structure defined inside of class

Question

I'm newbie in c++, consider the following snippet:

class myClass
{
  ...
  struct EntryKeyBase
  {
     void setOperation(OpType oper)
     {
        operation = oper;
     }
     OpType getOperation() const
     {
        return operation;
     }
     virtual void serialize(std::ostream& os) const = 0;
     protected:
        OpType operation;
    };

  struct ProtoEntryKey: EntryKeyBase
  {
     // some methods here

     ProtoEntryKey(uint8_t l4proto) : proto(l4proto)  // ???
     {
        operation = Inserted;
     }

     protected:
        uint8_t proto;
  };

  // here more structs defined...


  public:
    ...

};

What does the line marked ??? do? I understand that we declare structure inheriting from EntryKeyBase, but whatever follows ':' I don't understand, what does this syntax really mean? Thanks!

It's a constructor. Please find a good C++ text book; this is one of the most fundamental concepts of the language and well learnable without asking a question here. — Kerrek SB, Sep 04 '13 at 13:07
Also, it is usually a bad idea to have identifiers like `l4` where the lower case L can be easily confused with a number one. — David Rodríguez - dribeas, Sep 04 '13 at 13:10
Also note that this has absolutely nothing to do with the structure being nested. — juanchopanza, Sep 04 '13 at 13:20

score 3 · Answer 1 · answered Sep 04 '13 at 13:14

3

It is an initialization list. it assigns value of l4proto to proto variable in struct ProtoEntryKey.

answered Sep 04 '13 at 13:14

Igor Popov

2,588
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score 1 · Answer 2 · answered Sep 04 '13 at 13:08

1

It is simple constructor. If you write : somefield(somevalue) after constructor it will set value of somefileld to somevalue.

In your example it will set value of proto to l4proto (constructor argument).

answered Sep 04 '13 at 13:08

Ari

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It is not simple. It's also not a constructor. It's a (base/member) initializer list. Which can be a part of a constructor. Yes – sehe Sep 04 '13 at 13:09
So it would have the same meaning if I put `proto=l4proto` inside of the body of `ProtoEntryKey` ? – Mark Sep 04 '13 at 13:17
@Mark: No. If you put it inside the constructor, the default constructor gets called for each member, and then the copy-assignment will get called. Putting it in an intializer-list makes it so only the copy-constructor gets called for the members in the initializer-list. – Zac Howland Sep 04 '13 at 13:19

score 0 · Answer 3 · edited May 23 '17 at 11:56

0

proto is a unit18_t member. It makes sense to initialise it in the constructor.
The : indicates the initialiser list. You can initialise base classes and member here. See here for further details.

edited May 23 '17 at 11:56

Community

1
1

answered Sep 04 '13 at 13:14

doctorlove

18,872
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score 0 · Answer 4 · edited May 23 '17 at 12:05

0

It's part of the constructor that sets proto variable to l4proto value.

This link may be useful.

edited May 23 '17 at 12:05

Community

1
1

answered Sep 04 '13 at 13:14

W4lker

51
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structure defined inside of class

4 Answers4