Class pointer inside struct C++

Question

The question is simple, I guess I cannot do this: (Im in the header file)

typedef struct {
     myclass *p;
     ...
 } mystruct;

class myclass {
     private:
          mystruct *s;
          ...
}

Because when the compiler reaches the struct it doesnt know what myclass is, but I cannot do the reverse either for the same reason:

class myclass {
     private:
          mystruct *s;
          ...
}

typedef struct {
     myclass *p;
     ...
 } mystruct;

How can I make that possible ? Im guessing there is some way to say the compiler "There is an struct called mystruct" before the class (2 example) so it knows that mystruct *s it's possible but I cant manage to do it right. Thanks

Please, no more typedef struct { ...} xyz; In clean C++ you write struct xyz { ...}; and the type is defined... — Christophe
– Christophe, Commented May 24, 2015 at 13:47

flogram_dev · Accepted Answer · 2015-05-24 14:03:23Z

10

You need to use a forward declaration:

class myclass;

typedef struct {
     myclass *p;
     ...
 } mystruct;

class myclass {
     private:
          mystruct *s;
          ...
}

Now the compiler knows that myclass is a class when it first sees it.

Also, no need to typedef structs in C++. You can simply write:

struct mystruct {
     myclass *p;
     ...
};

Which does the same.

edited May 24, 2015 at 14:03

answered May 24, 2015 at 13:45

flogram_dev

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A.S.H · Accepted Answer · 2015-05-24 13:49:04Z

1

In general, it is possible to declare a pointer to a class that has not been defined yet. You need only to have "declared" the class (class myclass).

This is the general solution to define classes with cross-references.

answered May 24, 2015 at 13:49

A.S.H

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Comments

FToDance · Accepted Answer · 2015-05-24 13:51:39Z

0

First of all, typedefs shouldn't be used anymore in modern C++ (there is better syntax). For structs just use:

struct mystruct {
...
}

Also maybe consider a redesign so that you don't have 2 types dependent on each other. If it's not possible, then as others have said forward declare the types before.

answered May 24, 2015 at 13:51

FToDance

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3 Comments

Ediolot Over a year ago

Sorry for that, Im comming from C and new to C++. Is typedef struct { ... } mystruct; just the same as struct mystruct { ... } ? Do they have any difference ?

flogram_dev Over a year ago

@Ediolot They are exactly the same. In C++, you can simply write struct mystruct {…}; and refer to the struct by its name: mystruct, no need to prepend struct before the name as in C. And consequently, no need to typedef it.

FToDance Over a year ago

It's not the same, but they are similar. You will struggle to forward declare a typedef struct.

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