// file: X.h
class X {
    public:
        X() {};
        ~X() {};

        void fooX() { // do stuff };
        void barX() { // do more stuff };
}

// file: X.cpp
#include "X.h";

// file: Y.h
#include "X.h"
class Y {
    public:
        Y() {x = new X()};
        ~Y() {};

        fooY() { // do stuff };
        barY() { // do more stuff };

    private:
        X x;    // object of class X
}

// file: Y.cpp
#include "Y.h"

// file: Z.h
#include "Y.h"
class Z {
    public:
        Z(uint8_t, std::string, std::vector<uint8_t>);
        ~Z() {};

        fooZ() { // do stuff };
        barZ() { // do more stuff };

    private:
        Y y;    // object of class Y
}

// file: Z.cpp
#include "Z.h"
Z::Z(uint8_t a, std::string b, std::vector<uint8_t> c)
{
     /// do stuff and create an object of Y
     Y y = new Y();
}

// file api.h
#include "Z.h"
void accessZ(uint8_t, std::string, std::vector<uint8_t>);

// file api.cpp
#include "api.h"
void accessZ(uint8_t a_uint,
     std::string b_string,
     std::vector<uint8_t> c_vector)
{
     // create object of Z
     Z z = new Z(a_uint, b_string, c_vector);
     z->fooZ();

     delete z;
     z = NULL;
}