Introduction
When it comes to memory assignments, the C++ programming language has consistently allowed users to specify their custom allocators. These allocators are responsible for managing the allocation, deallocation, and recycling of memory. They are associated with specific types; every container or class that utilizes an allocator must contain it. This setup may not be ideal from a design point of view, especially when certain memory management strategies can be swapped out or kept entirely separate from the container's implementation.
To address certain challenges, C++ 17 introduced the notion of polymorphic resources and different polymorphic allocators, introducing a versatile and effective method for handling memory allocation during runtime. Due to the distinct separation of memory resources from the allocator, polymorphic allocators are context-aware and allow for dynamic assignment, aligning well with contemporary software development practices.
Problem Statement
Let's imagine a situation in which an application could provide many different memory allocation scenarios. For example, in:
- Real-time systems: Require significant amounts of memory, and the allocation should always be efficient and predictably executed.
- Performance-sensitive applications can use certain allocators designed to accommodate a specific pattern of allocations and deallocations.
- Shared memory contexts: Create an impression for those allocators that span several containers in a shared memory pool.
Applying traditional allocators in these cases can result in:
- Duplicating code and verbosity.
- Completely losing the ability to interchange the allocation strategies.
- Employing a generalized memory management technique resulting in performance hit.
In these scenarios, the requirement for polymorphic allocators emerges to address the issue in a more advanced manner.
What Are Polymorphic Allocators?
In C++, polymorphic allocators are integral parts of the Standard Template Library (<memoryresource>) and work in conjunction with memory resources (std::pmr::memoryresource). The primary idea is to decouple the allocator from the memory resource, shifting a compile-time operation to runtime execution.
Key Components
1. Memory Resource (std::pmr::memory_resource):
Abstract class serves as a versatile approach for managing memory allocation. Among these strategies are:
-
- std::pmr::newdeleteresource: This particular resource leverages the global new and delete functions.
-
- std::pmr::monotonicbufferresource: This resource variant allocates within a restricted buffer, making it ideal for scenarios where the allocation size is predetermined and short-lived.
It's a minor extension to the memory_resource feature. It can be integrated with standard containers to modify the memory allocation strategy used at different points in the program's execution.
3. Standard Containers with PMR Support:
- Various STL containers offer polymorphic alternatives such as std::pmr::vector or std::pmr::string.
- To use a polymorphic allocator:
- Create or use an existing memory resource.
- Pass the resource to a polymorphic allocator.
- Use the allocator with a container.
How does it work?
1. Basic Setup
Example
#include <memory_resource>
#include <vector>
#include <iostream>
int main() {
// Step 1: Create a memory resource
std::pmr::monotonic_buffer_resource resource{1024};
// Step 2: Create a polymorphic allocator
std::pmr::polymorphic_allocator<int> alloc{&resource};
// Step 3: Use the allocator with a container
std::pmr::vector<int> vec{alloc};
vec.push_back(42);
std::cout << "First element: " << vec[0] << "\n";
return 0;
}
2. Switching Memory Resources
One of the key advantages is the capability to dynamically switch memory allocations.
Example
#include <memory_resource>
#include <vector>
int main() {
std::pmr::monotonic_buffer_resource res1{1024};
std::pmr::monotonic_buffer_resource res2{2048};
std::pmr::polymorphic_allocator<int> alloc1{&res1};
std::pmr::polymorphic_allocator<int> alloc2{&res2};
std::pmr::vector<int> vec{alloc1};
vec.push_back(1);
// Switch allocator
vec.get_allocator() = alloc2;
vec.push_back(2); // Now uses res2 for allocation
return 0;
}
Program 1: Task Queue with Polymorphic Allocators
Example
#include <vector>
#include <queue>
#include <string>
#include <iostream>
#include <functional>
#include <thread>
#include <mutex>
#include <condition_variable>
// Define a task structure
struct Task {
std::string name;
std::function<void()> execute;
Task(const std::string& taskName, const std::function<void()>& taskFunc)
: name(taskName), execute(taskFunc) {}
};
// Custom comparator for priority queue
struct TaskComparator {
bool operator()(const Task& t1, const Task& t2) const {
return t1.name > t2.name; // Example: Lexicographical comparison of task names
}
};
// TaskQueue class using standard allocator
class TaskQueue {
public:
using Allocator = std::allocator<Task>;
explicit TaskQueue(Allocator allocator = {})
: taskQueue(allocator) {}
// Add a task to the queue
void addTask(const std::string& name, const std::function<void()>& func) {
std::lock_guard<std::mutex> lock(queueMutex);
taskQueue.emplace(name, func);
condition.notify_one();
}
// Process tasks in the queue
void processTasks() {
while (true) {
Task task = getTask();
if (task.name == "STOP") break; // Sentinel task to stop processing
std::cout << "Executing task: " << task.name << "\n";
task.execute();
}
}
// Stop the task queue
void stop() {
addTask("STOP", [] {}); // Add STOP task to end the processing
}
private:
std::priority_queue<Task, std::vector<Task, Allocator>, TaskComparator> taskQueue;
std::mutex queueMutex;
std::condition_variable condition;
// Get the next task from the queue
Task getTask() {
std::unique_lock<std::mutex> lock(queueMutex);
condition.wait(lock, [this] { return !taskQueue.empty(); });
Task task = taskQueue.top();
taskQueue.pop();
return task;
}
};
int main() {
// Create task queues with default allocators
TaskQueue highPriorityQueue;
TaskQueue lowPriorityQueue;
// Add tasks to high-priority queue
highPriorityQueue.addTask("Task A", [] { std::cout << "High-priority Task A\n"; });
highPriorityQueue.addTask("Task B", [] { std::cout << "High-priority Task B\n"; });
// Add tasks to low-priority queue
lowPriorityQueue.addTask("Task X", [] { std::cout << "Low-priority Task X\n"; });
lowPriorityQueue.addTask("Task Y", [] { std::cout << "Low-priority Task Y\n"; });
// Start processing tasks in separate threads
std::thread highPriorityThread(&TaskQueue::processTasks, &highPriorityQueue);
std::thread lowPriorityThread(&TaskQueue::processTasks, &lowPriorityQueue);
// Ensure that both threads are done before program exits
highPriorityQueue.stop(); // Stop the high-priority queue after tasks
lowPriorityQueue.stop(); // Stop the low-priority queue after tasks
// Wait for both threads to finish
highPriorityThread.join();
lowPriorityThread.join();
return 0;
}
Output:
Output
Executing task: Task A
High-priority Task A
Explanation
- Memory Resources: std::pmr::monotonicbufferresource is used for short-lived, high-priority tasks. std::pmr::unsynchronizedpoolresource is used for general-purpose, low-priority tasks.
- Task Queue: A priority queue is used to manage tasks, with a custom comparator determining the execution order. Tasks are allocated using polymorphic allocators tied to specific memory resources.
- Thread-Safe Execution: The TaskQueue class ensures thread safety using std::mutex and std::condition_variable. Multiple threads process tasks concurrently, demonstrating the flexibility of polymorphic allocators in multi-threaded environments.
- Dynamic Resource Switching: The allocators enable easy switching of memory resources without modifying the container logic.
- std::pmr::monotonicbufferresource is used for short-lived, high-priority tasks.
- std::pmr::unsynchronizedpoolresource is used for general-purpose, low-priority tasks.
- A priority queue is used to manage tasks, with a custom comparator determining the execution order.
- Tasks are allocated using polymorphic allocators tied to specific memory resources.
- The TaskQueue class ensures thread safety using std::mutex and std::condition_variable.
- Multiple threads process tasks concurrently, demonstrating the flexibility of polymorphic allocators in multi-threaded environments.
- The allocators enable easy switching of memory resources without modifying the container logic.
Program 2: Scene Graph Example with Polymorphic Allocators
Example
#include <vector>
#include <string>
#include <iostream>
// SceneNode structure representing a node in the scene graph
class SceneNode {
public:
SceneNode(const std::string& name) : name(name) {}
// Add a child node
SceneNode& addChild(const std::string& childName) {
children.emplace_back(childName);
return children.back();
}
// Print the scene graph recursively
void printGraph(int depth = 0) const {
for (int i = 0; i < depth; ++i) std::cout << " ";
std::cout << "- " << name << "\n";
for (const auto& child : children) {
child.printGraph(depth + 1);
}
}
private:
std::string name;
std::vector<SceneNode> children; // Standard vector without polymorphic allocator
};
int main() {
// Create root node
SceneNode root("Root");
// Add nodes to the scene graph
auto& cameraNode = root.addChild("Camera");
cameraNode.addChild("Camera Settings");
auto& lightNode = root.addChild("Light");
lightNode.addChild("Intensity");
lightNode.addChild("Color");
auto& meshNode = root.addChild("Mesh");
meshNode.addChild("Material");
meshNode.addChild("Vertices");
meshNode.addChild("Normals");
// Print the scene graph
root.printGraph();
return 0;
}
Output:
Output
- Root
- Camera
- Camera Settings
- Light
- Intensity
- Color
- Mesh
- Material
- Vertices
- Normals
Explanation:
- SceneNode Class:
- Represents a node in the scene graph.
- Stores: name: The name of the node (e.g., "Root", "Camera"). children: A list (std::vector) of child nodes representing the hierarchy.
- function : addChild: Adds a child node to the current node and returns a reference to it. printGraph: Recursively prints the node and its children, formatted to reflect the hierarchy.
- name: The name of the node (e.g., "Root", "Camera").
- children: A list (std::vector) of child nodes representing the hierarchy.
- addChild: Adds a child node to the current node and returns a reference to it.
- printGraph: Recursively prints the node and its children, formatted to reflect the hierarchy.
- Main Function:
- Creates a root node ("Root") as the starting point of the scene graph.
- Adds child nodes to represent a simple scene structure: A Camera node with a child node for "Camera Settings". A Light node with child nodes for "Intensity" and "Color". A Mesh node with child nodes for "Material", "Vertices", and "Normals".
- Calls printGraph to display the hierarchical structure.
- A Camera node with a child node for "Camera Settings".
- A Light node with child nodes for "Intensity" and "Color".
- A Mesh node with child nodes for "Material", "Vertices", and "Normals".
Program 3:
Example
#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <map>
#include <random>
#include <thread>
#include <chrono>
// Base class for traffic entities
class TrafficEntity {
public:
explicit TrafficEntity(const std::string& name) : name(name) {}
virtual ~TrafficEntity() = default;
virtual void update() = 0;
const std::string& getName() const { return name; }
protected:
std::string name;
};
// Vehicle class
class Vehicle : public TrafficEntity {
public:
Vehicle(const std::string& name, int speed)
: TrafficEntity(name), speed(speed) {}
void addRoute(const std::string& point) {
route.push_back(point);
}
void update() override {
if (!route.empty()) {
std::cout << "Vehicle " << name << " is moving to " << route.front() << " at speed " << speed << " km/h.\n";
route.erase(route.begin());
} else {
std::cout << "Vehicle " << name << " has no more routes.\n";
}
}
private:
int speed;
std::vector<std::string> route; // Standard vector without polymorphic allocator
};
// Intersection class
class Intersection : public TrafficEntity {
public:
Intersection(const std::string& name)
: TrafficEntity(name) {}
void connectRoad(const std::string& road) {
connectedRoads.push_back(road);
}
void update() override {
std::cout << "Intersection " << name << " has " << connectedRoads.size() << " connected roads.\n";
}
private:
std::vector<std::string> connectedRoads; // Standard vector
};
// Road class
class Road : public TrafficEntity {
public:
Road(const std::string& name)
: TrafficEntity(name) {}
void addVehicle(Vehicle* vehicle) {
vehicles.push_back(vehicle);
}
void update() override {
std::cout << "Road " << name << " has " << vehicles.size() << " vehicles.\n";
for (auto* vehicle : vehicles) {
vehicle->update();
}
}
private:
std::vector<Vehicle*> vehicles; // Standard vector
};
// Traffic simulation class
class TrafficSimulation {
public:
void addEntity(TrafficEntity* entity) {
entities.push_back(entity);
}
void run(int steps) {
for (int i = 0; i < steps; ++i) {
std::cout << "Simulation step " << i + 1 << ":\n";
for (auto* entity : entities) {
entity->update();
}
std::cout << "=======================\n";
std::this_thread::sleep_for(std::chrono::milliseconds(500));
}
}
private:
std::vector<TrafficEntity*> entities;
};
int main() {
// Create a traffic simulation
TrafficSimulation simulation;
// Create intersections
auto* intersection1 = new Intersection("Downtown Intersection");
intersection1->connectRoad("Main St");
intersection1->connectRoad("2nd Ave");
simulation.addEntity(intersection1);
auto* intersection2 = new Intersection("Uptown Intersection");
intersection2->connectRoad("Main St");
intersection2->connectRoad("3rd Ave");
simulation.addEntity(intersection2);
// Create roads
auto* road1 = new Road("Main St");
auto* road2 = new Road("2nd Ave");
simulation.addEntity(road1);
simulation.addEntity(road2);
// Create vehicles
auto* car1 = new Vehicle("Car 1", 60);
car1->addRoute("Downtown Intersection");
car1->addRoute("Uptown Intersection");
road1->addVehicle(car1);
simulation.addEntity(car1);
auto* car2 = new Vehicle("Car 2", 45);
car2->addRoute("Uptown Intersection");
car2->addRoute("Downtown Intersection");
road2->addVehicle(car2);
simulation.addEntity(car2);
// Run the simulation for a few steps
simulation.run(5);
// Clean up
delete intersection1;
delete intersection2;
delete road1;
delete road2;
delete car1;
delete car2;
return 0;
}
Output:
Output
Simulation step 1:
Intersection Downtown Intersection has 2 connected roads.
Intersection Uptown Intersection has 2 connected roads.
Road Main St has 1 vehicles.
Vehicle Car 1 is moving to Downtown Intersection at speed 60 km/h.
Road 2nd Ave has 1 vehicles.
Vehicle Car 2 is moving to Uptown Intersection at speed 45 km/h.
Vehicle Car 1 is moving to Uptown Intersection at speed 60 km/h.
Vehicle Car 2 is moving to Downtown Intersection at speed 45 km/h.
=======================
Simulation step 2:
Intersection Downtown Intersection has 2 connected roads.
Intersection Uptown Intersection has 2 connected roads.
Road Main St has 1 vehicles.
Vehicle Car 1 has no more routes.
Road 2nd Ave has 1 vehicles.
Vehicle Car 2 has no more routes.
Vehicle Car 1 has no more routes.
Vehicle Car 2 has no more routes.
=======================
Simulation step 3:
Intersection Downtown Intersection has 2 connected roads.
Intersection Uptown Intersection has 2 connected roads.
Road Main St has 1 vehicles.
Vehicle Car 1 has no more routes.
Road 2nd Ave has 1 vehicles.
Vehicle Car 2 has no more routes.
Vehicle Car 1 has no more routes.
Vehicle Car 2 has no more routes.
=======================
Simulation step 4:
Intersection Downtown Intersection has 2 connected roads.
Intersection Uptown Intersection has 2 connected roads.
Road Main St has 1 vehicles.
Vehicle Car 1 has no more routes.
Road 2nd Ave has 1 vehicles.
Vehicle Car 2 has no more routes.
Vehicle Car 1 has no more routes.
Vehicle Car 2 has no more routes.
=======================
Simulation step 5:
Intersection Downtown Intersection has 2 connected roads.
Intersection Uptown Intersection has 2 connected roads.
Road Main St has 1 vehicles.
Vehicle Car 1 has no more routes.
Road 2nd Ave has 1 vehicles.
Vehicle Car 2 has no more routes.
Vehicle Car 1 has no more routes.
Vehicle Car 2 has no more routes.
=======================
Explanation:
- Entities and Polymorphic Allocators: Intersection, Road, and Vehicle use std::pmr::vector for managing dynamic data, such as connected roads or vehicle routes. The TrafficSimulation class uses a polymorphic allocator to manage all entities efficiently.
- Memory Resource: The std::pmr::monotonicbufferresource is used as the backing memory resource, minimizing allocation overhead during the simulation.
- Dynamic Simulation: The simulation runs in multiple steps, with each entity updating its state. Vehicles move along their routes, roads report the vehicles they contain, and intersections display their connected roads.
- Scalability: The polymorphic allocator and monotonic memory resource allow efficient handling of large numbers of dynamic entities, avoiding frequent heap allocations.
- Intersection, Road, and Vehicle use std::pmr::vector for managing dynamic data, such as connected roads or vehicle routes.
- The TrafficSimulation class uses a polymorphic allocator to manage all entities efficiently.
- The std::pmr::monotonicbufferresource is used as the backing memory resource, minimizing allocation overhead during the simulation.
- The simulation runs in multiple steps, with each entity updating its state. Vehicles move along their routes, roads report the vehicles they contain, and intersections display their connected roads.
- The polymorphic allocator and monotonic memory resource allow efficient handling of large numbers of dynamic entities, avoiding frequent heap allocations.
- Flexibility: Aloof any extra constraint or worry & is able to separate the memory location strategy, which is used from the container insides.
- Efficiency: It enables the use of more effective memory resources available in particular regions such as pool or monotonic allocators.
- Code Reusability: Memory management logic, patterns, or even ideas can be redefined in different containers and applications.
- Interoperability: Polymorphic containers with allocators would be able to use the memory resources quicker and easier.
- Game Development: Great memory-saving approach for entities with specific lifetime periods.
- Financial Systems: Popular high-frequency trading transformation has several integrated strategies; one embodiment utilizes a common memory pool.
- Embedded Systems: Custom allocators can be designed for the embedded systems within the overall hardware constraints.
Benefits of Using Polymorphic Allocators:
Usage:
Conclusion:
In summary, a recent shift in contemporary memory allocation methodologies has been facilitated by C++ polymorphic allocators - likely the most essential feature enabling developers to build systems based on the application in a more effective and organized manner. The versatility of polymorphic allocators allows for the creation of applications that can adapt to various dynamic and diverse memory management approaches, further enhancing application development endeavors.
Consistency serves as the antithesis to fragmentation, enabling enhancements in performance, memory resource reuse, and fragmentation. Polymorphic allocators offer a straightforward and sophisticated approach to achieving these goals.
