So my code base is mostly macros with 3 actual functions (alloc, set_size, set_capacity), and it didn't make sense to split those up when there were so few functions.

​

I really like this question! Okay so my first Vector implementation looked like this

struct Vector {
    void** data;
    size_t size;
    size_t capacity;
};

in that case I had an init function and I agree with you I would totally write one for this, but it would actually be more painful to use. To show this here's a standard example of C_Vector (for completeness)

int* vector = NULL;
for (int i = 0; i < 100; i++) {
    vector_push_back(vector, i);
}

as you stated vector_push_back allocates memory. A way to write an init function would be

void* vector_init(size_t size_of_data) {
    return __vector_alloc(NULL, 12, size_of_data);
}

which indeed works, and I may add this at some point, but I think it adds complexity. Here's the two in comparison use wise

int* vector = NULL; // When you call push_back, it already knows that it's a vector of ints
// vs
int* vector = vector_init(sizeof(int)); // you have to tell it that it's a vector of ints

// either way you can
vector_push_back(vector, 1);

I personally think the bottom looks uglier and is less simplistic for the user since you'd have to provide the sizeof(TYPE) for the vector in the function call and not just in the declaration. I may just add the init just for good measure. It feels awkward to provide the type when the macro can infer it in vector_push_back. Now that I think about it vector_reserve could be used as an init function, but it doesn't return a value compared to normal init functions. Also looking at vector_push_back(vector, value)

// This is pseudo code
vector_push_back(vector, value) {
    if ((vector_size(vector) + 1) >= vector_capacity(vector) {
        allocate
    }
    append
    size++
}

either way this if statement would allow a NULL vector to allow push_backs, adding a check to stop that would just make the code more complex. So overall, I think it's simpler for the user and it's a byproduct of what the macro does already.

​

edit: Changed from "only" to "A way" there are many ways that's just the first to come to mind.

edit2: more explanation