CODEWITHSUNDEEP
c++
Nyaruko
Nyaruko

Reputation: 4489

How to make a list and its content const in c++?

If I have a:

class myClass 
{
...
list<myObject*> objectList;
};

How can I write a getObjectListPointer() function that return a pointer to this objectList in a way, that itself and its content cannot be modified?

Which means, if I have 

list<myObject*>* pointer = getObjectListPointer();

then pointer itself and pointer->at(i) for any i cannot be modified? Like where should I put const?

Ok, my purpose is to able to modify objectList with in myClass but not other places, but I need to access the value of objectList and its content's value in other class, how should design the code?

Upvotes: 1

Views: 86

Answers (2)

weltensturm
weltensturm

Reputation: 2162

#include <list>
#include <iostream>

class myClass {
    std::list<float*> container;

    public:

        myClass(){
            float* f = new float;
            *f = 0.5f;
            container.push_back(f);
        }

        const std::list<const float*>* containerConst(){
            return (std::list<const float*>*)&container;
        }
};

int main(){
    myClass instance;
    std::cout << *instance.containerConst()->front() << std::endl;
    return 0;
}

Outputs 0.5.

Pointers can be freely cast to const and the compiler usually does so implicitly, but the template system can't handle that by itself. Casting isn't the nicest solution since it results in a new template instantiation, but if the resulting class has the same data layout it works.

Upvotes: 1

Jarod42
Jarod42

Reputation: 217593

With

struct myClass 
{
    std::list<const myObject*> getObjectListPointer() const
    {
        // Create a copy with const pointers
        return {std::begin(objectList), std::end(objectList)};
    }

    std::list<myObject*> objectList;
};

You may then do

myClass instance;
const auto& pointer = instance.getObjectListPointer();
// cannot modify list `objectList`, and all `myObject` would be `const`

Upvotes: 2

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