Forward declaring and using in one step

Question

As an optimisation, or to avoid include looping, a type may be forward declared, This leads to code like:

class A;

class B
{
    A *a;
};

If the number of forward declaration becomes large, it can take up a lot of space at the top of the header file. Is there a way of forward declaring and using at the same time? Sort of like:

class B
{
    extern A *a;
};

I've never really thought about this before, but I have a header with a bunch of forward declarations and I would like to make it tidier (without farming them off to another include file).

EDIT: I changed 'a' to a pointer, as it was rightly pointed out that you can only use forward declare on pointers and references.

Galik · Accepted Answer · 2016-04-22T01:58:38.637

5

What you're asking isn't completely clear but, if I understand you right, you can forward declare at the same time as declaring your variables:

class B
{
    class A* a; // declaring A as class is an in-place forward declaration
};

Is that what you mean?

edited Apr 22 '16 at 01:58

answered Apr 21 '16 at 03:54

Galik

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Interesting, I have never seen this syntax used before. – Remy Lebeau Apr 22 '16 at 02:01
Thank you. That was exactly what I was looking for, and, no, I've never seen anything like it before. It helped clean up my header file. – Simon Parker Apr 22 '16 at 02:05

Jack · Answer 2 · 2016-04-21T03:53:36.223

A forward declaration wouldn't allow you to do

class A;

class B
{
    A a;
};

unless A is a reference or pointer type, since a forward declaration doesn't give any additional information on the size of the object (unless for enum class in C++11). So are you using pointers/references? Otherwise it means you are including the definition of A for sure.

Regarding your problem there is no way to forward declare and use a type since we're talking about two different things. A variable declaration doesn't define a type, it defines a variable.

A simple solution to your problem would be to gather all forward declaration in a single header file and include it in the project (or in your eventual precompiled header). This wouldn't create too much problems, since forward declarations don't expose anything nor they are heavyweight.

Sorry, dumb mistake in my example. – Simon Parker Apr 22 '16 at 02:05 — Simon Parker, Apr 22 '16 at 02:05

score 3 · Answer 3 · edited May 23 '17 at 12:31

3

No, you can't do what you want. This answer about forward declarations should give you all the gory details, but in summary you need the full definition of a type if you want to use it (as including more or less does); not just the fact that it exists (as forward declaring more or less does).

edited May 23 '17 at 12:31

Community

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answered Apr 21 '16 at 03:50

fetherolfjd

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Sorry, my example was broken. – Simon Parker Apr 22 '16 at 02:06

Forward declaring and using in one step

3 Answers3