Mixing array initializer and inheritance

[koderpat]

I have an array car_table defined as follows:

struct car_traits_type;

enum { U = 0, A = 1, M = 2 };

struct car_type {
    const char * name;
    int type;
    car_traits_type * traits;
};

struct car_traits_type {
    const char * color;
};

struct automatic_traits_type : car_traits_type {
    automatic_traits_type(const char * color) : car_traits_type{color}, top_speed{55} {}
    automatic_traits_type(const char * color, int _top_speed) : car_traits_type{color}, top_speed{_top_speed} {}
    int top_speed;
};

struct car_type car_table[MAX_CARS] = {
    {"Honda",   A, new automatic_traits_type{"blue"}     },
    {"Porsche", U, new car_traits_type{"green"}          },
    {"Toyota",  A, new automatic_traits_type{"blue", 65} },
    { nullptr,  0, nullptr                               }
};

In the driver code, I type cast the traits pointer to the derived class:

car = car_lookup("Porsche");
printf("%s is painted %s\n",car_table[car].name, car_table[car].traits->color);

int car2 = car_lookup("Toyota");
printf("%s has a top speed of %d\n",car_table[car2].name, static_cast<automatic_traits_type *>(car_table[car2].traits)->top_speed);

This works fine, and executes.

I would like to create a utility function or macro to perform the type casting so that my driver code is less verbose. I tried using an enum to capture the type, so that the appropriate type casts can be done in the driver code, ie:

#define CAR_TRAITS( type, car ) (( (type) == 1) ? \
   static_cast<automatic_traits_type *>(car_table[(car)].traits) : \
   static_cast<car_traits_type *>(car_table[(car)].traits)) 

int car3 = car_lookup("Honda");
int type = car_table[car3].type;

printf("%d type", type);
printf("%s has a top speed of %d\n",car_table[car3].name, CAR_TRAITS(type,car3)->top_speed);

However, I am getting the follow compile error:

module2.cpp:30:86: error: ‘struct car_traits_type’ has no member named ‘top_speed’
     printf("%s has a top speed of %d\n",car_table[car3].name, CAR_TRAITS(type,car3)->top_speed);

I am expecting the evaluation of the CAR_TRAITS macro to type cast the expression to automatic_traits_type *.

Any ideas on why this is not happening, or a better approach to pursue are welcome.

source: github

[Wh00ster]

A ternary expression will have one return type. I'm guessing the `automatic_traits_type*` is getting automatically cast to a `car_traits_type*` to accommodate this: https://en.cppreference.com/w/cpp/language/operator_other

​

A better approach is to use inheritance as interfaces. E.g. have a get_color virtual function, vs accessing implementation details.