Thread: Mutually dependent classes

A forward declaration is only sufficient if you are using a pointer or reference to the type being forward-declared. Passing by value, in the example you provide, is not possible because the full definition of the type is not available yet.

To expand on what Brewbuck says: You litterally CAN NOT DO THAT.

Use references, pointers or come up with another way of solving the entire problem.

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Mats

I'm making a C++ regular expression wrapper class (wraps the C regex library) that is similiar to the Java.regex package.

It's got two classes: Pattern and Matcher.

The Pattern class has a function like:
Matcher Pattern::match(std::string pattern)

and the Matcher class needs to have a method such as:
void Matcher::setPattern(Pattern)

As you can see, I can't define the Pattern class without first defining the Matcher class, and vice-versa.
Of course, I could change the argument into setPattern to a const reference and define the Matcher class before the Pattern class. I can't think of any other way to implement this design without circular dependency.

Originally Posted by MacNilly

I'm making a C++ regular expression wrapper class (wraps the C regex library) that is similiar to the Java.regex package.

It's got two classes: Pattern and Matcher.

The Pattern class has a function like:
Matcher Pattern::match(std::string pattern)

and the Matcher class needs to have a method such as:
void Matcher::setPattern(Pattern)

In that case, setPattern() should take a const Pattern &, since it will presumably compile it down internally, and there's no real need to pass by value. On the other hand, returning a Matcher by value from Pattern::match() is the right way to go.

Originally Posted by MacNilly

I'm making a C++ regular expression wrapper class (wraps the C regex library) that is similiar to the Java.regex package.

Is the Boost Regex library insufficient for your needs?

You only need a forward declaration of a type to write a declaration of a function that takes or returns that type by value. You only need the full definition when you define the function. In other words, this is perfectly fine:

Code:

struct A;

struct B {
  A frabuzzle(A);
};

But you need the full definition of A before you can implement B::frabuzzle.