C++ forward declaration in Qt

Question

I have 3 classes which depend on each other:

class Channel
{
public:

enum ChannelType {
    DIMMER, RED, GREEN, BLUE, STROBE
};

Channel(ChannelType type);
const ChannelType type;

void x(Fixture fixture);
};

class Fixture
{
    public:
    Fixture(int channel, FixturePattern pattern);

    Channel getChannel(const Channel::ChannelType);

    private:
        const int channel;
        FixturePattern pattern;
};

class FixturePattern
{

public:
    FixturePattern(QList<Channel> channels);
    Channel getChannel(Channel::ChannelType type);

private:
    QList<Channel> channels;
};

These classes are in separate header files. I tried to connect them with #include but I always end up with incomplete types or the XY was not declared in this scope error. Can someone explain me what I am doing wrong?
I didn't add the #includes since I completely screwed it up yesterday. Recently I already found a question with this topic but I don't want to put it in the same file. Is it possible?

Just google your title and you will find a lot of Q/As regarding this, e.g.: [SO: What are forward declarations in C++?](https://stackoverflow.com/a/4757718/7478597) — Scheff's Cat, Jun 12 '18 at 05:46
Possible duplicate of [What are forward declarations in C++?](https://stackoverflow.com/questions/4757565/what-are-forward-declarations-in-c) — eyllanesc, Jun 12 '18 at 05:52

score 0 · Accepted Answer · answered Jun 12 '18 at 06:03

Without going into a lot of details you should use forward declaration for your classes. You'll need to modify your code to be able to do it. The code should look something like this. I didn't test it but it should work.

class Fixture; // the forward deceleration 
class Channel
{
public:

enum ChannelType {
    DIMMER, RED, GREEN, BLUE, STROBE
};

Channel(ChannelType type);
const ChannelType type;

void x(const Fixture &fixture); // or void x(Fixture *fixture);
};

#include "FixturePattern.h" 
class Fixture
{
    public:
    Fixture(int channel,const FixturePattern &pattern);

    Channel getChannel(const Channel::ChannelType); 

    private:
        const int channel;
        FixturePattern pattern;
};

#include "Channel.h"
class FixturePattern
{

public:
    FixturePattern(QList<Channel> channels);
    Channel getChannel(Channel::ChannelType type);

private:
    QList<Channel> channels;
};

You can do a forward declaration only to pointers and references. In general it's a good practice to pass class instances as const &. — Yakir E, Jun 13 '18 at 07:32

C++ forward declaration in Qt

1 Answers1