How do I get over this forward declaration error?

I'm trying to do a barebones example of a class A that provides callback methods to a dependent class B by passing a reference to itself. The best I've produced is the following:

a.h

#ifndef A_H
#define A_H

#include "b.h"

namespace b {
    class ClassB;
}

namespace a {

    class ClassA {
    public:
        ClassA();
        b::ClassB* m_b;
        void callback(const std::string& msg);
    };
}

#endif 

a.cc

#include "a.h"

a::ClassA::ClassA() {
    m_b = new b::ClassB(this);
}

void a::ClassA::callback(const std::string& msg) {
    std::cout << msg << '\n';
}

int main(void)
{
    a::ClassA* doit = new a::ClassA();
    delete doit;
    return 0;
}

b.h

#ifndef B_H
#define B_H

#include <iostream>
#include <string>

namespace a {
    class ClassA;
}

namespace b {
    class ClassB{
    public:
        ClassB(a::ClassA* a_inst);
    };
}

#endif

b.cc

#include "b.h"

namespace b {

    ClassB::ClassB(a::ClassA* a_inst) {
        a_inst->callback("Call me maybe");
    }
}

This is the closest I can get to a compilation, but I'm left with the following errors:

b.cc: In constructor 'b::ClassB::ClassB(a::ClassA*)':
b.cc:6:9: error: invalid use of incomplete type 'struct a::ClassA'
b.h:8:8: error: forward declaration of 'struct a::ClassA'

How can I fix this? And is there a better way to construct this relationship?