digitalmars.D.learn - Modify const reference data
- Heinz (66/66) Dec 08 2013 [DMD 2.064.2]
- Heinz (2/2) Dec 08 2013 Duhhh! i got the pink avatar by default. That sucks. Pink is just
- Adam D. Ruppe (6/23) Dec 08 2013 Since abc isn't initialized in this constructor, abc.ptr is null.
- =?UTF-8?B?QWxpIMOHZWhyZWxp?= (14/37) Dec 08 2013 Apparently the OP intended to set it in foo(). If the data is actually
[DMD 2.064.2]
Hello,
I've been strugling with a solution before bothering you guys
here (again). I have my own complex code but i made a VERY simple
test case to reproduce my problem, here's the code:
private import std.stdio;
class A
{
private const ubyte* data;
private ubyte[] abc;
this()
{
abc = [1,2,3,4];
data = cast(const ubyte*)abc.ptr;
}
public void print()
{
for(size_t i = 0; i < 4; i++)
{
writefln("%d", data[i]);
}
}
}
class B
{
private const ubyte* data;
private ubyte[] abc;
this()
{
data = cast(const ubyte*)abc.ptr;
}
public void foo()
{
abc = [1,2,3,4];
}
public void print()
{
for(size_t i = 0; i < 4; i++)
{
writefln("%d", data[i]);
}
}
}
void main()
{
A a = new A();
a.print(); // OK.
B b = new B();
b.print(); // Crash.
}
The thing is that i can not use the data that const variable
"data" in class B is referencing because in my original code i
get "Invalid Memory Operation", in the test case i get "access
violation" but it doesn't matter the name of the exception, the
only thing that matters is that the exception raises for the same
reason. I understand that i should not modify the const data
outside a constructor but by stricts reason i can't avoid that.
Do you guys have any other aproach to get away with this? (modify
a const reference data and then manipulate that data). THANK YOU
very much in advance. D2 learner by the way.
Dec 08 2013
"Heinz" <thor587 yahoo.com> Duhhh! i got the pink avatar by default. That sucks. Pink is just not my color.
Dec 08 2013
Easy problem in class B: data is null! On Monday, 9 December 2013 at 02:53:01 UTC, Heinz wrote:class B { private const ubyte* data; private ubyte[] abc; this() { data = cast(const ubyte*)abc.ptr; }Since abc isn't initialized in this constructor, abc.ptr is null. So data is null too.public void print() { for(size_t i = 0; i < 4; i++) { writefln("%d", data[i]); } } }And since data is null, data[i] will be a memory err/segfault/access violation/whatever it is called.
Dec 08 2013
On 12/08/2013 07:24 PM, Adam D. Ruppe wrote:Easy problem in class B: data is null! On Monday, 9 December 2013 at 02:53:01 UTC, Heinz wrote:Apparently the OP intended to set it in foo(). If the data is actually mutable and there really is no way other than going against the type system, foo() must be called at least once and can be implemented like this: public void foo() { abc = [1,2,3,4]; cast(ubyte*)data = abc.ptr; } // ... B b = new B(); b.foo(); b.print(); // now OK Aliclass B { private const ubyte* data; private ubyte[] abc; this() { data = cast(const ubyte*)abc.ptr; }Since abc isn't initialized in this constructor, abc.ptr is null. So data is null too.public void print() { for(size_t i = 0; i < 4; i++) { writefln("%d", data[i]); } } }And since data is null, data[i] will be a memory err/segfault/access violation/whatever it is called.
Dec 08 2013
Apparently the OP intended to set it in foo(). If the data is
actually mutable and there really is no way other than going
against the type system, foo() must be called at least once and
can be implemented like this:
public void foo()
{
abc = [1,2,3,4];
cast(ubyte*)data = abc.ptr;
}
// ...
B b = new B();
b.foo();
b.print(); // now OK
Ali
Wow, i didn't know i could cast out constness from an lvalue.
This is what i needed. I'll try it in my code as soon as i can.
Thanks.
Dec 09 2013
Wow, i didn't know i could cast out constness from an lvalue. This is what i needed. I'll try it in my code as soon as i can. Thanks.Yep, it compiles and works! By the way, i forgot to call b.foo() in my example. Thanks for your help guys.
Dec 09 2013