CODEWITHSUNDEEP
c++classconstructorconstants
michael kova
michael kova

Reputation: 105

Default constructor with constant members

I have a class in C++, say "Customer", and I want to have a member of it for ID,

private:
const string x; // Customer's ID

now the normal constructor would look like this: 

Customer::Customer(const string& ID): x(ID){}

Now, I want to build a default constructor, without initializing x. Is it possible? Because if I initialize it to some random value, say "untitled" then I can't change it anymore because its a const.

So should I not build a default constructor at all, or is there a possibility of building one that doesn't initialize x and you can do that later?

Upvotes: 4

Views: 3738

Answers (2)

DarthRubik
DarthRubik

Reputation: 3985

Technically you can modify it later (but that is probably not a good idea). After you have thought about it for a good long time look at this code:

const string s = "";
*const_cast<string*>(&s) = "new string";

To use this in your code you could create a parameterless constructor like this:

Customer() : x("") { }

And then modify it later in your code:

*const_cast<string*>(&x) = "new string"

Although this might be a bad idea to do in practice. You should look at this post for more info about the dangers of this

Another alternative to this is to create a cool struct:

template<class T>
class OneTimeAssign{
        T data;
        bool alreadyAssigned;
    public:
        OneTimeAssign() : data(), alreadyAssigned(false){ }
        void operator=(T&& newData){
            if (alreadyAssigned) throw "Error";
            data = newData;
            alreadyAssigned = true;
        }
        operator const T&() const{
            return data;
        }
        const T* const operator->(){
            return &data;
        }
};

You could then use it like this:

OneTimeAssign<std::string> Mine;
Mine = "First assign";
//Mine = "Second assign"; //Runtime Error
std::cout << (std::string)Mine << std::endl;
system("pause");

Upvotes: 0

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

Now, I want to build a default constructor, without initializing x. Is it possible?

No, it's not possible.

Because if I initialize it to some random value, say "untitled" then I can't change it anymore because its a const.

That's exactly the purpose of const.

So should I not build a default constructor at all, or is there a possibility of building one that doesn't initialize x and you can do that later?

Well, if x really should be const (which I doubt is actually needed), you shouldn't provide a default constructor, unless you could manage to initialize x uniquely from some constexpr.

Upvotes: 3

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