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readonly C++ class member
Posted by Erik on Jan 14 2006 @ 16:39 :: 4108 unique visits
Sometimes you want a class member which can only be read by the outside. Most of the time this is done with a simple get... function. Here is another simpler way to do this:CODE: CPP
class someclass {
private:
int _themember;
public:
const int &themember;
someclass() : _themember(0), themember(_themember) {
;
}
void somefunction() {
_themember = 42;
}
};
private:
int _themember;
public:
const int &themember;
someclass() : _themember(0), themember(_themember) {
;
}
void somefunction() {
_themember = 42;
}
};
Inside the class you can use _themember to write and read from.
The outside can only read themember:
CODE: CPP
someclass foo;
int a = foo.themember; // works
foo.themember = 1337; // error
int a = foo.themember; // works
foo.themember = 1337; // error
15 comments posted so far
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2. On Jan 14 2006 @ 22:11 Erik wrote:
It isn't a way to make data private, it is a way to make data readonly. It perfectly fits into the principles of data encapsulation. and why shouldn't it be intuitive?3. On Jan 15 2006 @ 22:08 Revvy wrote:
It's not intuitive because someone else looking at the code would assume that if it can be read like a public variable, it would follow that it can be also be written to like a public variable.4. On Jan 20 2006 @ 16:44 hxa7241 wrote:
It takes twice the memory!And, as mentioned, it breaks encapsulation. If you wanted to change the representation of _themember -- to float, say -- you would run into extra work and awkwardness...
5. On Feb 15 2006 @ 00:20 Shalom wrote:
1. It doesn't use twice the memory, the reference "themember" is given the same memory address as the int "_themember" -- and if the compiler is really smart, this gets done during link-time (As long as you don't do something really special, like re-cast "themember" and use it in some kind of l-value expression.)2. I don't really like to discuss philosophy in C/C++ - my attitude is more of: "if you can do it and it's defined in the language spec, it's supposed to do what you told it to." So Please take the next comment with a grain of salt.
I don't think there's any difference between this declaration of a "changble const" that's public, and the actual "plain-vanialla" public const. If there's a constant in a class, it's a constant. If it's public, then the user of that class can read it. It doesn't expect to be able to change it, but since code of that user's makes use of the const's value, it shouldn't care if the value changes. It should simply keep working as if the new value has always been there (or not. depends on what you're doing.)
But I have to admit one big reservation: most compilers are far more competent at optimizing-away calls to value-getting-functions (even if they aren't inline) than correctly interpreting the above. You much more likely to have any call to such a function simply replaced with a simple stack push of the returned value, i.e. the value of _themember.
6. On Mar 07 2006 @ 19:14 hxa7241 wrote:
1. That's an optimization. Refs must in general be pointers. I wouldn't trust it unless I had: either the clause in the standard guaranteeing it (I don't think there is one), or results from compilers showing it is common.7. On Mar 10 2006 @ 15:27 guest wrote:
Well, should this class be non-copyable or the default copy behavior is intentional? What about the copy constructor?8. On Aug 14 2006 @ 18:08 guest wrote:
how, if the member is a vector and i want every element in the vector to be readonly by outside?if i use "get()" to do this, will the performance be very poor?
9. On Dec 08 2008 @ 05:23 guest wrote:
I think trick is very good!!!!!!10. On May 19 2009 @ 09:33 guest wrote:
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11. On Jun 12 2009 @ 10:05 guest wrote:
i know this thread is old...but think abut that:every constant reference can be easily converted to a non-constant reference which gives full access to the private data members:
CODE: CPP
int main(int argc,char *argv[]) {
someclass object;
int & full_access_member=(int &)object.themember;
full_access_member=42;
}
someclass object;
int & full_access_member=(int &)object.themember;
full_access_member=42;
}
so, no real read-only;D
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good!!!!!!!!!!!!14. On Sep 17 2009 @ 10:03 Steffen wrote:
I prefer using C#-style properties without those get/set keywords in c++ too!@Jun 12 2009:
Access to memory-addresses is one of the major powers
or weaknesses (...as some consultants say) of c++.
Your ptr-cast would also work for a public member variable
declared as "const int iVal".
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1. On Jan 14 2006 @ 21:23 guest wrote:
The problem with this is that it is completely unintuitive for someone reading the code. It also violates the principles of data encapsulation; if you're going to use to make data private, you should do it so that it's obviously private.