Deleting an object from a dynamically allocated array in C++

Question

In the first stage, i've created an object Planet with some attributes, like name, type and distanceToEarth. I've created a Repository then, basically a structure consisting of a dynamic array elems and its length and maximum capacity.

typedef enum { NEPTUNE_LIKE, GAS_GIANT, TERRESTRIAL, SUPER_EARTH, UNKNOWN }PlanetType; typedef struct { char name[30]; PlanetType type; float distanceToEarth; }Planet; Planet createPlanet(char name[], PlanetType type, double distance) { Planet pl; strcpy(pl.name, name); pl.distanceToEarth = distance; pl.type = type; return pl; } typedef struct { Planet* elems; /** dynamic array containing the planets */ int length; /** actual length of the array */ int capacity; /** maximum capacity of the array */ } PlanetRepo; PlanetRepo createPlanetRepo(int capacity) { /// create a new planet repo; the elems field must be dynamically allocated (malloc) PlanetRepo r; r.capacity = capacity; r.length = 0; r.elems = (Planet*) malloc(sizeof(Planet)*capacity); return r; } bool remove(PlanetRepo* repo, Planet pl) { /// @todo remove planet pl from the repository /// return false if the planet does not exist in the repository return false; }

My problem is related to the function remove(). I can't figure out how I am supposed to remove that object from a dynamically allocated array.

Of course, this is not the entire code, but I've selected only the relevant parts. If I forgot to include something, let me know.

Are you sure you are not writing a C program instead of a C++ program? Usage of malloc, typedef struct, etc. are indications that you are writing C, not C++. — PaulMcKenzie
– PaulMcKenzie, Commented Mar 28, 2020 at 20:32
@PaulMcKenzie You're right. I've simultaneously studied both C and C++ and now I tend to mix them up — Severienne Bianca
– Severienne Bianca, Commented Mar 28, 2020 at 20:35
Well, with C++, all you need is std::vector<Planet> and be done. — PaulMcKenzie
– PaulMcKenzie, Commented Mar 28, 2020 at 20:36
@SeverienneBianca That is a really bad idea. Proper style and idioms in both languages are completely different. The code you are showing would be a badly written C++ program and would technically even have undefined behavior in C++ prior to C++20. You should focus on one of the two languages and what you are showing right now is how one writes C, not C++. — walnut
– walnut, Commented Mar 28, 2020 at 20:36

walnut · Accepted Answer · 2020-03-28 20:54:01Z

Since you insisted on tagging C++, rather than C:

In C++ you wouldn't define the PlanetRepo and the associated functions at all. Instead you would simply declare a variable of type

std::vector<Planet>

or maybe depending on the use case (but less likely)

std::list<Planet>

Both of these already have member functions .erase that remove elements from them.

In C++ you also wouldn't write

typedef struct { char name[30]; PlanetType type; float distanceToEarth; }Planet;

but instead

struct Planet { char name[30]; PlanetType type; float distanceToEarth; };

and you would most likely use std::string instead of char[30] as type for name.

Instead of a function Planet createPlanet(char name[], PlanetType type, double distance) you would define a constructor for Planet:

struct Planet { std::string name; PlanetType type; float distanceToEarth; Planet(std::string name, PlanetType type, double distance) : name(name), type(type), distance(distance) {} };

and probably make the members private.

You also wouldn't define an unscoped enum, but a scoped one instead (since C++11), see Why is enum class preferred over plain enum?.

Thank you so much! I've discovered that I actually have huge gaps in my C++ knowledge, so I will just stick to C instead.
@SeverienneBianca Notice that C++ isn't just "C with classes", it is very different, and if you want to use it, you should invest more time in learning just C++. It is very confusing at the start, so stick to one of these until you feel comfortable to learn a new language. Good luck!

Blub · Accepted Answer · 2020-03-28 20:41:06Z

Since this is rather a C than a C++ program, you could use a linked list which makes it possible for you to delete elements in a dynamically allocated "array".

This might be of interest.

Victor Hugo · Accepted Answer · 2020-03-28 22:36:09Z

Like mentioned before C++ has implemented data structures so you can easily store your planets. But you can do something like:

bool remove(PlanetRepo* repo, Planet pl) { PlanetRepo* temp = (PlanetRepo*) malloc(sizeof(Planet)*repo->capacity); if(!temp){ return false; } temp->capacity = repo->capacity; temp->size = repo->size-1; for(int i = 0; i < repo->size; ++i){ if(repo[i] != pl){ //This won't work. How to compare Planets is left as an exercise to the reader temp[i] = repo[i]; } } free(repo); repo = temp; return true; }

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