CRTP implements static polymorphism through base classes using derived classes as template parameters to avoid virtual function overhead. 1. Used for object counting: Each derived class has an independent static counter, and counts are added or decreased during destruction; 2. Implement static polymorphism: The base class calls the derived class method through static_cast, determines the call target during compilation, and improves performance; 3. Common uses include Mixin function injection, performance optimization, operator generation, EBO utilization, etc., which support zero-cost abstraction and code reuse.
CRTP (Curiously Recurring Template Pattern) is a template programming technique in C. It allows a base class to "know" its derived class type with derived class as template parameters. This pattern implements static polymorphism at compile time to avoid virtual function overhead. It is often used to implement mixin classes, tool classes (such as counting, serialization, comparison operations, etc.) or performance-sensitive scenarios.
Here is a simple CRTP example: implementing a base class that automatically counts object construction and destructuring.
1. Basic usage: Object counter
#include <iostream>
template <typename Derived>
class ObjectCounter {
private:
static int count; // Each derived class has its own count public:
ObjectCounter() {
count;
}
ObjectCounter(const ObjectCounter&) {
count;
}
~ObjectCounter() {
--count;
}
static int get_count() {
return count;
}
};
// Static member definition template <typename Derived>
int ObjectCounter<Derived>::count = 0;
// Use CRTP for specific classes
class Widget : public ObjectCounter<Widget> {
// Widget automatically obtains counting function};
class Gadget : public ObjectCounter<Gadget> {
// Gadget also has independent counts};
int main() {
std::cout << "Widgets: " << Widget::get_count() << "\n"; // 0
std::cout << "Gadgets: " << Gadget::get_count() << "\n"; // 0
{
Widget w1, w2;
Gadget g1;
std::cout << "Widgets: " << Widget::get_count() << "\n"; // 2
std::cout << "Gadgets: " << Gadget::get_count() << "\n"; // 1
}
std::cout << "After scope:\n";
std::cout << "Widgets: " << Widget::get_count() << "\n"; // 0
std::cout << "Gadgets: " << Gadget::get_count() << "\n"; // 0
return 0;
}2. Implement static polymorphism (replace virtual functions)
CRTP can be used to implement "static polymorphism", that is, to decide which function to call at compile time to avoid runtime overhead.
template <typename Derived>
class Shape {
public:
void draw() {
// Cast to a derived class and call the specific implementation static_cast<Derived*>(this)->draw();
}
};
class Circle : public Shape<Circle> {
public:
void draw() {
std::cout << "Drawing a circle.\n";
}
};
class Rectangle : public Shape<Rectangle> {
public:
void draw() {
std::cout << "Drawing a rectangle.\n";
}
};
void render(Shape<Circle>& circle) {
circle.draw(); // Call Circle::draw()
}
int main() {
Circle c;
Rectangle r;
c.draw(); // Drawing a circle.
r.draw(); // Drawing a rectangle.
return 0;
}Note: This method of calling is not polymorphic (cannot point to different subclasses with
Shape*), but it has high performance and is suitable for template context.
3. Summary of common uses
CRTP is commonly used for:
- Mixin mode : Add common functions to derived classes (such as counting, logging, cloning)
- Performance optimization : replace virtual function tables to implement compile-time binding
- Interface enhancement : Automatically generate operators or methods for derived classes
- EBO (Empty Base Optimization) utilizes : zero overhead abstraction
For example, it can also be implemented using CRTP:
- Automatically generate
==and!=(as long as) - Object registration mechanism
- Singleton template
- Access control or permission checking
Basically that's it. The key to CRTP is "the base class knows who inherits it", and calls derived class methods through static_cast<derived>(this)</derived> , to achieve powerful code reuse and zero-cost abstraction. Although the name is "strange", it is very practical to use.
The above is the detailed content of C CRTP (Curiously Recurring Template Pattern) example. For more information, please follow other related articles on the PHP Chinese website!
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