Field has incomplete type in forward declaration

Question

I reproduce an error with the following simple file.

It says:

field has incomplete type 'Foo'

bar.h:

class Foo;
class Bar{
    private:
        int x_;
        Foo foo_; // error: incomplete type
    public:
        void setx(int x) {x_ = x;};
        void increment(int);
};
class Foo{
public:
    void run(int y,Bar& bar) {bar.setx(y);};
};
void Bar::increment(int i){foo_.run(i,*this);}

Member foo_ must not be a reference or a pointer. The reason for this is that in my actual code, I cannot initialize Foo in the initialization list of Bar.

Solution 1

In this case it's simple enough: since Foo only uses a reference to Bar, flipping them around will do the trick:

class Bar; 
class Foo{
public:
    void run(int y,Bar& bar);
};
class Bar { ... };
void Foo::run(int y, Bar& bar) {
    bar.setx(y);
}

You also need to move the body of Foo::run below, because it's actually using the Bar member function.

Solution 2

Your problem can be reduced to this:

class Foo;
class Bar{
    Foo foo_; // error: incomplete type
};

Here you did a forward declaration for type Foo, i.e. a declaration without a complete definition: that's enough in C++ to declare a pointer, but not a concrete instance as you did in Bar.

Either give a complete definition to your class:

class Foo{
    // put details here
};
class Bar{
    Foo foo_; // OK
};

Or use (smart) pointers, e.g.:

class Foo;
class Bar{
    std::unique_ptr<Foo> foo_; // OK
};

Or change order declaration as pointed by Bartek Banachewicz.

Comments

[beginneR]

was helpful, thanks! ;)