I've been told that value structs can have default constructors (I'm using
VS C++.NET 2005 PRO using clr:/pure syntax). But the following code
generates the following error:
value struct ValueStruct
{
double x ;
double y ;
ValueStruct() { x = y = double(0) ; } // error *
} ;
* error C3417: ValueStruct::Va lueStruct(void) ' : value types cannot contain
user-defined special member functions
Was I misinformed, or am I doing something wrong in the above code?
[==P==] 9 5793
If you look in "CSharpFinalWor kingDraftApril2 005.pdf", you read :
Because every value type implicitly has a public parameterless instance
constructor, it is not possible for a
struct type to contain an explicit declaration of a parameterless
constructor. A struct type is however
permitted to declare parameterized instance constructors (§18.3.8).
jmd
"Peter Oliphant" <po*******@Roun dTripInc.com> wrote in message
news:Ol******** ******@TK2MSFTN GP09.phx.gbl... I've been told that value structs can have default constructors (I'm using VS C++.NET 2005 PRO using clr:/pure syntax). But the following code generates the following error:
value struct ValueStruct { double x ; double y ; ValueStruct() { x = y = double(0) ; } // error * } ;
* error C3417: ValueStruct::Va lueStruct(void) ' : value types cannot contain user-defined special member functions
Was I misinformed, or am I doing something wrong in the above code?
[==P==]
If you also look in "C++-CLI Standard.pdf", you read :
12.2.1 Value classes
A value class is a data structure that contains fields, function members, and nested types. Unlike other class
types, value classes do not support user-defined destructors, finalizers, default constructors, copy
constructors, or copy assignment operators. Value classes are designed to allow the CLI execution engine to
efficiently copy value class objects.
Hope that helps.
jmd
"Peter Oliphant" <po*******@Roun dTripInc.com> wrote in message news:Ol******** ******@TK2MSFTN GP09.phx.gbl... I've been told that value structs can have default constructors (I'm using VS C++.NET 2005 PRO using clr:/pure syntax). But the following code generates the following error: value struct ValueStruct { double x ; double y ; ValueStruct() { x = y = double(0) ; } // error * } ; * error C3417: ValueStruct::Va lueStruct(void) ' : value types cannot contain user-defined special member functions Was I misinformed, or am I doing something wrong in the above code? [==P==]
> Because every value type implicitly has a public parameterless instance constructor, it is not possible for a struct type to contain an explicit declaration of a parameterless constructor. A struct type is however permitted to declare parameterized instance constructors (§18.3.8).
How does the implicit parameterless (i.e., default) constructor determine
the values it gives it members?
Anyway, thanks! This clarifies it for me. I got the exact opposite advice,
or more likely, I misinterpreted the advice (previously) given to me.
So, as I NOW understand it, I can create any number of constructors for a
value struct EXCEPT the default one. That is, I can create constructors
which HAVE parameters. I just tested this, and it does work... : )
[==P==]
"jmd.msdn" <jm******@newsg roup.nospam> wrote in message
news:uZ******** ******@TK2MSFTN GP11.phx.gbl... If you look in "CSharpFinalWor kingDraftApril2 005.pdf", you read :
Because every value type implicitly has a public parameterless instance constructor, it is not possible for a struct type to contain an explicit declaration of a parameterless constructor. A struct type is however permitted to declare parameterized instance constructors (§18.3.8).
jmd
"Peter Oliphant" <po*******@Roun dTripInc.com> wrote in message news:Ol******** ******@TK2MSFTN GP09.phx.gbl... I've been told that value structs can have default constructors (I'm using VS C++.NET 2005 PRO using clr:/pure syntax). But the following code generates the following error:
value struct ValueStruct { double x ; double y ; ValueStruct() { x = y = double(0) ; } // error * } ;
* error C3417: ValueStruct::Va lueStruct(void) ' : value types cannot contain user-defined special member functions
Was I misinformed, or am I doing something wrong in the above code?
[==P==]
"Peter Oliphant" <po*******@Roun dTripInc.com> wrote So, as I NOW understand it, I can create any number of constructors for a value struct EXCEPT the default one. That is, I can create constructors which HAVE parameters. I just tested this, and it does work... : )
Value types can't have special member functions. You therefore
also cannot provide a copy constructor for a value type.
-hg
I guess I then need the definition of 'special member functions'.
Based on what you've said, a copy constructor is a special member function.
But a constructor which, I guess, DOESN'T have as its only parameter a
pointer or reference to the same class type (i.e., isn't a copy constructor)
IS allowed. Why the destinction? Or, like I said, and more importantly what
defines a 'special member function' in contrast to one that isn't 'special'?
[==P==]
"Holger Grund" <ho**********@r emove.ix-n.net> wrote in message
news:OC******** ******@TK2MSFTN GP11.phx.gbl... "Peter Oliphant" <po*******@Roun dTripInc.com> wrote
So, as I NOW understand it, I can create any number of constructors for a value struct EXCEPT the default one. That is, I can create constructors which HAVE parameters. I just tested this, and it does work... : )
Value types can't have special member functions. You therefore also cannot provide a copy constructor for a value type.
-hg
Look at http://www.plumhall.com/ecma/index.html
and download the 1.15 C++/CLI draft.
"Peter Oliphant" <po*******@Roun dTripInc.com> wrote in message
news:uF******** ******@TK2MSFTN GP12.phx.gbl... I guess I then need the definition of 'special member functions'.
Based on what you've said, a copy constructor is a special member function. But a constructor which, I guess, DOESN'T have as its only parameter a pointer or reference to the same class type (i.e., isn't a copy constructor) IS allowed. Why the destinction? Or, like I said, and more importantly what defines a 'special member function' in contrast to one that isn't 'special'?
[==P==]
"Holger Grund" <ho**********@r emove.ix-n.net> wrote in message news:OC******** ******@TK2MSFTN GP11.phx.gbl... "Peter Oliphant" <po*******@Roun dTripInc.com> wrote
So, as I NOW understand it, I can create any number of constructors for a value struct EXCEPT the default one. That is, I can create constructors which HAVE parameters. I just tested this, and it does work... : )
Value types can't have special member functions. You therefore also cannot provide a copy constructor for a value type.
-hg
Peter Oliphant wrote: How does the implicit parameterless (i.e., default) constructor determine the values it gives it members?
By memset'ing (filling) the entire class with all 0 bytes, regardless of
the actual type of the member variables. Think of value types as PODs
(plain old data, like a C struct). When a value type is copied, the CLR
is simply doing a memcpy on the underlying byte-content (as a raw
octet-stream). If this doesn't satisfy your goals, you must use a ref class.
Tom
"Peter Oliphant" <po*******@Roun dTripInc.com> wrote I guess I then need the definition of 'special member functions'.
Special member functions are defined by the C++ standard.
These are:
- default constructor
- copy constructor
- copy assignment operator
- destructor
Based on what you've said, a copy constructor is a special member function. But a constructor which, I guess, DOESN'T have as its only parameter a pointer or reference to the same class type (i.e., isn't a copy constructor) IS allowed. Why the destinction? Or, like I said, and more importantly what defines a 'special member function' in contrast to one that isn't 'special'?
The CLR doesn't have concept of copy construction during in ordinary
control flow.
Therefore, there's no way for the C++ compiler to emit information
that other language implementations will understand to implement the
C++ semantics. The language designers apparently figured that this
would cause confusion (and FWIW I disagree) and hence decided
not to allow special member functions. Only special member functions
are called implicitly by the compiler-generated code.
For instance,
// C++
value struct X {
X(){ /*..*/ }
X( const X%){/*..*/}
X% operator=(const X%){ /*...*/};
}
// C# client
void foo() {
X x; // doesn't invoke default ctor
new X[] ( 10 ); // doesn't invoke default ctor for elements
X y = x; // doesn't invoke copy ctor
y = x; // doesn't invoke copy assignment operator
} // doesn't invoke destructor for x or y
-hg
This is good. That's exactly how I personally think of as the 'natural' way
to default-fill data types one doesn't know anything about. So in this I am
happy! : )
[==P==]
"Tamas Demjen" <td*****@yahoo. com> wrote in message
news:uj******** ******@TK2MSFTN GP12.phx.gbl... Peter Oliphant wrote:
How does the implicit parameterless (i.e., default) constructor determine the values it gives it members?
By memset'ing (filling) the entire class with all 0 bytes, regardless of the actual type of the member variables. Think of value types as PODs (plain old data, like a C struct). When a value type is copied, the CLR is simply doing a memcpy on the underlying byte-content (as a raw octet-stream). If this doesn't satisfy your goals, you must use a ref class.
Tom This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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