C++ header file referencing a struct defined below

Question

What is considered the best practice for handling this situation?

class A {
    private:
        std::vector<B> derp;
    public:
        struct B { ... };
        void foo(B b);
}

(The problem is that this code would say "error: use of undeclared identifier 'B'" I think I could solve the problem by doing something like

class A {
    public:
        struct B { ... };
        void foo(B b);
    private:
        std::vector<B> derp;
}

But that seems strange and not like the proper solution. Also as a side note, if I were to write that should I write it like this?

struct A {
    struct B { ... };
    void foo(B b);

    private:
        std::vector<B> derp;
}

i would say to create the B class and include it later in the Class A declaration. — Abel
– Abel, Commented May 20, 2015 at 18:39
Do you ant people to be able to create instances of B or should it be something only accessible to your A? — NathanOliver
– NathanOliver, Commented May 20, 2015 at 18:44

R Sahu · Accepted Answer · 2015-05-20 18:46:47Z

1

You could use a forward declaration of B before declaring derp if you don't want to move the definition of B before declaring derp.

class A {
   public:
      struct B;
   private:
      std::vector<B> derp;
   public:
      struct B { ... };
      void foo(B b);
};

However, from a physical layout perspective, the public section of a class shoud be before its private section. You want the public section to be seen by a user first.

Given that, I think it'll be better to use:

class A {
    public:
        struct B { ... };
        void foo(B b);
    private:
        std::vector<B> derp;
};

That solves the problem of B not being declared/defined before the declaration of the member variable derp. It also puts the public section ahead of the private section.

answered May 20, 2015 at 18:46

R Sahu

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1 Comment

Anton Savin Over a year ago

It's undefined behavior to create std::vector of incomplete type.

Ken P · Accepted Answer · 2015-05-20 18:44:13Z

0

Just like variables, you can forward-declare classes and structures. For example, in your case you can do the following:

class A {
public:
    struct B;
private:
    std::vector<B> derp;
public:
    struct B { ... };
    void foo( B b );
};

Keep in mind if you forward-declare a class or structure within a class, you need to declare it with the same access level (private/protected/public) as you define it later.

answered May 20, 2015 at 18:44

Ken P

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