Hope this is not a duplicate. If so, please point me to it in a comment and I'll remove the question again.
I have a data object with data that's only valid in a bundle - i.e. there's no sense in changing the value of one member without invalidating the other members.
This data object describes some image information:
struct ImageInfo
{
ImageInfo(const double &ppix, ...)
: PpiX(ppix),
...
{ }
const double PpiX;
const double PpiY;
const int SizeX;
const int SizeY;
};
In my image object I have a non-const member of type ImageInfo:
class MyImageObject
{
...
private:
ImageInfo mMyInfo;
}
I want to be able to change mMyInfo at runtime, but only so that it will take a new ImageInfo(...) instance.
In the MyImageObject::Load() function, I'd like to read this data from the file info and then create an ImageInfo instance with the correct set of data:
double ppix = ImageFile.GetPpiX();
...
mMyInfo = ImageInfo(ppix, ...);
But I couldn't manage to write a valid assignment operator (copy constructor is possible of course). My solution left mMyInfo empty, because I didn't reference this:
ImageInfo operator=(const ImageInfo &other)
{
// no reference to *this
return ImageInfo(other);
}
Out of curiosity I'd like to know how the assignment operator for such a class would need to look like.
I'm using plain C++.
EDIT
Possible solutions (the goal is to keep the data transportable, but coherent):
Use private members together with Get...() functions -> simple, but I'd like to avoid the parentheses.
Store a pointer to ImageInfo: ImageInfo *mpMyInfo; (I'd like to avoid the heap.)
Use serialization and store the serialized ImageInfo, then create local instances from the serialized data.
I don't think you can have const member variables that aren't static. If you need const variables that change with the instance, you could do something like this:
struct ImageInfo
{
private:
double myPpiX;
double myPpiY;
int mySizeX;
int mySizeY
public:
ImageInfo(const double &ppix, ...)
: myPpiX(ppix),
PpiX(myPpiX),
...
{ }
ImageInfo( const ImageInfo &other)
: myPpiX( other.myPpiX),
PpiX(myPpiX)
...
{ }
const double &PpiX;
const double &PpiY;
const int &SizeX;
const int &SizeY;
// EDIT: explicit assignment operator was missing
ImageInfo& operator=(const ImageInfo &other)
{
myPpiX = other.myPpiX;
myPpiY = other.myPpiX;
mySizeX = other.mySizeX;
mySizeX = other.mySizeX;
return *this;
}
};
The values are stored in the private variables that can be set at construction, and their values accessed by const references. You're also not dependent on the references passed into the constructor living as long as the ImageInfo instance.
As they are data fields that can be modified, they are not const.
If you want to restrict post-construct access to them to const, you need to wrap them in accessors as follows:
struct ImageInfo
{
ImageInfo(const double &ppix, /*...*/)
: PpiX_(ppix),
/*...*/
{ }
double const& PpiX() const {return PpiX_; };
double const& PpiY() const {return PipY_; };
int const& SizeX() const {return SizeX_; };
int const& SizeY() const {return SizeY_; };
private:
double PpiX_;
double PpiY_;
int SizeX_;
int SizeY_;
};
That allows move/copy assignment and construction, while blocking non-const access outside of said construction.
Avoiding the () is tricky, but could be done with pseudo-references, something like this:
struct pseudo_const_reference_to_Ppix {
ImageInfo const* self;
operator double() const { return self->Ppix; }
void reseat( ImageInfo const* o ) { self = o; }
};
plus a whole pile of boilerplate to overload every const operator on the left and right such that the above pseudo_const_reference_* is just as valid as double.
Generic versions can be written (either taking a functor or a std::function if you are willing to suffer type erasure overhead).
Then you maintain these pseudo-const references on assignment and copy/move construction.
I think the () is the better option.
Note that the overhead of a pointer (or more) per pseudo-reference is basically unavoidable: member variables do not have access to the this from which they are invoked, even though the accesing site has it right there plain as day.
If something is const, you can't change it. Full stop.
So you must adjust the design somewhere, either not have those ImageInfo members const, not have ImageInfo as member, or best: not do the assignment.
Normally const members are set in constructor. You can make a load function that creates a MyImageObject object with all its content, so avoiding to have a half-done thing and load the sate in a second phase.
An alternative is to have the mMyInfo indirectly, say using unique_ptr, then you can replace it with another instance. I would not do that without a really good reason.
Immutable value objects are great. But the variable holding the value object (mMyInfo in MyImageObject) should be (non-constant) a pointer. In other languages (e.g. Java) this is automatically the case, but not in C++, where you need the * operator. Also, there is no need to override/implement the = operator for value objects. To change the image data within the image object, you assign a newly constructed ImageInfo object to the myImageInfo pointer. This way, none of the internal variables of the value object are changed.
Related
In the library I am designing, I sometimes need read-access to large member variables of classes. Because of their size I don't want to make a getter that returns by copying the member. I don't want them to be modifiable from the outside, so I can't have them public or return references to them. So I thought I would use a "reader":
class TestClass
{
public:
explicit TestClass(double d): d_(d){}
const double& readD() const { return d_; }
private:
double d_;
};
(this is not really meant for doubles)
But here somebody could const_cast the reference and access the data directly. Even without assuming malicious intent, somebody could safe a reference to the data-member and keep it around after the original object has gone out of scope. I know const references can keep a temporary viable, but that doesn't remove the const_cast-problem.
So I came up with a workaround:
#include <iostream>
template<class T>
class SafeMemberReference
{
public:
using type = T;
SafeMemberReference(const T& t) :t(t) {}
explicit SafeMemberReference(T&& t) = delete;
operator const T& () && {return t; }
T get() && {return t; }
private:
const T& t;
};
class TestClass
{
public:
explicit TestClass(double d): d_(d){}
SafeMemberReference<double> readD() const { return d_; }
private:
double d_;
};
int main()
{
TestClass foo(1.2);
// temporary from read can be used as temporary in expressions
std::cout << foo.readD() << std::endl;
// temporary can be used to copy from
auto x = foo.readD().get();
// lvalue can not be used, so a possible dangling reference is no problem
auto ref = foo.readD();
//std::cout << ref << std::endl;
}
I have several questions to this:
Q1) How necessary is this from an efficiency POV? the largest objects I am returning are dense complex matrices with dimensions of maybe 1000x1000. These copies may happen frequently
Q2) Are my concerns about returning by const& valid?
Q3) Does this seem like a good solution? which drawbacks does it have?
Any solution that attempts to fight the language itself is not a good solution.
They should get a wrap on the knuckles if they used a const_cast in that way: the behaviour on trying to change an object via a const_cast on an object that was originally declared as const is undefined. Even if you manage to conjure up a solution to prevent that, a hostile programmer could still take the address of your object, offset that address (using unsigned char* pointer arithmetic) and modify a data member through that pointer!
So if I were you I wouldn't fight the language. Return a const reference if I were you, as per your original suggestion.
Code static analysis tools / compiler warnings / code reviews / human resource departments will help you keep other collaborative programmers on the straight and narrow.
I have const-overloaded methods in my class:
class A{
public:
typedef int data[10];
data& operator[](int index);
const data& operator[](int index) const;
}
This class is implementing copy-on-write for its internal data. I figured that since i am allowing to access data directly, i must create copy of shared data (if it is shared obviously) on every use of operator[], but not operator[] const. However, even if code is using operator[] for reading data, but object itself is not declared as const, it will still cause creating copy, as operator[] will be used. Is there any syntax that would let me allow to choose which of those operators i am calling?
Yes: const_cast<A const&>(anAObj)[5].
Why does the operator return a data& rather than an int& that references a single item?
That being said your options include:
Always do the copy even if it turns out to be unneeded.
Use the operator for read or write, and a named method (GetRef for example) for the other one.
Cast the object to const at the use point: static_cast<const A&>(obj)[index].
You need to create a Proxy class to substitute the return value of both methods. Then, in the Proxy class you can handle reads and writes appropriately. Here's code that should compile to demonstrate the idea (working with an array of unsigned integers):
typedef unsigned int UINT;
class A {
class Proxy {
public:
Proxy(const UINT &number): _number(number) {}
const Proxy &operator=(const Proxy &obj) {
cout << "Writting...\n";
_number = obj._number;
return *this;
}
operator const UINT &() const {
cout << "Reading...\n";
return _number;
}
private:
UINT _number;
};
public:
A(UINT *array): _array(array) {}
Proxy operator[](int index) {
return _array[index];
}
const Proxy operator[](int index) const {
return _array[index];
}
private:
UINT *_array;
};
int main(int argc, char** argv) {
UINT myArray[] = {0, 1, 2, 3, 4};
A a(myArray); // Normal A object
UINT num1 = a[1]; // Reading fine
a[1] = num1; // Writting fine
const A ca(myArray); // Constant A object
UINT num2 = ca[1]; // Reading fine
ca[1] = num2; // Writting NOT fine (compilation error)
return 0;
}
implement data& at(size_t i); and const data& cat(size_t); functions, so you can invoke cat() on non-const objects to enforce constness of returned data.
No, unless you cast the reference/pointer to a constant. Or create a constant copy.
A a;
const_cast<const A &> (a)[0] /*.foo ()*/;
If you are implementing copy-on-write, then you should not define those subscription operators at all. A client that calls the const version of the subscription method for an object x is not allowed to subsequently use that reference to modify a component of x, but the reference should still reflect the changes that other clients make to (that component of) x. But his is not going to happen with a copy-on-write strategy, since the change will happen in a different copy than the reference points to.
Also two clients that call the non-const subscription operator with the same index should get (modifyable) references to the same data object; but they won't, because two seperate copies will have been made when the subscription operators were called.
Instead you could have a subscription method that returns data by value. This avoids creating the illusion of obtaining an alias for a component of x (which as I argued must be illusory when copy-on-write is implemented). You could also provide a single method that modifies a component of x (rather than splitting this operation into a subscription followed by assignment to the reference so obtained), which would either internally make a copy, or just modify a component in place if this copy was already made. This would hide from clients the use of a copy-on-write implementation.
More generally, exposing methods that return references to internal objects, whether const or not, is exposing the implementation detail that such objects exist at all. This limits the freedom of changing the implementation later, for instance to compress the data or to store it elsewhere than in memory.
It is an open ended question.
Effective C++. Item 3. Use const whenever possible. Really?
I would like to make anything which doesn't change during the objects lifetime const. But const comes with it own troubles. If a class has any const member, the compiler generated assignment operator is disabled. Without an assignment operator a class won't work with STL. If you want to provide your own assignment operator, const_cast is required. That means more hustle and more room for error. How often you use const class members?
EDIT: As a rule, I strive for const correctness because I do a lot of multithreading. I rarely need to implemented copy control for my classes and never code delete (unless it is absolutely necessary). I feel that the current state of affairs with const contradicts my coding style. Const forces me to implement assignment operator even though I don't need one. Even without const_cast assignment is a hassle. You need to make sure that all const members compare equal and then manually copy all non-const member.
Code. Hope it will clarify what I mean. The class you see below won't work with STL. You need to implement an assignment for it, even though you don't need one.
class Multiply {
public:
Multiply(double coef) : coef_(coef) {}
double operator()(double x) const {
return coef_*x;
}
private:
const double coef_;
};
You said yourself that you make const "anything which doesn't change during the objects lifetime". Yet you complain about the implicitly declared assignment operator getting disabled. But implicitly declared assignment operator does change the contents of the member in question! It is perfectly logical (according to your own logic) that it is getting disabled. Either that, or you shouldn't be declaring that member const.
Also, providing you own assignment operator does not require a const_cast. Why? Are you trying to assign to the member you declared const inside your assignment operator? If so, why did you declare it const then?
In other words, provide a more meaningful description of the problems you are running into. The one you provided so far is self-contradictory in the most obvious manner.
As AndreyT pointed out, under these circumstances assignment (mostly) doesn't make a lot of sense. The problem is that vector (for one example) is kind of an exception to that rule.
Logically, you copy an object into the vector, and sometime later you get back another copy of the original object. From a purely logical viewpoint, there's no assignment involved. The problem is that vector requires that the object be assignable anyway (actually, all C++ containers do). It's basically making an implementation detail (that somewhere in its code, it might assign the objects instead of copying them) part of the interface.
There is no simple cure for this. Even defining your own assignment operator and using const_cast doesn't really fix the problem. It's perfectly safe to use const_cast when you get a const pointer or reference to an object that you know isn't actually defined to be const. In this case, however, the variable itself is defined to be const -- attempting to cast away the constness and assign to it gives undefined behavior. In reality, it'll almost always work anyway (as long as it's not static const with an initializer that's known at compile time), but there's no guarantee of it.
C++ 11 and newer add a few new twists to this situation. In particular, objects no longer need to be assignable to be stored in a vector (or other collections). It's sufficient that they be movable. That doesn't help in this particular case (it's no easier to move a const object than it is to assign it) but does make life substantially easier in some other cases (i.e., there are certainly types that are movable but not assignable/copyable).
In this case, you could use a move rather than a copy by adding a level of indirection. If your create an "outer" and an "inner" object, with the const member in the inner object, and the outer object just containing a pointer to the inner:
struct outer {
struct inner {
const double coeff;
};
inner *i;
};
...then when we create an instance of outer, we define an inner object to hold the const data. When we need to do an assignment, we do a typical move assignment: copy the pointer from the old object to the new one, and (probably) set the pointer in the old object to a nullptr, so when it's destroyed, it won't try to destroy the inner object.
If you wanted to badly enough, you could use (sort of) the same technique in older versions of C++. You'd still use the outer/inner classes, but each assignment would allocate a whole new inner object, or you'd use something like a shared_ptr to let the outer instances share access to a single inner object, and clean it up when the last outer object is destroyed.
It doesn't make any real difference, but at least for the assignment used in managing a vector, you'd only have two references to an inner while the vector was resizing itself (resizing is why a vector requires assignable to start with).
I very rarely use them - the hassle is too great. Of course I always strive for const correctness when it comes to member functions, parameters or return types.
Errors at compile time are painful, but errors at runtime are deadly. Constructions using const might be a hassle to code, but it might help you find bugs before you implement them. I use consts whenever possible.
I try my best to follow the advice of using const whenever possible, however I agree that when it comes to class members, const is a big hassle.
I have found that I am very careful with const-correctness when it comes to parameters, but not as much with class members. Indeed, when I make class members const and it results in an error (due to using STL containers), the first thing I do is remove the const.
I'm wondering about your case... Everything below is but supposition because you did not provide the example code describing your problem, so...
The cause
I guess you have something like:
struct MyValue
{
int i ;
const int k ;
} ;
IIRC, the default assignment operator will do a member-by-member assignment, which is akin to :
MyValue & operator = (const MyValue & rhs)
{
this->i = rhs.i ;
this->k = rhs.k ; // THIS WON'T WORK BECAUSE K IS CONST
return *this ;
} ;
Thus, this won't get generated.
So, your problem is that without this assignment operator, the STL containers won't accept your object.
As far I as see it:
The compiler is right to not generate this operator =
You should provide your own, because only you know exactly what you want
You solution
I'm afraid to understand what do you mean by const_cast.
My own solution to your problem would be to write the following user defined operator :
MyValue & operator = (const MyValue & rhs)
{
this->i = rhs.i ;
// DON'T COPY K. K IS CONST, SO IT SHOULD NO BE MODIFIED.
return *this ;
} ;
This way, if you'll have:
MyValue a = { 1, 2 }, b = {10, 20} ;
a = b ; // a is now { 10, 2 }
As far as I see it, it is coherent. But I guess, reading the const_cast solution, that you want to have something more like:
MyValue a = { 1, 2 }, b = {10, 20} ;
a = b ; // a is now { 10, 20 } : K WAS COPIED
Which means the following code for operator =:
MyValue & operator = (const MyValue & rhs)
{
this->i = rhs.i ;
const_cast<int &>(this->k) = rhs.k ;
return *this ;
} ;
But, then, you wrote in your question:
I would like to make anything which doesn't change during the objects lifetime const
With what I supposed is your own const_cast solution, k changed during the object lifetime, which means that you contradict yourself because you need a member variable that doesn't change during the object lifetime unless you want it to change!
The solution
Accept the fact your member variable will change during the lifetime of its owner object, and remove the const.
you can store shared_ptr to your const objects in STL containers if you'd like to retain const members.
#include <iostream>
#include <boost/foreach.hpp>
#include <boost/make_shared.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/utility.hpp>
#include <vector>
class Fruit : boost::noncopyable
{
public:
Fruit(
const std::string& name
) :
_name( name )
{
}
void eat() const { std::cout << "eating " << _name << std::endl; }
private:
const std::string _name;
};
int
main()
{
typedef boost::shared_ptr<const Fruit> FruitPtr;
typedef std::vector<FruitPtr> FruitVector;
FruitVector fruits;
fruits.push_back( boost::make_shared<Fruit>("apple") );
fruits.push_back( boost::make_shared<Fruit>("banana") );
fruits.push_back( boost::make_shared<Fruit>("orange") );
fruits.push_back( boost::make_shared<Fruit>("pear") );
BOOST_FOREACH( const FruitPtr& fruit, fruits ) {
fruit->eat();
}
return 0;
}
though, as others have pointed out it's somewhat of a hassle and often easier in my opinion to remove the const qualified members if you desire the compiler generated copy constructor.
I only use const on reference or pointer class members. I use it to indicate that the target of the reference or pointer should not be changed. Using it on other kinds of class members is a big hassle as you found out.
The best places to use const is in function parameters, pointers and references of all kinds, constant integers and temporary convenience values.
An example of a temporary convenience variable would be:
char buf[256];
char * const buf_end = buf + sizeof(buf);
fill_buf(buf, buf_end);
const size_t len = strlen(buf);
That buf_end pointer should never point anywhere else so making it const is a good idea. The same idea with len. If the string inside buf never changes in the rest of the function then its len should not change either. If I could, I would even change buf to const after calling fill_buf, but C/C++ does not let you do that.
The point is that the poster wants const protection within his implementation but still wants the object assignable. The language does not support such semantics conveniently as constness of the member resides at the same logical level and is tightly coupled with assignability.
However, the pImpl idiom with a reference counted implementation or smart pointer will do exactly what the poster wants as assignability is then moved out of the implementation and up a level to the higher level object. The implementation object is only constructed/destructed whence assignment is never needed at the lower level.
I think your statement
If a class has const any member, the
compiler generated assignment operator
is disabled.
Might be incorrect. I have classes that have const method
bool is_error(void) const;
....
virtual std::string info(void) const;
....
that are also used with STLs. So perhaps your observation is compiler dependent or only applicable to the member variables?
I would only use const member iff the class itself is non-copyable. I have many classes that I declare with boost::noncopyable
class Foo : public boost::noncopyable {
const int x;
const int y;
}
However if you want to be very sneaky and cause yourself lots of potential
problems you can effect a copy construct without an assignment but you have to
be a bit careful.
#include <new>
#include <iostream>
struct Foo {
Foo(int x):x(x){}
const int x;
friend std::ostream & operator << (std::ostream & os, Foo const & f ){
os << f.x;
return os;
}
};
int main(int, char * a[]){
Foo foo(1);
Foo bar(2);
std::cout << foo << std::endl;
std::cout << bar<< std::endl;
new(&bar)Foo(foo);
std::cout << foo << std::endl;
std::cout << bar << std::endl;
}
outputs
1
2
1
1
foo has been copied to bar using the placement new operator.
It isn't too hard. You shouldn't have any trouble making your own assignment operator. The const bits don't need to be assigned (as they're const).
Update
There is some misunderstanding about what const means. It means that it will not change, ever.
If an assignment is supposed to change it, then it isn't const.
If you just want to prevent others changing it, make it private and don't provide an update method.
End Update
class CTheta
{
public:
CTheta(int nVal)
: m_nVal(nVal), m_pi(3.142)
{
}
double GetPi() const { return m_pi; }
int GetVal() const { return m_nVal; }
CTheta &operator =(const CTheta &x)
{
if (this != &x)
{
m_nVal = x.GetVal();
}
return *this;
}
private:
int m_nVal;
const double m_pi;
};
bool operator < (const CTheta &lhs, const CTheta &rhs)
{
return lhs.GetVal() < rhs.GetVal();
}
int main()
{
std::vector<CTheta> v;
const size_t nMax(12);
for (size_t i=0; i<nMax; i++)
{
v.push_back(CTheta(::rand()));
}
std::sort(v.begin(), v.end());
std::vector<CTheta>::const_iterator itr;
for (itr=v.begin(); itr!=v.end(); ++itr)
{
std::cout << itr->GetVal() << " " << itr->GetPi() << std::endl;
}
return 0;
}
Philosophically speaking, it looks as safety-performance tradeoff. Const used for safety. As I understand, containers use assignment to reuse memory, i.e. for sake of performance. They would may use explicit destruction and placement new instead (and logicaly it is more correct), but assignment has a chance to be more efficient. I suppose, it is logically redundant requirement "to be assignable" (copy constructable is enough), but stl containers want to be faster and simpler.
Of course, it is possible to implement assignment as explicit destruction+placement new to avoid const_cast hack
Rather than declaring the data-member const, you can make the public surface of the class const, apart from the implicitly defined parts that make it (semi)regular.
class Multiply {
public:
Multiply(double coef) : coef(coef) {}
double operator()(double x) const {
return coef*x;
}
private:
double coef;
};
You basically never want to put a const member variable in a class. (Ditto with using references as members of a class.)
Constness is really intended for your program's control flow -- to prevent mutating objects at the wrong times in your code. So don't declare const member variables in your class's definition, rather make it all or nothing when you declare instances of the class.
Given a class like this:
class Foo
{
const int a;
};
Is it possible to put that class in a vector? When I try, my compiler tells me it can't use the default assignment operator. I try to write my own, but googling around tells me that it's impossible to write an assignment operator for a class with const data members. One post I found said that "if you made [the data member] const that means you don't want assignment to happen in the first place." This makes sense. I've written a class with const data members, and I never intended on using assignment on it, but apparently I need assignment to put it in a vector. Is there a way around this that still preserves const-correctness?
I've written a class with const data members, and I never intended on using assignment on it, but apparently I need assignment to put it in a vector. Is there a way around this that still preserves const-correctness?
You have to ask whether the following constraint still holds
a = b;
/* a is now equivalent to b */
If this constraint is not true for a and b being of type Foo (you have to define the semantics of what "equivalent" means!), then you just cannot put Foo into a Standard container. For example, auto_ptr cannot be put into Standard containers because it violates that requirement.
If you can say about your type that it satisfies this constraint (for example if the const member does not in any way participate to the value of your object, but then consider making it a static data member anyway), then you can write your own assignment operator
class Foo
{
const int a;
public:
Foo &operator=(Foo const& f) {
/* don't assign to "a" */
return *this;
}
};
But think twice!. To me, it looks like that your type does not satisfy the constraint!
Use a vector of pointers std::vector<Foo *>. If you want to avoid the hassle of cleaning up after yourself, use boost::ptr_vector.
Edit: My initial stab during my coffee break, static const int a; won't work for the use case the OP has in mind, which the initial comments confirm, so I'm rewriting and expanding my answer.
Most of the time, when I want to make an element of a class constant, it's a constant whose value is constant for all time and across all instances of the class. In that case, I use a static const variable:
class Foo
{
public:
static const int a;
};
Those don't need to be copied among instances, so if it applied, that would fix your assignment problem. Unfortunately, the OP has indicated that this won't work for the case the OP has in mind.
If you want to create a read-only value that clients can't modify, you can make it a private member variable and only expose it via a const getter method, as another post on this thread indicates:
class Foo
{
public:
int get_a() const { return a; }
private:
int a;
};
The difference between this and
class Foo
{
public:
const int a;
};
is:
The const int gives you assurance that not even the implementation of the class will be able to muck with the value of a during the lifetime of the object. This means that assignment rightfully won't work, since that would be trying to modify the value of a after the object's been created. (This is why, btw, writing a custom operator=() that skips the copy of a is probably a bad idea design-wise.)
The access is different – you have to go through a getter rather than accessing the member directly.
In practice, when choosing between the two, I use read-only members. Doing so probably means you'll be able to replace the value of an object with the value of another object without violating semantics at all. Let's see how it would work in your case.
Consider your Grid object, with a width and height. When you initially create the vector, and let's say you reserve some initial space using vector::reserve(), your vector will be populated with initial default-initialized (i.e. empty) Grids. When you go to assign to a particular position in the vector, or push a Grid onto the end of the vector, you replace the value of the object at that position with a Grid that has actual stuff. But you may be OK with this! If the reason you wanted width and height to be constant is really to ensure consistency between width and height and the rest of the contents of your Grid object, and you've verified that it doesn't matter whether width and height are replaced before or after other elements of Grid are replaced, then this assignment should be safe because by the end of the assignment, the entire contents of the instance will have been replaced and you'll be back in a consistent state. (If the lack of atomicity of the default assignment was a problem, you could probably get around this by implementing your own assignment operator which used a copy constructor and a swap() operation.)
In summary, what you gain by using read-only getters is the ability to use the objects in a vector or any container with value semantics. However, it then falls to you to ensure that none of Grid's internal operations (or the operations of friends of Grid) violate this consistency, because the compiler won't be locking down the width and height for you. This goes for default construction, copy construction, and assignment as well.
I'm considering making the data member non-const, but private and only accessible by a get function, like this:
class Foo
{
private:
int a;
public:
int getA() const {return a;}
};
Is this 'as good' as const? Does it have any disadvantages?
As of c++20, using const member variables are legal without restrictions that had made it virtually unusable in containers. You still have to define a copy assignment member function because it continues to be automatically deleted when a const object exists in the class. However, changes to "basic.life" now allow changing const sub-objects and c++ provides rather convenient functions for doing this. Here's a description of why the change was made:
The following code shows how to define a copy assignment member function which is useable in any class containing const member objects and uses the new functions std::destroy_at and std::construct_at to fulfil the requirement so the new "basic.life" rules. The code demonstrates assignment of vectors as well as sorting vectors with const elements.
Compiler explorer using MSVC, GCC, CLANG https://godbolt.org/z/McfcaMWqj
#include <memory>
#include <vector>
#include <iostream>
#include <algorithm>
class Foo
{
public:
const int a;
Foo& operator=(const Foo& arg) {
if (this != &arg)
{
std::destroy_at(this);
std::construct_at(this, arg);
}
return *this;
}
};
int main()
{
std::vector<Foo> v;
v.push_back({ 2 });
v.push_back({ 1 });
v.insert(v.begin() + 1, Foo{ 0 });
std::vector<Foo> v2;
v2 = v;
std::sort(v2.begin(), v2.end(), [](auto p1, auto p2) {return p1.a < p2.a; });
for (auto& x : v2)
std::cout << x.a << '\n';
}
I want to implement a Swap() method for my class (let's call it A) to make copy-and-swap operator=(). As far as I know, swap method should be implemented by swapping all members of the class, for example:
class A
{
public:
void swap(A& rhv)
{
std::swap(x, rhv.x);
std::swap(y, rhv.y);
std::swap(z, rhv.z);
}
private:
int x,y,z;
};
But what should I do if I have a const member? I can't call std::swap for it, so I can't code A::Swap().
EDIT: Actually my class is little bit more complicated. I want to Serialize and Deserialize it. Const member is a piece of data that won't change (its ID for example) within this object. So I was thinking of writing something like:
class A
{
public:
void Serialize(FILE* file) const
{
fwrite(&read_a, 1, sizeof(read_a), file);
}
void Deserialize(FILE* file) const
{
size_t read_a;
fread(&read_a, 1, sizeof(read_a), file);
A tmp(read_a);
this->Swap(tmp);
}
private:
const size_t a;
};
and call this code:
A a;
FILE* f = fopen(...);
a.Deserialize(f);
I'm sorry for such vague wording.
I think what you really want is to have an internal data structure that you can easily exchange between objects. For example:
class A
{
private:
struct A_Data {
int x;
int y;
const int z;
A_Data(int initial_z) : z(initial_z) {}
};
std::auto_ptr<A_Data> p_data;
public:
A(int initial_z) : p_data(new A_Data(initial_z)) {}
void swap(A& rhv) {
std::swap(p_data, rhv.p_data);
}
};
This keeps the z value constant within any instance of A object internal data, but you can swap the internal data of two A objects (including the constant z value) without violating const-correctness.
After a good nights sleep I think the best answer is to use a non-const pointer to a const value -- after all these are the semantics you are trying to capture.
f0b0s, a good design principle is to design your objects to be immutable. This means that the object can't change once created. To "change" the object, you must copy the object and make sure to change the elements you want.
That being said, in this case you should look at using a copy constructor instead to copy the objects you want to swap, and then actually swap the references to the object. I can understand it'd be tempting just to be able to change the elements of an object under the hood, but it'd be better to make a copy of the object and replace the references to that object with the NEW object instead. This gets you around any const nastiness.
Hope this helps.
I suggest you use pointers to the instances. The pointers can be swapped much easier than the data in the class or struct.
The only way to swap a constant value is to create another object, or clone the current object.
Given a struct:
struct My_Struct
{
const unsigned int ID;
std::string name;
My_Struct(unsigned int new_id)
: ID(new_id)
{ ; }
};
My understanding is that you want to swap instances of something like My_Struct above. You can copy the mutable (non-const) members but not the const member. The only method to alter the const member is to create a new instance with a new value for the const member.
Perhaps you need to rethink your design.
IMHO you must consider not to swap CONST members.
PD: I think you could consider to use reflection in your approach. so you don't have to maintain the function.
This is why const_cast was created. Just remember not to shoot your foot off.
Edit: OK, I concede - const_cast wasn't made for this problem at all. This might work with your compiler, but you can't count on it and if demons come flying out of your nostrils, please don't blame me.
tl;dr; : It's Undefined Behavior.
Reference/reason: CppCon 2017: Scott Schurr “Type Punning in C++17: Avoiding Pun-defined Behavior, #24m52s +- ”
My interpretation, by example:
Suppose you create an object of type T, which have some const members. You can pass this object as a non-const reference to a function f(&T) that manipulates it, but you'd expect the const members to remain unalterable after the call. swap can be called in non-const references, and it can happen inside the function f, breaking the premise of const members to the caller.
Every part of your code that uses swap would have to assert that the object of type T being swapped does not belong to any context where the const members are assumed constant. That is impossible to automatically verify*.
*I just assumed that this is impossible to verify because it seems like an extension of the undecidability of the halting problem.