C++ classes - getting from one class while in another

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C++

I have 2 classes (recipe in recipe.cpp and ingredients in ingredients.cpp). From

void Recipe::show_recipe( std::ostream &out ) const
{
//print out receipe with ingredients
}

Ingredient Recipe::get_ingredient( int location) const
{
return Ingredient();
}

I am trying to print recipe ingredients from the ingredient class. How do I access them?

What I have tried:

Pointers - passing addresses - I am at a loss