CODEWITHSUNDEEP
c++classstruct
Sweeney Todd
Sweeney Todd

Reputation: 880

Details and benefits of using a struct in a class definition

Consider the following code: 

class myclass 
{
    public:
        //some public stuff
    private:
        struct classitem  
        {
            int x;
            classitem *next;
        };
}

What I do not understand is this;
Is classitem just a definition, or will it already be a member of an object of this class? In other words, will it be filling any memory when we create an object of this class? If it is just a definition, how would we use it in future?
And what would be the benefits of using this struct in class definition instead of defining it outside of the class?

Upvotes: 1

Views: 997

Answers (1)

ScarletAmaranth
ScarletAmaranth

Reputation: 5101

It will be just a definition.

If you want an object, use: 

struct T { ... } instance;

The inner struct will be scoped in the outer struct definition - as to why it's useful, you can make the type (class) definition private for instance, if you don't want that type to be used outside of your class.

class NHeadedBeast {
   struct Head {
       Head() : numHeads{2}, eyesPerHead{4} { }
       int numHeads;
       int eyesPerHead;
   } head;
public:
   int getEyeCount() const {
       return head.eyesPerHead * head.numHeads;
   }
};

Notice how specific the Head class is - and also, it's called Head, which has a decent chance of colliding with some other type name. In other words, noone would ever want to use that Head type without my NHeadedBeast class, so might as well make the type inaccessible and isolate it in the NHeadedBeast scope.

Upvotes: 3

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