x.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <ccan/list/list.h>

struct context {
    struct list_head connections;
	int num_connections;
};

struct connection {
	int id;
	struct list_node list;
};

int main(int argc, char *argv[])
{
	struct context ctx;
	struct connection *conn;

	list_head_init(&ctx.connections);
	srand(time(NULL));

	const int N = argc > 1 ? atoi(argv[1]) : 10;

	/* Create a bunch of connections */
	for (int i = 0; i < N; i++) {
		conn = malloc(sizeof(*conn));
		conn->id = i;
		list_add(&ctx.connections, &conn->list);
		ctx.num_connections++;
	}

	/* Simulate servicing connections by iterating
	 * down the list and randomly removing and inserting
	 * connections
	 */
	printf("%d\n", ctx.num_connections);
	for (int i = 0; i < 10; i++) {
		list_for_each(&ctx.connections, conn, list) {
			if (rand()%23 == 0) {
				list_del_from(&ctx.connections, &conn->list);
				ctx.num_connections--;
			}
		}

		list_for_each(&ctx.connections, conn, list) {
			if (rand()%11 == 0) {
				struct connection *new;
				new = malloc(sizeof(*new));
				new->id = 42;
				list_add(&ctx.connections, &new->list);
				ctx.num_connections++;
			}
		}
	}
	printf("%d\n", ctx.num_connections);
}

y.cpp

#include <iostream>
#include <vector>
#include <ctime>

struct connection {
    int id;
};

struct context {
    std::vector<connection> connections;
};

int main(int argc, char** argv)
{
    context ctx;

    srand(time(nullptr));

    const int N = argc > 1 ? atoi(argv[1]) : 10;

    for (int i = 0; i < N; i++)
        ctx.connections.emplace_back(connection{i});

    std::cout << ctx.connections.size() << '\n';
    for (int i = 0; i < 10; i++) {
        int removed = 0;
        for (auto& conn : ctx.connections) {
            if (rand()%23 == 0) {
                conn = std::move(ctx.connections.back());
                removed++;
            }
        }
        ctx.connections.resize(ctx.connections.size() - removed);

        const int n = ctx.connections.size();
        for (int i = 0; i < n; i++) {
            if (rand()%11 == 0)
                ctx.connections.emplace_back(connection{i+n});
        }
    }
    std::cout << ctx.connections.size() << '\n';
}