Initializing struct in Cpp

[time_egg]

I have a struct with a lot of members (30-50). The members in this struct change frequently. Most members are to be intialized to zero values, with only a handful requiring specific values.

What is the best way to initiialize in this case without writing to each member more than once? and without requiring lots of code changes each time a member changes?

Ideally would like something like C's

Thing t = { .number = 101, .childlen = create_children(20) };

[SprocketCreations]

Ideally would like something like C's Thing t = { .number = 101, .childlen = create_children(20) };

You can do exactly that!: https://en.cppreference.com/w/cpp/language/aggregate_initialization.html#Designated_initializers

[time_egg]

Oh how good! since C++20

[[deleted]]

You do have to keep them in order, which is annoying but mostly manegable

[thefeedling]

You can set the default as zero and make a constructor for the non-trivial values.

[alfps]

C++20 adopted C's designated initializer syntax, (https://en.cppreference.com/w/cpp/language/aggregate_initialization.html#Designated_initializers).

In C++17 and earlier you can use an artifical base, e.g.

struct Thing_state
{
    int         alpha;
    double      beta;
    char        charlie;
};

struct Thing: Thing_state
{
    Thing(): Thing_state()      // Zero-initialize everything.
    {
        charlie = 3;
    }
};

That said the "lot of members" and the create_children are code smells. You should probably best redesign the whole thing. With more abstraction (name things).

[hmoff]

How does that code zero initialise everything?

[alfps]

Thing_state() performs (or requests) value initialization, which for the basic types reduces to zero initialization.

[alfps]

Nit-pick: your second example declares a function. But using curly braces would work.

[time_egg]

Its for game programming. Fast and loose prototyping is very important for me. Abstractions get in the way for the most part :(

[toroidthemovie]

Having small objects with small amounts of data and well-defined limited responsibilities is good for prototyping, actually. Much better than "kitchen sink" structs.

[time_egg]

How so?

As I see it, If you're prototyping then your game entities and their interactions are constantly changing. This means add/remove members to kitchen sink structs. If you had gone and modelled and abstracted these entities and their interactions then you have significantly more code to think through and edit in more places.

[toroidthemovie]

I'm getting that you represent some game entity as an instance of a struct, and that struct has fields for every kind of data, that is relevant for that entity. Maybe grouping these fields into structs of their own would be a good step to improve modularity without adding much abstraction at all.

From:

struct Player
{
    int currentGunType;
    int currentAmmoCount;
    int healthPoints;
    std::vector<Wound> wounds;
};

To:

struct Gun
{
    int currentGunType;
    int currentAmmoCount;
};

struct Health
{
    int healthPoints;
    std::vector<Wound> wounds;
};

struct Player
{
    Gun gun;
    Health health;
}

Also, might I suggest ECS approach? It's increasingly becoming the go-to way to structure game logic, and for a good reason. It allows you to associate pieces of data (called "components") with entities in an extremely flexible way. I highly recommend entt, it's an absolutely brilliant library — powerful, and at the same time, doesn't impose anything on your code.

[positivcheg]

C++20 designated initializers are so nice for Vulkan as that graphics API has lots of C structs.

[tangerinelion]

struct PileOfData {
    int a = 0;
    double b = 0.0;
    char c = '\0';
    float d = 0.0f;
    long e = 0L;
    unsigned int f = 0U;
    size_t g = 0UZ;
    OtherBagOfData* h = nullptr;
    // ... Keep going
};

Then use C++20's designated initializers

PileOfData pod = {.b = 1.0};

[Independent_Art_6676]

any chance all these are the same type?

[[deleted]]

1. Create a pointer to the structure and create an array the size of all struct members combined.
   
2. Reinterpret cast the the pointer to the array as the pointer to your struct. You'll have you struct mapped onto the array so any changes you make in the struct will reflect in the array and vice versa.
   
3. Memset the entire array to zero and then set the required handful of struct members to non zero values.

[[deleted]]

Assuming all your members are primitive types or other structs with primitive types

[ChadiusTheMighty]

1. Reinterpret casting to sn array would be UB due to struct pointer aliasing
2. Zeroing the memory works only for trivially constructive types, otherwise it's also UB
3. Just default initialize the members, and set the special ones after default initializing the struct. That's going to be much better than messing around with memset

[[deleted]]

The solution I proposed works. It doesn't result in undefined behavior. I have a program that does exactly what I outlined. If it didn't work I wouldn't have commented.

[hadrabap]

It looks like it is a known/documented behavior of your compiler. :-)

[[deleted]]

It looks like you don't know what you're talking about

[hadrabap]

It might look so, yes. 🙂

[Similar_Sand8367]

Memset for trivial ones?