How does polymorphism in C++ affect runtime performance?

Polymorphism affects performance at runtime, mainly because virtual function calls require indirect calls through the virtual function table, which is more expensive than direct calls. Optimization methods include: 1. Use inline functions; 2. Avoid deep inheritance; 3. Use interfaces (C++11).

The impact of polymorphism on runtime performance in C++

Polymorphism is a key in object-oriented programming Traits that allow programs to bind to methods and properties of different classes at runtime. While polymorphism provides flexibility and code reusability, it also introduces some runtime overhead.

Virtual function calls

When a virtual function is called, the compiler cannot determine at compile time which method version to call. Therefore, it must use a virtual function table (VFT) at runtime. The VFT is a pointer table containing pointers to actual functions. When a virtual function is called, the compiler looks up the appropriate method pointer in the VFT and then makes the indirect call.

This indirect call is more expensive than a direct call because it involves additional memory lookup. Although this overhead is usually small, it can accumulate in code that requires frequent calls to virtual functions.

Example: Shape Class Hierarchy

Consider a shape class hierarchy where there are different shape classes (such as Circle, Square, and Rectangle). These classes all derive from a Shape base class that defines the getArea() virtual function.

class Shape {
public:
    virtual double getArea() const = 0;
};

class Circle : public Shape {
public:
    Circle(double radius) : radius(radius) {}
    double getArea() const override { return M_PI * radius * radius; }

private:
    double radius;
};

class Square : public Shape {
public:
    Square(double side) : side(side) {}
    double getArea() const override { return side * side; }

private:
    double side;
};

class Rectangle : public Shape {
public:
    Rectangle(double width, double height) : width(width), height(height) {}
    double getArea() const override { return width * height; }

private:
    double width;
    double height;
};

When we create a Shape object and call getArea(), the compiler cannot determine which specific implementation version to call. Therefore, it will look for the corresponding function pointer in the VFT, as shown below:

Shape* shape = new Circle(5);
double area = shape->getArea(); // 间接调用

Performance Optimization

If virtual functions need to be called frequently, we can consider the following Methods to optimize performance:

• Use inline functions: Inline functions can be replaced with direct calls at compile time, thus eliminating the overhead of indirect calls.
• Avoid deep inheritance hierarchies: Deep inheritance hierarchies require more VFT lookups, thus increasing overhead.
• Using interfaces (C++11): Interfaces allow dynamic binding without virtual functions. This can reduce the overhead of VFT lookups.

Conclusion

While polymorphism is a powerful feature, its runtime performance impact needs to be considered when choosing to use it. By understanding the overhead of virtual function calls and implementing appropriate optimizations, we can balance flexibility with performance.

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