CODEWITHSUNDEEP
c++templatescastingconstants
yoni
yoni

Reputation: 1364

template casting operator, 'const' keyword required

I have a template class, called OneCell, here the definition:

template <class T>
class OneCell
{
.....
}

I have a cast operator from OneCell to T, here

operator T() const
{
    getterCount++;
    return value;
}

As you see, I want to increment variable in this method, but i get an error because the keyword const.

On the other hand, if I remove this keyword, the casting doesn't work at all.

What can I do?

Thank you, and sorry about my poor English.

Upvotes: 0

Views: 118

Answers (2)

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361762

Actually operator T() const is a const-member function, inside which this pointer refers to a const object, which in turns makes the getterCount const as well.

The solution is to declare getterCount as mutable, as shown below:

mutable size_t getterCount;

Now getterCount can be incremented in const member function, which also means, it can be incremented even if the object is const:

void f(OneCell const & cell)
{
    //incrementing getterCount!
    ++cell.getterCount; //ok, even though cell is const!
}

Upvotes: 5

K-ballo
K-ballo

Reputation: 81399

As you see, I want to increment variable in this method, but i get an error because the keyword const. On the other hand, if I remove this keyword, the casting doesn't work at all.

It will work, but only on mutable instances of OneCell.

What can I do?

Go mutable, assuming that you change the bitwise constness but not the logical constness.

mutable int getterCount;

Upvotes: 3

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