I have this code:
template <class T>
class Something
{
T val;
public:
inline Something() : val() {}
inline Something(T v) : val(v) {}
inline T& get() const { return val; }
inline Something& operator =(const Something& a) { val = a.val; return *this; }
};
typedef Something<int> IntSomething;
typedef Something<const int> ConstIntSomething;
class Other
{
public:
IntSomething some_function()
{
return IntSomething(42);
}
ConstIntSomething some_function() const
{
return ConstIntSomething(42);
}
};
void wtf_func()
{
Other o;
ConstIntSomething s;
s = o.some_function();
}
However, the compiler picks the wrong overload of Other::some_function() in wtf_func() (i.e. the non-const one). How can I fix this? Note that for certain reasons I cannot change the name of Other::some_function().
o is not const-qualified, so the non-const some_function is selected. If you want to select the const-qualified overload, you need to add the const qualifier to o:
Other o;
Other const& oref(o);
ConstIntSomething s;
s = oref.some_function();
When overload resolution occurs, the compiler only looks at the o.some_function() subexpression; it does not look at the context around the function call to decide to pick something else. Further, the return type of a member function is not considered during overload resolution.
Note that it may make more sense for IntSomething to be implicitly convertible to ConstIntSomething, either using an operator ConstIntSomething() overload in IntSomething (less good) or using a non-explicit ConstIntSomething(IntSomething const&) constructor in ConstIntSomething (more good).
It doesn't pick the wrong overload; const-ness is resolved by whether this is const or not. In your case, o is non-const, so the non-const overload is picked.
You can hack this by creating a const-reference to o, e.g.:
const Other &o2 = o;
s = o2.some_function();
But really, you should probably be considering your overloads in Something. For instance, you can't currently do this:
IntSomething x;
ConstIntSomething y;
y = x;
which doesn't sound correct. Why shouldn't you be allowed to take a const ref to a non-const ref?
Your object o needs to be const object for a const function to be called on it. Otherwise the compiler rightly picks up the non const version of the function.
The compiler picks the overload to use based on the constness of the object that will become this. You can make it call the desired version with static_cast: s = static_cast<const Other&>(o.some_function());
You might also want to copy the new behaviour found in the containers of the C++0x standard library. Containers such as vector now have members cbegin() and cend() that return a const_iterator whether the container is const or not unlike begin() and end()
class Other {
// Rest of other
public:
// No overload for non-const
// Even if called with a non const Other, since this member is marked
// const, this will be of type Other const * in all cases and will call
// the const qualified overload of some_function.
ConstIntSomething csome_function() const
{
return some_function();
}
};
Related
I tried to wrap something similar to Qt's shared data pointers for my purposes, and upon testing I found out that when the const function should be called, its non-const version was chosen instead.
I'm compiling with C++0x options, and here is a minimal code:
struct Data {
int x() const {
return 1;
}
};
template <class T>
struct container
{
container() {
ptr = new T();
}
T & operator*() {
puts("non const data ptr");
return *ptr;
}
T * operator->() {
puts("non const data ptr");
return ptr;
}
const T & operator*() const {
puts("const data ptr");
return *ptr;
}
const T * operator->() const {
puts("const data ptr");
return ptr;
}
T* ptr;
};
typedef container<Data> testType;
void testing() {
testType test;
test->x();
}
As you can see, Data.x is a const function, so the operator -> called should be the const one. And when I comment out the non-const one, it compiles without errors, so it's possible. Yet my terminal prints:
"non const data ptr"
Is it a GCC bug (I have 4.5.2), or is there something I'm missing?
If you have two overloads that differ only in their const-ness, then the compiler resolves the call based on whether *this is const or not. In your example code, test is not const, so the non-const overload is called.
If you did this:
testType test;
const testType &test2 = test;
test2->x();
you should see that the other overload gets called, because test2 is const.
test is a non-const object, so the compiler finds the best match: The non-const version. You can apply constness with static_cast though: static_cast<const testType&>(test)->x();
EDIT: As an aside, as you suspected 99.9% of the time you think you've found a compiler bug you should revisit your code as there's probably some weird quirk and the compiler is in fact following the standard.
It doesn't matter whether Data::x is a constant function or not. The operator being called belongs to container<Data> class and not Data class, and its instance is not constant, so non-constant operator is called. If there was only constant operator available or the instance of the class was constant itself, then constant operator would have been called.
But testType is not a const object.
Thus it will call the non const version of its members.
If the methods have exactly the same parameters it has to make a choice on which version to call (so it uses the this parameter (the hidden one)). In this case this is not const so you get the non-const method.
testType const test2;
test2->x(); // This will call the const version
This does not affect the call to x() as you can call a const method on a non const object.
I have an interesting predicament here where the code will compile if I pass a variable by value to the function but not if I pass it by reference, and I'm not sure why. In header.h:
#include <iostream>
#include <string>
void get_name(std::string &name)
{
getline(std::cin, name);
return;
}
template <class T, class U>
class readonly
{
friend U;
private:
T data;
T& operator=(const T& arg) {data = arg; return data;}
public:
operator const T&() const {return data;}
};
class myClass
{
private:
typedef readonly<std::string, myClass> RO_string;
public:
RO_string y;
void f()
{
get_name(y); // compile error
}
};
The main.cpp implementation file simply includes this header file, creates an instance of myClass and then calls f(). When doing so, it won't compile properly. The problem lies in the fact that I'm passing the variable y by reference to the get_name function. If I change the function so that I pass by value instead, everything compiles properly and works as expected (except I'm obviously not making changes to y anymore). However, I don't understand this behavior. Why does this happen and is there an optimal way to fix it in this situation?
Your conversion operator:
operator const T&() const {return data;}
Only allows implicit conversion to a const reference. This makes sense since you are interested in making something read only. But get_name requires a non-const reference to a string. That is, get_name expects to be able to mutate its input. The type being passed in cannot convert to a mutable string reference, only to a constant string reference, so it doesn't compile.
When you pass by value, it simply constructs a new string from the const reference to pass into the function, which is fine.
Intuitively, a function called get_name should probably take a const reference since it shouldn't need to mutate its input just to get the name.
As explained by Nir Friedman, get_name(y); invoques the implicit conversion operator which gives you a const reference.
But, as you wrote friend U;, myClass has access to the private member data of y and allows you to do:
get_name(y.data);
I have some code which has been written without regard for const correctness. Are there any circumstances in which changing this
class X
{
public:
X(X& rhs); // does not modify rhs
...
};
to this
class X
{
public:
X(const X& rhs);
...
};
would change the behaviour of an existing program? I know this change will allow code which doesn't currently compile to compile, but I'm interested if there is any circumstance in which code that already compiles would change it's behaviour.
Similar question, is there any merit in making this change instead?
class X
{
public:
X(X& rhs); // does not modify rhs
X(const X& rhs);
...
};
For copy constructor I don't think so. But note that in general, yes, declaration of const can affect which method is called. The only example that comes to mind is with array overloading - see e.g. this question.
In fact a copy constructor should, imho, always take a const reference as its argument.
X(X& rhs) { } // does not modify rhs
This does not allow to copy const objects and hence the following code will not compile.
While non-const objects can serve as const arguments, the other way round is impossible
X const test;
X new_x(test);
I can't imagine why someone should preclude the copy of a const object.
Concerning the changes you want to make:
Does the copy constructor rely on any X member function that is defined non-const?
This will work like a charm but permit copying const objects:
class X
{
private:
int a;
public:
X(X &rhs) { a = rhs.value(); }
int& value (void) { return a; }
};
The next example will not compile since rhs is const but value() is not const.
class X
{
private:
int a;
public:
X(X const &rhs) { a = rhs.value(); }
int& value (void) { return a; }
};
If you want to make your class const correct you'll probably have to examine the whole class.
It should only affect your in-class-implementations. Since I don't know a case where external code should rely on "non-constness" of a class member function.
Except when non-const-references are returned by any public member-functions as in my example.
The following snippet will do as intended.
class X
{
private:
int a;
public:
X(int const &b) : a(b) { }
X(X const &rhs) { a = rhs.value(); }
int const & value (void) const { return a; }
};
But be aware that this will interfere with any code like:
X test(100);
test.value() = 12;
This would work using int& value (void) { return a; } but fails with int const & value (void) const { return a; }.
You could of course provide both to be on the safe side.
Since you are changing the class that has the copy constructor I am assuming you can inspect the copy constructors code. If you can make this change and the copy constructor does not give a compiler error you are probably good. One conor case to consider is what the copy assignment operator does as there is no assurance which will be called especially in optimized code. So also make sure that your copy assignment will work with a const parameter.
djechlin is making an important point in his answer, although somewhat unclear, so I will try to explain better.
The constness of an object or reference affects overload resolution. For instance, if you have an object with a const based overload of a member function, different overloads will be chosen:
struct foo {
void do_stuff();
void do_stuff() const;
};
int main() {
foo f;
const foo& fr = f;
f.do_stuff(); // calls non-const version
fr.do_stuff(); // calls const version
}
Now, if one of the overloads has side-effects the other doesn't, you'd get different behavior after changing the signature, given that (or rather even though) the program compiles fine.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How do I remove code duplication between similar const and non-const member functions?
i have two members
A &B::GetA (int i)
{
return *(m_C[m_AtoC[i]]);
}
const A &B::GetA (int i) const
{
return *(m_C[m_AtoC[i]]);
}
for now i just duplicate code, but may be there exists nice way to do it. I certanly dont want to deal with type cast from const to non-const.
EDIT: So i want to call one member frm another to avoid code duplication.
[Note the correction; sorry for getting it the wrong way round initially.]
This is an appropriate situation for using a const_cast, and it allows you to deduplicate code by forwarding the call from the non-const function to the corresponding const function:
A & B::GetA(int index)
{
return const_cast<A &>(static_cast<B const *>(this)->GetA(index));
}
It's important to note that this should only be done in one direction: You can implement the non-const method in terms of the constant one, but not the other way round: The constant call to GetA() gets a constant reference to the object in question, but since we have additional information that it's actually OK to mutate the object, we can safely cast away the constness from the result.
There's even a chapter on precisely this technique in Scott Meyer's Effective C++.
You could do something like:
class B {
public:
A& GetA (int index) { return GetA_impl(index); }
const A& GetA (int index) const { return GetA_impl(index); }
private:
A& GetA_impl (int index) const { return *(m_C[m_AtoC[i]]); }
};
I'm not sure it's really worth the effort in this case, but this can be useful if the potentially duplicated code gets more complicated.
You can avoid const_cast with a little template metaprogramming.
// This meta function returns a const U if T is const, otherwise returns just U.
template <typename T, typename U>
make_same_const<T, U>
{
typedef typename std::conditional<
std::is_const<T>::value,
typename std::add_const<U>::type,
U
>::type type;
};
// Generic version of a function. Constness of return type depends on
// constness of T.
// This is a static member template of B.
template <typename T>
make_same_const<T, A>::type& B::GetA(T& obj, int i)
{
return *(obj.m_C[obj.m_AtoC[i]]);
}
A& B::GetA(int i) { return B::GetA(*this, i); }
A const& B::GetA(int i) const { return B::GetA(*this, i); }
IMO this isn't enough code (or complexity) to be worth de-duplicating.
As you can see in both the const_cast-based solutions, the cast expression is actually longer than the original code.
If you have a longer or more complex expression you're really worried about, though, please show it.
Assuming the bodies of GetA() and GetA() const are identical (which means GetA() doesn't modify *this), you can safely use one const_cast to implement the const version using the non-const one:
const A& B::GetA() const {
return const_cast<B*>(this)->GetA();
}
The non-const GetA() doesn't modify the object, so calling it on a const B object is not undefined. The non-const reference returned by non-const GetA() is converted to a const& before being returned out of GetA() const, so there's no danger of undefined behaviour there either.
How about
const A &B::GetA (int index) const
{
return *(const_cast<B*>(this)->GetA(index));
}
I define two versions of overloaded operator[] function in a class array. ptr is a pointer to first element of the array object.
int& array::operator[] (int sub) {
return ptr[sub];
}
and
int array::operator[] (int sub) const {
return ptr[sub];
}
Now, if I define a const object integer1 the second function can only be called..... but if I make a non-const object and then invoke as below:
cout << "3rd value is" << integer1[2];
which function is called here?
In your second example, the non-const version will be called, because no conversion is required, and a call that requires no conversion is a better match than one that requires a conversion.
Ultimately, however, you have a basic problem here: what you really want is behavior that changes depending on whether you're using your object as an rvalue or an lvalue, and const doesn't really do that. To make it work correctly, you normally want to return a proxy object, and overload operator= and operator T for the proxy object:
template <class T>
class myarray {
T *ptr;
class proxy {
T &val;
proxy &operator=(proxy const &p); // assignment not allowed.
public:
proxy(T &t) : val(t) {}
operator T() const { return val; }
proxy &operator=(T const&t) { val = t; return *this; }
};
proxy const operator[](int sub) const { return proxy(ptr[sub]); }
proxy operator[](int sub) { return proxy(ptr[sub]); }
// obviously other stuff like ctors needed.
};
Now we get sane behavior -- when/if our array<int> (or whatever type) is const, our operator[] const will be used, and it'll give a const proxy. Since its assignment operators are not const, attempting to use them will fail (won't compile).
OTOH, if the original array<int> was not const, we'll get a non-const proxy, in which case we can use both operator T and operator=, and be able to both read and write the value in the array<int>.
Your const version should return const int& not int, so that the semantics are just the same between the two functions.
Once you've done that, it doesn't matter which one is used. If the const version has to be used because your object has a const context, then it will be... and it won't matter as you're not trying to modify anything. Otherwise, it'll use the non-const version... but with just the same effect.