const to Non-const Conversion in C++ - c++

I'm really annoyed by const keyword these days, as I'm not quite familiar with it. I had a vector that stores all const pointers like vector<const BoxT<T> *> *Q_exclude, and in the constructor of another class, I need an element in this queue to be passed in as a parameter and assign it to a non-const member. My question is:
How do I assign a const variable to a non-const variable? I know this doesn't make sense because after all, a const is a const, and should not be changed by any mean. But that annoying member variable REALLY has to be changed during the process! I might also change the data type in the vector to be non-const, but that would be too much work. Or does anyone know how to avoid such situation?

You can assign a const object to a non-const object just fine. Because you're copying and thus creating a new object, constness is not violated.
Like so:
int main() {
const int a = 3;
int b = a;
}
It's different if you want to obtain a pointer or reference to the original, const object:
int main() {
const int a = 3;
int& b = a; // or int* b = &a;
}
// error: invalid initialization of reference of type 'int&' from
// expression of type 'const int'
You can use const_cast to hack around the type safety if you really must, but recall that you're doing exactly that: getting rid of the type safety. It's still undefined to modify a through b in the below example:
int main() {
const int a = 3;
int& b = const_cast<int&>(a);
b = 3;
}
Although it compiles without errors, anything can happen including opening a black hole or transferring all your hard-earned savings into my bank account.
If you have arrived at what you think is a requirement to do this, I'd urgently revisit your design because something is very wrong with it.

Changing a constant type will lead to an Undefined Behavior.
However, if you have an originally non-const object which is pointed to by a pointer-to-const or referenced by a reference-to-const then you can use const_cast to get rid of that const-ness.
Casting away constness is considered evil and should not be avoided. You should consider changing the type of the pointers you use in vector to non-const if you want to modify the data through it.

The actual code to cast away the const-ness of your pointer would be:
BoxT<T> * nonConstObj = const_cast<BoxT<T> *>(constObj);
But note that this really is cheating. A better solution would either be to figure out why you want to modify a const object, and redesign your code so you don't have to.... or remove the const declaration from your vector, if it turns out you don't really want those items to be read-only after all.

Leaving this here for myself,
If I get this error, I probably used const char* when I should be using char* const.
This makes the pointer constant, and not the contents of the string.
const char* const makes it so the value and the pointer is constant also.

void SomeClass::changeASettingAndCallAFunction() const {
someSetting = 0; //Can't do this
someFunctionThatUsesTheSetting();
}
Another solution is to call said function in-between making edits to variables that the const function uses. This idea was what solved my problem being as I was not inclined to change the signature of the function and had to use the "changeASettingAndCallAFunction" method as a mediator:
When you call the function you can first make edits to the setting before the call, or (if you aren't inclined to mess with the invoking place) perhaps call the function where you need the change to the variable to be propagated (like in my case).
void SomeClass::someFunctionThatUsesTheSetting() const {
//We really don't want to touch this functions implementation
ClassUsesSetting* classUsesSetting = ClassUsesSetting::PropagateAcrossClass(someSetting);
/*
Do important stuff
*/
}
void SomeClass::changeASettingAndCallAFunction() const {
someFunctionThatUsesTheSetting();
/*
Have to do this
*/
}
void SomeClass::nonConstInvoker(){
someSetting = 0;
changeASettingAndCallAFunction();
}
Now, when some reference to "someFunctionThatUsesTheSetting" is invoked, it will invoke with the change to someSetting.

Related

Reference from literal

I am calling a function with the signature
void setValue(int& data)
I would like to pass a literal number to it:
setValue(1);
But I get:
error: invalid initialization of non-const reference of type 'int&' from an rvalue of type 'int'
Is there a way I can make this work without changing the function (it's in a library) and without assigning each literal value to a variable?
Assuming setValue does not actually modify its argument and just has a wrong signature which you cannot change, here is an approach which is not thread-safe among other things:
#include <iostream>
void setValue(int &i)
{
std::cout << "i = " << i << std::endl;
}
int& evil(int i)
{
static int j;
j = i;
return j;
}
int main()
{
setValue(evil(1));
setValue(evil(2));
}
When you declare the argument as being an int&, you are saying that the function called can change the value and the caller will see the change.
So it is no longer valid to pass a literal value then because how could the function possibly change the given value of a literal?
If you don't want the setValue to be able to change the given value, make the argument either be an int or const int&. And if you do want the setValue function to be able to change the value, then the caller must declare a non-const variable to hold the int and pass in that.
Can I change something at the call site to make it work
The problem with your code is that you declared your function to expect a reference, which means the compiler has to prepare the code to allow the function to change whatever you pass into it at the call site. So yes, sure, you can declare a variable, set it to 1 and call your function with it.
Contrast this with a constant reference in the declaration, where the compiler knows you won't change it inside the function, and then you can pass a literal in without issues. In fact, any logical, thought out design will make setters accept constant parameters because it won't change them, it will just store a possibly processed value in its state.
The answer to „what do I do if a library has a bad interface and I can't change it“ is usually „write a wrapper“. Assuming this is a method of some class BadLibraryClass, you could do something like:
class Wrapper {
public:
BadLibraryClass inner;
setValue(int i) {
inner.setValue(i); // i is an lvalue
}
};
This is just a crude example. Perhaps inner is better off being a pointer, a reference or even a smart pointer. Perhaps you want a conversion operator to BadLibraryClass. Perhaps you can use inheritance to expose other methods of BadLibraryClass.
Two options:
Use the result of assignment:
static int _data;
void myCall() {
setValue((_data = 3));
}
Write a wrapper:
struct setValueW {
int _data;
// constructor
setValueW(int _data) : _data(_data) {
setValue(_data);
}
// if you want to call it again
void operator()() {
setValue(_data);
}
};
void myCall2() {
setValueW(3);
}
AFAIK, references keeps the addresses of the variable. 1 is not variable. It is temporary.
Take a look this article(this is a quote from this site)
c++11 introduced a new kind of reference variable -- an r-value reference
To declare one, use && after a type
int & // type designation for an L-value reference
int && // type designation for an R-value reference
L-value references can only refer to L-values
R-value references can reference to R-values (temporaries)
int x, y, z; // regular variables
int & r = x; // L-value reference to the variable x
int & r2 = x + y; // This would be ILLEGAL, since x + y is an R-value
int && r3 = x + y; // LEGAL. R-value reference, referring to R-value
So you can use (But this is not useful. It may be more useful if you write this in plain without rvalue or lvalue.):
void setValue(int&& data)
setValue(1);
Or you can use that:
void setValue(int& data)
int a = 11;
setValue(a);
Don't forget for second example. If you change the value of data parameter. You will have change the a variable value.
No, you can't.
An lvalue reference like that binds to a variable (roughly speaking).
Your literal is not such a thing. It never had a name, and may not even have a home in memory.
Your two options are the two things you ruled out, I'm afraid.
For what it's worth, this is not your fault: that is a rather poor setter. It should take const int& (which will automatically create a nice temporary variable for you out of the literal!), or even just const int.

Using const_cast to add const-ness - bad idea?

as we all know the usage of const_cast to remove the const-ness of a pointer should be avoided.
But how is it about the other way around?
For my use case I have a function that copies data (bytes) from a non-const source buffer.
I thought a good design decision would be to declare the parameter according to that source buffer fully const.
void copyfunction(const char* const data) { ... }
For a function call like below this would lead to a pointer-type error 'const char* const <-> char*'.
void main() {
char sourcebuffer[] = {0x00};
copyfunction(sourcebuffer);
}
Sure, now I could simply declare the sourcebuffer as const but in my case I don't have access to that variable because it's from a different code location (external library).
void main() {
char sourcebuffer[] = {0x00};
copyfunction(const_cast<const char* const>(sourcebuffer));
}
However the code beyond would work but is it good style (according to my use case)?
I thought declaring the parameter of the copyfunction as const assures the user of not modifying (read-only) the pointer or the location of the source buffer itself.
So in this case the const_cast would only be a necessary evil to enable to function call and not willfully remove the const-ness of a pointer...
Greets
You should not use const_cast to add const, because:
it's unnecessary. T* converts implicitly to const T*. Your question states that char sourcebuffer[] = {0x00}; copyfunction(sourcebuffer); is an error, but that's not true.
it's potentially (albeit unlikely) harmful. It can remove volatile from the pointer type, which is not the intention here and would result in undefined behavior if sourcebuffer were declared as volatile sourcebuffer[].
You should not use const_cast to add const because
In the cases where the operation is safe, it is almost always not required. int* turns into a const int* implicitly.
It can do something you don't want it to do. It can strip volatile, or make you miss the fact that const was added somewhere else in your variables and your const_cast now silently strips them.
In the cases where it is required to add const, its use is dangerous in hard to reason about ways.
There are cases where you need to call const_cast in order to add const that will not happen implicitly.
void assign_ptr( int const*& lhs, int const* rhs ) { lhs = rhs; }
int const foo = 7;
int* bar = nullptr;
assign_ptr( const_cast<int const*&>(bar), &foo );
*bar = 2; // undefined behavior!
std::cout << foo << "#" << &foo << "\n"; // will print probably 7#something
std::cout << *bar << "#" << bar << "\n"; // will print probably 2#same address as above!
the above call to assign_ptr only adds const, but it will not happen implicitly.
A side effect of it is that modification of *bar is undefined behavior, as it modifies a variable declared const (it makes bar, a int*, point at foo a const int).
So while const_cast is required to make the assign_ptr call compile, it is because it was unsafe. The const_cast doesn't make it safer, it just hides the error.
This is a specific case of the rectangle-square problem. Squares are not Rectangles, because if you change the width of a Square its height also changes, and this does not happen when you modify a Rectangle. Similarly, int** are not int const**. (Note that immutable Squares are a kind of immutable Rectangle; it is the mutation that causes the issue. In the case of pointers, a int*const* is a int const*const*: the mutability of the "higher level" pointers causes the problem.)

c++ const member function that returns a const pointer.. But what type of const is the returned pointer?

I apologize if this has been asked, but how do I create a member function in c++ that returns a pointer in the following scenerios:
1. The returned pointer is constant, but the junk inside can be modified.
2. The junk inside is constant but the returned pointer can be modified.
3. Neither the junk, nor the pointer can be modified.
Is it like so:
int *const func() const
const int* func() const
const int * const func() const
All of the tutorials I've read don't cover this distinction.
Side note:
If my method is declared const then the tutorials say that I'm stating that I won't modify the parameters.. But this is not clear enough for me in the case when a parameter is a pointer. Do my parameters need to be like:
a. void func(const int* const x) const;
b. void func(const int* x) const;
c. void func(const int* const x) const;
I don't know what book you have read, but if you mark a method const it means that this will be of type const MyClass* instead of MyClass*, which in its turn means that you cannot change nonstatic data members that are not declared mutable, nor can you call any non-const methods on this.
Now for the return value.
1 . int * const func () const
The function is constant, and the returned pointer is constant but the 'junk inside' can be modified. However, I see no point in returning a const pointer because the ultimate function call will be an rvalue, and rvalues of non-class type cannot be const, meaning that const will be ignored anyway
2 . const int* func () const
This is a useful thing. The "junk inside" cannot be modified
3 . const int * const func() const
semantically almost the same as 2, due to reasons in 1.
HTH
Some uses of const don't really make much sense.
Suppose you have the following function:
void myFunction (const int value);
The const tells the compiler that value must not change inside the function. This information does not have any value for the caller. It's up to the function itself to decide what to do with the value. For the caller, the following two function definitions behave exactly the same for him:
void myFunction (const int value);
void myFunction (int value);
Because value is passed by value, which means that the function gets a local copy anyway.
On the other hand, if the argument is a reference or a pointer, things become very different.
void myFunction (const MyClass &value);
This tells the caller that value is passed by reference (so behind the screens it's actually a pointer), but the caller promises not to change value.
The same is true for pointers:
void myFunction (const MyClass *value);
We pass a pointer to MyClass (because of performance reasons), but the function promises not to change the value.
If we would write the following:
void myFunction (MyClass * const value);
Then we are back int he first situation. myFunction gets a pointer, which is passed by value, and which is const. Since MyFunction gets a copy of the pointer value, it doesn't matter for the caller whether it is const or not. The most important thing is that myFunction can change the contents of value, because the pointer variable itself is const, but the contents in it isn't.
The same is true for return values:
const double squareRoot(double d);
This doesn't make any sense. squareRoot returns a const double but since this is passed 'by value', and thus needs to be copied to my own local variable, I can do whatever I want with it.
On the other hand:
const Customer *getCustomer(char *name);
Tells me that getCustomer returns me a pointer to a customer, and I am not allowed to change the contents of the customer.
Actually, it would be better to make the char-pointer-contents const as well, since I don't expect the function to change the given string:
const Customer *getCustomer(const char *name);
int *const func() const
You cannot observe the const here except for a few cases
Taking the address of func.
In C++0x, directly calling func with the function-call syntax as a decltype operand, will yield int * const.
This is because you return a pure pointer value, that is to say a pointer value not actually stored in a pointer variable. Such values are not const qualified because they cannot be changed anyway. You cannot say obj.func() = NULL; even if you take away the const. In both cases, the expression obj.func() has
the type int* and is non-modifiable (someone will soon quote the Standard and come up with the term "rvalue").
So in contexts you use the return value you won't be able to figure a difference. Just in cases you refer to the declaration or whole function itself you will notice the difference.
const int* func() const
This is what you usually would do if the body would be something like return &this->intmember;. It does not allow changing the int member by doing *obj.func() = 42;.
const int * const func() const
This is just the combination of the first two :)
Returning a pointer to const makes a lot of sense, but returning a const pointer (you cannot modify) usually adds no value (although some say it can prevent user errors or add compiler optimisation).
That is because the return value belongs to the caller of the function, i.e. it is their own copy so it doesn't really matter if they modify it (to point to something else). The content however does not "belong" to the caller and the implementor of the function may make a contract that it is read-only information.
Const member functions promise not to change the state of the class, although this is not necessarily enforced in reality by the compiler. I am not referring here to const_cast or mutable members so much as the fact that if your class itself contains pointers or references, a const member function turns your pointers into constant pointers but does not make them pointers to const, similarly your references are not turned into references-to-const. If these are components of your class (and such components are often represented by pointers) your functions can change their state.
Mutable members are there for the benefit of allowing your class to change them whilst not changing internal state. These can typically be applied to:
Mutexes that you wish to lock even for reading.
Data that is lazy-loaded, i.e. filled in the first time they are accessed.
Reference-counted objects: You want to increase the reference count if it has another viewer, thus you modify its state just to read it.
const_cast is generally considered a "hack" and is often done when someone else has not written their code properly const-correct. It can have value though in the following situations:
Multiple overloads where one is const and one non-const and the const returns a const-reference and the non-const returns a non-const reference, but otherwise they are the same. Duplicating the code (if it is not a simple data member get) is not a great idea, so implement one in terms of the other and use const_cast to get around the compiler.
Where you want in particular to call the const overload but have a non-const reference. Cast it to const first.
The const method prevents you from modifying the members. In case of pointers, this means you can't reassign the pointer. You can modify the object pointed at by the pointer to your heart's desire.
As the pointer is returned by value (a copy), the caller can't use it to modify the pointer member of the class. Hence adding const to the return value adds nothing.
Things are different if you were to return a reference to the pointer. Now, if the pointer weren't const, this would mean that a function that doesn't have rights to modify a value is granting this right to the caller.
Example:
class X
{
int* p;
public:
int* get_copy_of_pointer() const //the returned value is a copy of this->p
{
*p = 42; //this being const doesn't mean that you can't modify the pointee
//p = 0; //it means you can't modify the pointer's value
return p;
}
int* const& get_reference_to_pointer() const //can't return a reference to non-const pointer
{
return p;
}
};

What is a reference in C?

I have just started C++ and have come across references and have not understood completely.
References , as i read is an alternative name for an object.Why use that instead of directly accessing the object as any operation on references is directly reflected on the object ...?
Why and when are they used ?
Is ist like a constant pointer that is referenced each time it is used ... ?
And , it says
double& dr = 1; ---- says it is an error (some lavalue needed)
const double& cdr = 1; ---- says it is ok.
i dont understand it properly..So please explain why it is so ...
Thank You...:)
Why use that instead of directly
accessing the object as any operation
on references is directly reflected on
the object ...?
C++ passes parameters by value, meaning if you have a function such as:
void foo(MyObject o) { ... }
By default C++ will make a copy of a MyObject, not directly use the object being passed in. So, one use of references is to ensure you are working on the same object:
void foo(MyObject &o) { ...}
Or, if you aren't modifying o:
void foo(const MyObject &o) { ... }
References are another way of what was originally in C code like this
void fubarSquare(int *x){
int y = *x;
*x = y * y;
}
// typical invocation
int z = 2;
fubarSquare(&z);
// now z is 4
with references in C++ it would be like this
void fubarSquareCpp(int& x){
x = x * x;
}
// typical invocation
int z = 2;
fubarSquareCpp(z);
// now z is 4
It's a neater syntactical way of using a call-by-reference parameter instead of using the C's notation asterisk/star to indicate a pointer and as a call-by-reference parameter...and modifying the parameter directly outside of the function...
Have a look at Bjarne Stoustrap's page here which covers how C++ is and also here on the technical faq here
A reference is basically a pointer that looks like an object. It is very very hard to get a NULL reference though you can go through hoops and create one.
With regards to your example, 1 is an rvalue or a result. It is just a temporary variable and can not be modified. Thus you can't take a non const reference to it. However you can take a const reference to it. This means you can't change the value of the reference.
Here is an example of creating a NULL reference. Don't do it!
int * x = (int *)NULL;
int & y = *x;
I agree with you. using references as just an alias name is not very useful.
It is more useful if you consider it as an immutable pointer. But not that useful in fact.
Practically, it is used to define clean interfaces. For example when you define:
int foo(const int& param);
You say that param is a read-only parameter in foo.
Do not forget that you MUST assign a value to a reference.
See the C++ faqlite on references for more
my2c
References improve the syntax, so no pointer dereference needed.
Assuming Base is a class that may be derived from:
void someFunction(Base b)
{
b.function();
// b is a copy of what was passed - probably performance issues
// possible unintended object slicing - you only get the Base part of it
// no virtual function call
// no changes to b visible outside the function
}
void someFunction(Base* b)
{
b->function();
// a shortcut for (*b).function();
// b is the same object that was passed to the function
// possible virtual call
// changes visible outside the function
}
void someFunction(Base& b)
{
b.function();
// b is the same object that was passed to the function
// possible virtual call
// changes visible outside the function
}
References are like constant pointers (NOT pointers to constants - i.e. you can change the object, but you can't change to what you're pointing). const reference is a reference through which you can do things that can be done on const object.
References are also good, because you can't have a null reference
Give the wikipedia article a good read through. To sum it up, references are more friendly version of pointers which are commonly used to pass objects as references into functions without worrying about a null pointer.
To explain the example:
Think of the number 1 represented as a variable. When compiled, this number is put into the global section of the memory which can be referenced by the program, but not modified.
So it is of type: const int
double &dr = 1 is trying to assign dr (a reference to a double) to the const int 1. Since 1 is a constant, the compiler will not allow you to make a non-constant reference to it.
In the second line:
const double &dr = 1 is trying to assign dr (a constant reference to a double) the const int 1. This works because the reference is also const and therefore can point to a const int.
EDIT
The const int is converted to a const double before assigned.
References are language entitities that represent another object they refer to. Nonconst references are lvalues, and must be initialized with an lvalue. They can be useful like this:
int& x=condition ? array[1] : array[2];
int& y=condition ? array[0] : array[3];
x+=y;
y=0;
When used as a function parameter, they tell the caller he has to pass an lvalue that might be written to by the function:
void set1(int& x) { x=1; }
int foo;
set1(foo); // ok, foo is 1
set1(foo+1); // not OK, not lvalue
Const references, on the other hand, can be bound to rvalues. In function parameters, they are usually used to avoid excessive copies:
void niceness(std::string s); // the string would be copied by its copy-ctor
void niceness(const std::string& s); // the caller's string would be used
Note that this may or may not yield faster code.
When const-references are used in normal code, they can bind rvalues, too, and as a special rule, they extend the lifetime of the object they are bound to. This is what you saw in your code:
const double& d=1; // OK, bind a rvalue to a const-ref
double& d=1; // Bad, need lvalue
All references are polymorphic, like pointers:
class A { virtual void f(); }
class B : public A { void f(); }
B b;
A& ar=b;
ar.f(); // calls B::f()
and all references are aliases like pointers:
int f(int& a, const int& b)
{
a=1;
return b;
}
int x;
f(x, 42); // ==42, foo=1
x=42;
f(x, x); // ==1 (not 42), foo=1
double& dr = 1; // 1.0 would be more clear
Is invalid because 1 is viewed to be of type const double so if you want a reference to that variable you need to have a reference to a const double so
const double& dr = 1.0;
Is correct.
Utility of references is most visible in the context of passing parameters to functions.
I.e,
int a;
func definition: void foo (int& param) {param = 1;}
func call: foo(a);
The way as 'param' aliases 'a' is clean and its intention is easily understood by a reader of this code as well as compiler that may optimize away when inlining any additional memory allocation needed for the reference.
Passing a reference to a function and then having the function use the reference is almost like passing a pointer to the function and then having the function dereference the pointer. In many cases, the machine-code implementation will be identical. There are some differences, though, especially in the case of functions that get expanded inline. If a variable is passed by reference to an inline function, the compiler will often be able to substitute the variable itself--even if stored in a machine register--when expanding the function. By contrast, if one takes the address of a variable and passes that as a pointer to a function which then dereferences it, the compiler is less likely to figure out that optimization unless it determines not only that--at least for one particular expansion of the function--the pointer will always point to that variable, but also that the pointer will not be used anywhere else (if the pointer was used elsewhere, the variable could not be kept in a register).

Is const_cast safe?

I can't find much information on const_cast. The only info I could find (on Stack Overflow) is:
The const_cast<>() is used to add/remove const(ness) (or volatile-ness) of a variable.
This makes me nervous. Could using a const_cast cause unexpected behavior? If so, what?
Alternatively, when is it okay to use const_cast?
const_cast is safe only if you're casting a variable that was originally non-const. For example, if you have a function that takes a parameter of a const char *, and you pass in a modifiable char *, it's safe to const_cast that parameter back to a char * and modify it. However, if the original variable was in fact const, then using const_cast will result in undefined behavior.
void func(const char *param, size_t sz, bool modify)
{
if(modify)
strncpy(const_cast<char *>(param), sz, "new string");
printf("param: %s\n", param);
}
...
char buffer[16];
const char *unmodifiable = "string constant";
func(buffer, sizeof(buffer), true); // OK
func(unmodifiable, strlen(unmodifiable), false); // OK
func(unmodifiable, strlen(unmodifiable), true); // UNDEFINED BEHAVIOR
I can think of two situations where const_cast is safe and useful (there may be other valid cases).
One is when you have a const instance, reference, or pointer, and you want to pass a pointer or reference to an API that is not const-correct, but that you're CERTAIN won't modify the object. You can const_cast the pointer and pass it to the API, trusting that it won't really change anything. For example:
void log(char* text); // Won't change text -- just const-incorrect
void my_func(const std::string& message)
{
log(const_cast<char*>(&message.c_str()));
}
The other is if you're using an older compiler that doesn't implement 'mutable', and you want to create a class that is logically const but not bitwise const. You can const_cast 'this' within a const method and modify members of your class.
class MyClass
{
char cached_data[10000]; // should be mutable
bool cache_dirty; // should also be mutable
public:
char getData(int index) const
{
if (cache_dirty)
{
MyClass* thisptr = const_cast<MyClass*>(this);
update_cache(thisptr->cached_data);
}
return cached_data[index];
}
};
I find it hard to believe that that's the only information you could find about const_cast. Quoting from the second Google hit:
If you cast away the constness of an
object that has been explicitly
declared as const, and attempt to
modify it, the results are undefined.
However, if you cast away the
constness of an object that has not
been explicitly declared as const, you
can modify it safely.
What Adam says. Another example where const_cast can be helpful:
struct sample {
T& getT() {
return const_cast<T&>(static_cast<const sample*>(this)->getT());
}
const T& getT() const {
/* possibly much code here */
return t;
}
T t;
};
We first add const to the type this points to, then we call the const version of getT, and then we remove const from the return type, which is valid since t must be non-const (otherwise, the non-const version of getT couldn't have been called). This can be very useful if you got a large function body and you want to avoid redundant code.
The short answer is no, it's not safe.
The long answer is that if you know enough to use it, then it should be safe.
When you're casting, what you are essentially saying is, "I know something the compiler doesn't know." In the case of const_cast, what you are saying is, "Even though this method takes in a non-const reference or pointer, I know that it won't change the parameter I pass it."
So if you do actually know what you are claiming to know in using the cast, then it's fine to use it.
You're destroying any chance at thread-safety, if you start modifying things that the compiler thought were const.