c++
I was reading about const_cast operator in c++
1.First weird thing thing i can't understand is
const_cast operator syntax ie
-const_cast--<--Type-->--(--expression--)--------------------><
what i have understand about this syntax is that it helps to cast away constness of an expression of type Type .But consider this code
class ConstTest {
private:
int year;
public:
ConstTest() : year(2007) {}
void printYear() const;
};
int main() {
ConstTest c;
c.printYear();
return 0;
}
void ConstTest::printYear() const {
ConstTest *c = const_cast<ConstTest*>(this);
c->year = 42;
std::cout << "This is the year " << year << std::endl;
}
Here in line ConstTest *c = const_cast<ConstTest*>(this) , I think that the const of this pointer should be cast away, but the output shows that it is the object which this refers to that loses its const-ness.
I feel that the code should have been ConstTest *c = const_cast<ConstTest>(*this) , but this produces an error. I know i am wrong at many interpretations. Please correct them all.
2.my second problem is the statement given below
The result of a const_cast expression is an rvalue unless Type is a reference type. In this case, the result is an lvalue.
Why is this so, and why it is not true in case of pointers?
it helps to cast away constness of an expression of type Type
No, Type is the type of the result, not the type of the operand.
What i think is const of this pointer should be casted away
this has type const ConstTest* . const_cast<ConstTest*>(this) has type ConstTest* . That's what "casting away const" from a pointer-to-const means.
I feel code should have been ConstTest *c = const_cast<ConstTest>(*this)
The result of const_cast<T> has type T, that's how it's defined. Maybe you would have defined it differently, but tough luck, you don't get a ConstTest* by writing const_cast<ConstTest> , you get it by writing const_cast<ConstTest*> . Your preferred syntax is not available.
You can either do ConstTest &c = const_cast<ConstTest&>(*this) or ConstTest *c = const_cast<ConstTest*>(this) , so pick your favorite.
The result of a const_cast expression is an rvalue unless Type is a reference type. In this case, the result is an lvalue.
why so and why it is not true in case of pointers?
It is true of pointers. ConstTest* is not a reference type, and the result of const_cast<ConstTest*>(this) is an rvalue. You then assign that value to the variable c .
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