I have a snippet of code which compiles in C++ Builder XE8 using the classic BCC compiler. However, in Rad Studio 10 Seattle using the Clang compiler I get the error
'no matching constructor found for initialization of TChoiceItem'
Here is the snippet of code which causes the error.
LISTITEM_BEGIN( sch_TYPE_Choice )
LISTITEM_DATA( sch_TYPE_Daily, "Daily" )
LISTITEM_DATA( sch_TYPE_Weekly, "Weekly" )
LISTITEM_DATA( sch_TYPE_Monthly, "Monthly" )
LISTITEM_END()
Here is the code which defines TChoiceItem
//------------------------------------------------------------------------------
#define LISTITEM_BEGIN( Name ) TChoiceItem Name[] = {
//------------------------------------------------------------------------------
#define INT_LISTITEM_BEGIN( Name ) TIntChoiceItem Name[] = {
//------------------------------------------------------------------------------
#define LISTITEM_DATA( XCode, XText ) { XCode, 0, (char*)XText, 0 },
#define LISTITEM_DATA_NC( XShortText, XText ) { 0, (char*)XShortText, (char*)XText, 0 },
#define LISTITEM_DATA_EX( XCode, XShortText, XText ) { XCode, (char*)XShortText, (char*)XText, 0 },
#define LISTITEM_DATA_EX2( XCode, XShortText, XText, XDesc ) { XCode, (char*)XShortText, (char*)XText, (char*)XDesc },
#define LISTITEM_END() LISTITEM_DATA(0,0) };
I am fairly new to C++ so I am not exactly sure what to call the above method of defining a class/method.
Is this some sort of dated language feature not supported by the Clang compiler? Is there a way to modify the code or definition so the compiler will accept it?
Edit:
I found the actual declaration of the TChoiceItem class.
class TChoiceItem : public TChoiceBase
{
public:
char Code;
char *ShortText;
char *Text;
char *Desc;
};
It does't appear to have any sort of standard constructor at all. But somehow, everything still compiles and works with the classic BCC compiler.
Edit 2:
I found this question which looks to be describing a similar issue. Could it be that I need to include some kind of compiler flag when compiling the code? If so can I add a flag somehow in the embarcadero project compiler settings?
Using a list of values in braces to initialize the individual members of a class or struct is known as aggregate initialization.
As explained on cppreference.com, aggregate initialization isn't permitted if the class has a base class (among other restrictions). TChoiceItem inherits from TChoiceBase, so aggregate initialization isn't allowed (and the "classic" bcc32 compiler shouldn't have allowed it).
You have a couple of choices:
First, you can change the code to not inherit from TChoiceBase.
Second, you can define a constructor:
TChoiceItem(char code, char *short_text, char *text, char *desc)
: Code(code), ShortText(short_text), Text(text), Desc(desc) {}
C++11's uniform initialization means that your macros' syntax doesn't have to change: instead of braces meaning a list of values for individual members, the braces will mean a list of parameters to the constructor, but the result will be the same.
Related
I'm a bit confused about the implications of the using declaration. The keyword implies that a new type is merely declared. This would allow for incomplete types. However, in some cases it is also a definition, no? Compare the following code:
#include <variant>
#include <iostream>
struct box;
using val = std::variant<std::monostate, box, int, char>;
struct box
{
int a;
long b;
double c;
box(std::initializer_list<val>) {
}
};
int main()
{
std::cout << sizeof(val) << std::endl;
}
In this case I'm defining val to be some instantiation of variant. Is this undefined behaviour? If the using-declaration is in fact a declaration and not a definition, incomplete types such as box would be allowed to instantiate the variant type. However, if it is also a definition, it would be UB no?
For the record, both gcc and clang both create "32" as output.
Since you've not included language-lawyer, I'm attempting a non-lawyer answer.
Why should that be UB?
With a using delcaration, you're just providing a synonym for std::variant<whatever>. That doesn't require an instantiation of the object, nor of the class std::variant, pretty much like a function declaration with a parameter of that class doesn't require it:
void f(val); // just fine
The problem would occur as soon as you give to that function a definition (if val is still incomplete because box is still incomplete):
void f(val) {}
But it's enough just to change val to val& for allowing a definition,
void f(val&) {}
because the compiler doesn't need to know anything else of val than its name.
Furthermore, and here I'm really inventing, "incomplete type" means that some definition is lacking at the point it's needed, so I expect you should discover such an issue at compile/link time, and not by being hit by UB. As in, how can the compiler and linker even finish their job succesfully if a definition to do something wasn't found?
I'm working on a platform invoke call from F#, and I am getting a compiler error I really can't make that much sense out of. First, let me show the C signature of what I am doing:
int Foo(
ULONG_PTR *phHandle,
DWORD flags
);
In F#, I think the correct way to invoke this natively is as so:
[<DllImport("somedll.dll")>]
static extern int APlatformInvokeCall
(
[<Out>]nativeint& phHandle,
uint32 flags
)
If I try to call this in a class, I get a compilation error when calling it like so:
type Class1() =
[<DllImport("somedll.dll")>]
static extern int APlatformInvokeCall
(
nativeint& phHandle,
uint32 flags
)
member this.Foo() =
let mutable thing = nativeint 0
APlatformInvokeCall(&thing, 0u) |> ignore
thing
The error is:
A type instantiation involves a byref type. This is not permitted by the rules of Common IL.
Weirdly, when I do this all in a module, the compilation errors go away:
module Module1 =
[<DllImport("somedll.dll")>]
extern int APlatformInvokeCall
(
nativeint& phHandle,
uint32 flags
)
let Foo() =
let mutable thing = nativeint 0
APlatformInvokeCall(&thing, 0u) |> ignore
thing
Why does this compile as a module, but not as a class?
I don't think it's valid to define an extern method within a class in F#.
If you pull up the F# 3.0 language specification and search for DllImport, near the bottom is a table listing some special attributes and how they can be used. The text for [<DllImport>] says:
When applied to a function definition in a module, causes the F# compiler to ignore the implementation of the definition, and instead compile it as a CLI P/Invoke stub declaration.
That seems to indicate that it's only valid to declare extern methods (that use [<DllImport>]) on functions defined in a module; it doesn't say anything about class members though.
I think you're running into a compiler bug. Please submit this code to fsbugs#microsoft.com so they can fix the error message emitted by the compiler -- it should really be giving you an error about defining an extern method in a class since that's not allowed by the language spec.
Whether this is a bug not withstanding, maybe this is what's going on: If APlatformInvokeCall were considered a static member function, that member have a single argument of tuple type. Tuples are compiled into objects of generic type (see here, at the bottom, or 5.1.3 in the spec). In this case that tuple is
System.Tuple<nativeint&, uint32>
But ECMA 335 II.9.4 says you can't instantiate generic types at byref types. This explains the error reported.
This explanation fits the fact mentioned above that Class1 works (well, compiles) if you modify the extern declaration and call to take instead a single argument. It also fits the fact that the module version works, since in that version there is no considering APlatFormInvokeCall a member function.
The simple solution is to check the spec, here is the class definition grammar:
type type-name pat_opt as-defn)opt =
class
class-inherits-decl_opt
class-function-or-value-defns_opt
type-defn-elements
end
then we have
class-function-or-value-defn :
attributes_opt staticopt let rec_opt function-or-value-defns
attributes_opt staticopt do expr
which doesn't allow extern.
and
type-defn-element :
member-defn
interface-impl
interface-signature
which isn't what you want either.
As a result, we can see that using extern as you are trying to use it can't be done inside a class.
What is the C++ equivalent of this code
ImageEnView1.IEBitmap.VirtualBitmapProvider := TIESlippyMap.Create();
I get a compile error
[bcc32 Error] Unit1.cpp(12907): E2285 Could not find a match for 'TIESlippyMap::TIESlippyMap()'
on my code
ImageEnview1->IEBitmap->VirtualBitmapProvider = new TIESlippyMap();
ImageEnView1->IEBitmap->VirtualBitmapProvider = new TIESlippyMap();
Update: You are trying to call this constructor:
constructor Create(provider:TIESlippyMapProvider = iesmpMapQuest; const cachePath:string = '');
The compiler error you are getting means that the C++ compiler cannot find a constructor that has no parameters, or at least a constructor with parameters that all have default values assigned to them. Depending on which C++Builder version you are using, it is likely that the Delphi compiler included with it is not emitting the default parameter values when generating the C++ .hpp file for the class. Older Delphi compiler versions did not do that correctly, but newer versions do. In which case, it sounds like you are using an affected version, so you will have to fill in those parameter values explicitly:
ImageEnView1->IEBitmap->VirtualBitmapProvider = new TIESlippyMap(iesmpMapQuest, "");
Or else edit the .hpp file to include the default values correctly:
class TIESlippyMap : public ...
{
...
public:
__fastcall TIESlippyMap(TIESlippyMapProvider provider = iesmpMapQuest, const String cachePath = "");
...
};
I've got a program that is to become part of an already existing, larger product which is built using C++ Builder 2010.
The smaller program does not (yet) depend on C++ Builder. It works fine in MS Visual Studio, but with C++ Builder it produces strange access violations.
Please let me explain this.
Depending on the code and on compiler settings, access violations happen or do not happen. The access violations are reproducible: When the program is built then the access violation does never occur or it does always occur at the same place. If the program is rebuilt with the same settings, it will show the same behavior. (I'm really glad about that).
The access violation happens at places where the delete operator is called. This can happen (depending on compiler settings and exact code) inside certain destructors, including destructors of own classes and inside the destructor of std::string.
The following things make the access violation less likely:
Build with "Debug" settings (instead of "Release").
No compiler optimizations.
Compiler switch "Slow exception epilogues".
Static RTL instead of dynamic.
Derive exceptions from std::exception instead of Borland's Exception class.
Use less "complicated" expressions (e.g. use "string s = "..." + "..."; throw SomeException(s);" instead of "throw
SomeException(string("...") + "...");")
Use try... __finally with manual cleanup instead of automatic variables with destructors.
Use a small console application instead a VCL windows application.
The program makes use of several C++ features, including exceptions, STL, move constructors etc. and it of course uses the heap.
I already tried some tools, none of them reported problems:
Borland's CodeGuard.
Microsoft Application Verifyer.
pageheap/gflags.
As already mentioned, there is absolutely no problem when building with MS Visual Studio.
Use of precompiled headers and incremental linking (which both seem to me are prone to errors) are disabled.
Neither the C++ Builder compiler ("enable all warnings") nor the one of Visual Studio (/W4) produces a warning that might be related to this issue.
I do not have access to another version of C++ Builder.
As the program will become part of a larger product, it is not an option to switch to a different compiler, and it is not an option to tune the compiler settings until the access violation does no longer happen. (I fear if this really should a compiler bug, the bug might show up again.)
Putting this together, I'm guessing this might result from heap corruption that is related to some compiler bug. However, I was not able to find a bug on qc.embarcadero.com. I'm guessing further this is related to cleanup code that is executed upon stack rewinding when an exception has been thrown. But, well, maybe it's only a stupid code bug.
Currently, I do not have any idea how to proceed. Any help appreciated. Thank you in advance!
tl;dr I believe the bug is that code is generated to delete the std::string from both branches of the ternary operator during stack unwinding, however only one of them was actually created of course.
Here is a simpler MCVE, which shows the problem via outputs in XE5:
#include <vcl.h>
#include <tchar.h>
#include <stdio.h>
using namespace std;
struct S
{
S() { printf("Create: %p\n", this); }
S(S const &) { printf("Copy: %p\n", this); }
void operator=(S const &) { printf("Assign: %p\n", this); }
~S() { printf("Destroy: %p\n", this); }
char const *c_str() { return "xx"; }
};
S rX() { return S(); }
int foo() { return 2; }
#pragma argsused
int _tmain(int argc, _TCHAR* argv[])
{
try
{
throw Exception( (foo() ? rX() : rX()).c_str() );
}
catch (const Exception& e)
{
}
getchar();
return 0;
}
This version shows the problem via output strings on the console. Check the edit history for this post to see a version that uses std::string and causes the segfault instead.
My output is:
Create: 0018FF38
Destroy: 0018FF2C
Destroy: 0018FF38
In the original code, the segfault comes from the bogus Destroy ending up calling delete on the bogus value it obtains by trying to retrieve the internal data pointer for a std::string which was actually never created at that location.
My conjecture is that the code generation for stack unwinding is bugged and tries to delete the temporary string from both branches of the ternary operator. The presence of the temporary UnicodeString does have something to do with it; as the bug did not occur in any variations where I tried to avoid that temporary.
In the debugger you can see the call stack and it is during global stack unwinding that this happens.
Phew, that was so simple that it took me some time:
#include <vcl.h>
#include <tchar.h>
#include <string>
using namespace std;
struct B
{
B(const char* c) { }
string X() const { return "xx"; }
int Length() const { return 2; }
};
struct C
{
void ViolateAccess(const B& r)
{
try
{
throw Exception(string("aoei").c_str());
}
catch (const Exception&) { }
throw Exception(((string) "a" + (r.Length() < 10 ? r.X() : r.X() + "...") + "b").c_str());
}
};
#pragma argsused
int _tmain(int argc, _TCHAR* argv[])
{
try
{
C c;
c.ViolateAccess("11");
}
catch (const Exception& e) { }
return 0;
}
(Preemptive comment: No, this code does not make any sense.)
Create a new console application and make sure to use the VCL. It might depend on the project settings whether there will be an access violation or not; my debug builds always crashed, release builds didn't.
Crashes with C++ Builder 2010 and XE3 trial.
Thus, bug in compiler or in VCL or in STL or whatever.
I'm working on some ActionScript code that needs to juggle a bunch of similar-but-not-interchangeable types (eg, position-in-pixels, internal-position, row-and-column-position) and I'm trying to come up with a naming scheme to minimize the complexity.
Additionally, I don't yet know what the best format for the "internal position" is – using int, uint and Number all have advantages and disadvantages.
Normally I'd solve this with a typedef:
typedef float pixelPos;
typedef int internalPos;
typedef int rowColPos;
Is there any way of getting similar functionality in ActionScript?
If you're using Flex or another command-line compiler to build your project, you could add a pass from an external preprocessor to your build process.
Doesn't get the type-safety, but otherwise appears to do what you want.
I have found an article titled Typedefs in ActionScript 3, which suggests using:
const pixelPos:Class = int;
But that doesn't work – the compiler complains that "Type was not found or was not a compile-time constant: pixelPos" (note: this also happens when I use Object instead of int).
Here is an example of code which doesn't compile:
const pixelPos:Class = int;
function add3(p:pixelPos):void { // <-- type not found on this line
return p + 3;
}
Just make it static const and you can register your own class. Like this:
static const MyClass:Class = int;
And you can't make a variable with this type:
var ert:MyClass; //error
private function ert2():MyClass {}; //error
But you can make an instance:
var ert:* = new MyClass();