -1, uBound = -1;
if (FAILED(SafeArrayGetLBound(sa, 0, &lBound))) return false;
if (FAILED(SafeArrayGetUBound(sa, 0, &uBound))) return false;
if ((lBound > -1) && (uBound > -1))
{
// access the raw data of the array
BSTR *values = NULL;
if (FAILED(SafeArrayAccessData(sa, (void**)&values))) return false;
try
{
List->BeginUpdate();
try
{
// loop through the array adding the elements to the list
for (LONG idx = lBound; l <= uBound; ++idx)
{
String s;
if (values[idx] != NULL)
s = String(values[idx], SysStringLen(values[idx]));
List->Add(s);
}
}
__finally
{
List->EndUpdate();
}
}
__finally
{
// unaccess the raw data of the array
SafeArrayUnaccessData(sa);
}
}
return true;
}
VarClear(V);
TVarData(V).VType = varOleStr;
You don't need those at all. The VCL Variant class initializes itself to a blank state, and there is no need to assign the VType since you are assigning a new value to the entire Variant immediately afterwards.
V = ComFunction->GetValues(); //<<<<----- V is empty
If V is empty, then GetValues() is returning an empty Variant to begin with.
mystr = (wchar_t *)(TVarData(V).VString);
TVarData::VString is an AnsiString& reference, not a wchar_t* pointer. To convert a VCL Variant (not a COM VARIANT) to a String, just assign it as-is and let the RTL works out the detail for you:
String mystr = V;
I want to create light object data-package to pass between client and server applications.
It is a so simple task, that I can control with only 1 byte, so
each bit in a byte will have a different meaning,
Using only the bit
0 = False
1 = True
Itens I need now:
1 - Loaded from database
2 - Persisted
3 - Changed
4 - Marked to Delete
5 -
6 -
7 - Null Value
8 - Read Only
1) How do I use bit operators in Delphi to check each bit value?
2) How do I set the bit Values?
Solution
After all help, Ill use the next Set
TStateType = (
stLoaded = 0, // loaded from persistance
stNative = 2, // value loaded and converted to native type
stPersisted = 3, // saved
stChanged = 4, // object or member changed
stToDelete = 5, // marked to delete
stReadOnly = 6, // read only object, will not allow changes
stNull = 7 // value is null
);
TState = Set of TStateType;
And for stream -> persistance, this will be the record to be used:
TDataPackage = record
Data: TBytes;
TypeInfo: TMetaInfo;
State: Byte;
Instance: TBuffer;
end;
Thank you guys, for all the answers and comments.
I'd really use a set for this. However, I see you really want a byte. Use sets everywhere then typecast to a byte in the end.
This solution will require much less typing, has support for standard delphi operators and really carries no performance penalty as Barry Kelly has pointed out.
procedure Test;
type
TSetValues = (
TSetValue1 = 0,
TSetValue2 = 1,
TSetValue4 = 2,
TSetValue8 = 3,
TSetValue16 = 4,
TSetValue32 = 5,
TSetValue64 = 6,
TSetValue128 = 7
);
TMySet = set of TSetValues;
var
myValue: byte;
mySet: TMySet;
begin
mySet := [TSetValue2, TSetValue16, TSetValue128];
myValue := byte(mySet);
ShowMessage(IntToStr(myValue)); // <-- shows 146
end;
I would use a set for this:
type
TMyDatum = (mdLoaded, mdPersisted, mdChanged, mdMarkedToDelete, ...);
TMyData = set of TMyDatum;
var
Foo: TMyData;
begin
Foo := [mdLoaded, mdChanged];
if (mdPersisted in Foo) then ...
These are implemented as integers, so you can pass them easily. And I find the code much, much more readable than bitwise operators.
This page describes Delphi operators, including bitwise operators.
It sounds like you need to use the and operator. For example:
const
LOADED_FROM_DATABASE = 1;
PERSISTED = 2;
CHANGED = 4;
// etc...
//...
if (bitFlags and LOADED_FROM_DATABASE) <> 0 then
begin
// handle LOADED FROM DATABASE
end;
if (bitFlags and PERSISTED) <> 0 then
begin
// handle PERSISTED
end;
// etc...
In order to set the flags, you can use OR:
bitFlags := LOADED_FROM_DATABASE or PERSISTED or CHANGED;
I have an integer field in a ClientDataSet and I need to compare to some values, something like this:
I can use const
const
mvValue1 = 1;
mvValue2 = 2;
if ClientDataSet_Field.AsInteger = mvValue1 then
or enums
TMyValues = (mvValue1 = 1, mvValue2 = 2);
if ClientDataSet_Field.AsInteger = Integer(mvValue1) then
or class const
TMyValue = class
const
Value1 = 1;
Value2 = 2;
end;
if ClientDataSet_Field.AsInteger = TMyValues.Value1 then
I like the class const approach but it seems that is not the delphi way, So I want to know what do you think
Declaration:
type
TMyValues = class
type TMyEnum = (myValue1, myValue2, myValue3, myValue4);
const MyStrVals: array [TMyEnum] of string =
('One', 'Two', 'Three', 'Four');
const MyIntVals: array [TMyEnum] of integer =
(1, 2, 3, 4);
end;
Usage:
if ClientDataSet_Field.AsInteger = TMyValues.MyIntVals[myValue1] then
A cast would generally be my last choice.
I wouldn't say that class consts are not the Delphi way. It's just they have been introduced to Delphi quite recently, and a lot of books and articles you'll find on the internet were written before their introduction, and thus you won't see them widely used. Many Delphi developers (I'd say the majority) will have started using Delphi before they were made available, and thus they're not the first thing that one thinks about.
One thing to consider is backwards compatibility - class constants are relatively new to Delphi so if your code has to be sharable with previous versions than they are out.
I typically use enumerated types, with the difference from yours is that my first enumeration is usually an 'undefined' item to represent NULL or 0 in an int field.
TmyValues = (myvUndefined, myvDescription1, myvDescription2)
if ClientDataSet_Field.AsInteger = Ord(myvDescription1) then...
To use a little bit of Jim McKeeth's answer - if you need to display to the user a text viewable version, or if you need to convert their selected text into the enumerated type, then an array comes in handy in conjuction with the type:
const MYVALS: array [TmyValues ] of string = ('', 'Description1', 'Description2');
You can then have utility functions to set/get the enumerated type to/from a string:
Function MyValString(const pMyVal:TmyValues):string;
begin
result := MYVALS[Ord(pMyVal)];
end;
Function StringToMyVal(const pMyVal:String):TMyValues;
var i:Integer;
begin
result := myvUndefined;
for i := Low(MYVALS) to High(MYVALS) do
begin
if SameText(pMyVal, MYVALS[i]) then
begin
result := TMyValues(i);
break;
end;
end;
end;
Continuing on... you can have scatter routine to set a combo/list box:
Procedure SetList(const DestList:TStrings);
begin
DestList.Clear;
for i := Low(MYVALS) to High(MYVALS) do
begin
DestList.Insert(MYVALS[i]);
end;
end;
In code: SetList(Combo1.Items) or SetList(ListBox1.Items)..
Then if you are seeing the pattern here... useful utility functions surrounding your enumeration, then you add everything to it's own class and put this class into it's own unit named MyValueEnumeration or whaterver. You end up with all the code surrounding this enumeration in one place and keep adding the utility functions as you need them. If you keep the unit clean - don't mix in other unrelated functionality then it will stay very handy for all projects related to that enumeration.
You'll see more patterns as time goes and you use the same functionality over and over again and you'll build a better mousetrap again.
When using constants I recommend assigning the type when the data type is a numeric float.
Delphi and other languages will not always evaluate values correctly if the types do not match...
TMyValue = class
const
// will not compare correctly to float values.
Value1 = 1; // true constant can be used to supply any data type value
Value2 = 2; // but should only be compared to similar data type
// will not compare correctly to a single or double.
Value3 = 3.3; // default is extended in debugger
// will not compare correctly to a single or extended.
Value1d : double = Value1; // 1.0
Value2d : double = Value2; // 2.0
end;
Compared float values in if () and while () statements should be compared to values of the same data type, so it is best to define a temporary or global variable of the float type used for any comparison statements (=<>).
When compared to the same float data type this format is more reliable for comparison operators in any programming language, not just in Delphi, but in any programming language where the defined float types vary from variable to constant.
Once you assign a type, Delphi will not allow you to use the variable to feed another constant, so true constants are good to feed any related data type, but not for comparison in loops and if statements, unless they are assigned and compared to integer values.
***Note: Casting a value from one float type to another may alter the stored value from what you entered for comparison purposes, so verify with a unit test that loops when doing this.
It is unfortunate that Delphi doesn't allow an enumeration format like...
TController : Integer = (NoController = 0, ncpod = 1, nextwave = 2);
or enforce the type name for access to the enumeration values.
or allow a class constant to be used as a parameter default in a call like...
function getControllerName( Controller : TController = TController.NoController) : string;
However, a more guarded approach that provides both types of access would be to place the enumeration inside a class.
TController = class
//const
//NoController : Integer = 1;
//ncpod : Integer = 2;
//nextwave : Integer = 3;
type
Option = (NoController = 0, ncpod = 1, nextwave = 2);
public
Class function Name( Controller : Option = NoController) : string; static;
end;
implementation
class function TController.Name( Controller : Option = NoController) : string;
begin
Result := 'CNC';
if (Controller = Option.nextwave) then
Result := Result + ' Piranha'
else if (Controller = Option.ncpod) then
Result := Result + ' Shark';
Result := Result + ' Control Panel';
end;
This approach will effectively isolate the values, provide the static approach and allow access to the values using a for () loop.
The access to the values from a floating function would be like this...
using TControllerUnit;
function getName( Controller : TController.Option = TController.Option.NoController) : string;
implementation
function getName( Controller : TController.Option = TController.Option.NoController) : string;
begin
Result := 'CNC';
if (Controller = TController.Option.nextwave) then
Result := Result + ' Piranha'
else if (Controller = TController.Option.ncpod) then
Result := Result + ' Shark';
Result := Result + ' Control Panel';
end;
so many options! :-) i prefer enums and routinely use them as you describe. one of the parts i like is that i can use them with a "for" loop. i do use class constants as well but prefer enums (even private enums) depending on what i'm trying to achieve.
TMyType=class
private const // d2007 & later i think
iMaxItems=1; // d2007 & later i think
private type // d2007 & later i think
TMyValues = (mvValue1 = 1, mvValue2 = 2); // d2007 & later i think
private
public
end;
An option you haven't thought of is to use a lookup table in the database and then you can check against the string in the database.
eg.
Select value, Description from tbl_values inner join tbl_lookup_values where tbl_values.Value = tbl_lookup_values.value
if ClientDataSet_Field.AsString = 'ValueIwant' then