all: this is quoted from Effective C++ 3rd editiion
const_cast is typically used to cast away the constness of objects. It is the only C++-style cast that can do this.
My question is can const_cast add constness to a non-const object? Actually i wrote a small programme trying to approve my thought.
class ConstTest
{
public:
void test() {
printf("calling non-const version test const function \n");
}
void test() const{
printf("calling const version test const function \n");
}
};
int main(int argc,char* argv){
ConstTest ct;
const ConstTest cct;
cct.test();
const_cast<const ConstTest&>(ct).test();//it is wrong to write this statement without the '&',why
}
Omitting the '&' incurs error below:
error C2440: 'const_cast' : cannot convert from 'ConstTest' to 'const ConstTest'
It shows that const_cast can add constness,but seems like you have to cast to a object reference, what is the magic of this reference ?
You don't need const_cast
to add constness:
class C;
C c;
C const& const_c = c;
The other way around needs a const_cast
though
const C const_c;
C& c = const_cast<C&>(const_c);
but behavior is undefined if you try to use non-const operations on c
.
By the way, if you don't use a reference, a copy of the object is to be made:
C d = const_c; // Copies const_c
const_cast can also be used to add constness.
$5.2.11/3 - "For two pointer types T1 and T2 where
T1 is cv1 , 0 pointer to cv1 , 1 pointer to . . . cv1 ,n − 1 pointer to cv1 ,n T
and
T2 is cv2 , 0 pointer to cv2 , 1 pointer to . . . cv2 ,n − 1 pointer to cv2 ,n T
where T is any object type or the void type and where cv1 ,k and cv2 ,k may be different cv-qualifications, an rvalue of type T1 may be explicitly converted to the type T2 using a const_cast. The result of a pointer const_cast refers to the original object.
Consider:
int *t1 = NULL; // T = int, cv1, 0 = none
int * const t2 = const_cast<int * const>(t1); // T = int, cv2, 0 = const
As per the quote above, the above is fine and it adds constness to t
But as Herb Sutter says, it probably is not required explicitly to be done most of the times.
const_cast
can only be used to cast to pointers and references. It can't be used to cast to objects. Here's why: if you have a const object you can't make it non-const and vice versa - it's already const, you can't redeclare it. You can only try to access it through a pointer or reference without (or with) const.
const_cast
cannot modify the constness of the value. So it returns a const reference to the value.
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