Brief introduction function is a corresponding relationship in mathematics, from non-empty number set A to real number set B. Simply put, A changes with B, and A is a function of B. To be precise, let X be a non-empty set, Y be a non-empty number set, and F be the corresponding rule. If there is only one element Y corresponding to each X in X according to the corresponding rule F, it is said that the corresponding rule F is a function on X, denoted as Y = f(x), X is the domain of the function F (x), and the set {y | y = the corresponding rule and the domain are two elements of a function. Function-related concept independent variable, a variable related to other variables, any value in this quantity can be found in other variables. The dependent variable (function) changes with the change of the independent variable. When the independent variable takes a unique value, the dependent variable (function) has one and only one corresponding value. Geometric meaning function is related to inequality and equation (elementary function). Let the function value be equal to zero, and from the geometric point of view, the corresponding independent variable is the intersection of the image and the X axis; From the algebraic point of view, the corresponding independent variable is the solution of the equation. In addition, replacing "=" in the expression of a function (except a function without expression) with "",and then replacing "y" with other algebraic expressions, the function becomes an inequality, and the value range of the independent variable can be found. Set theory (relation) function definition If the binary relation F goes from X to Y? X×Y, for each x∈X, there is a unique y∈Y, so that < x, y & gt∈f is called as a function from x to y, which is denoted as: f: x→ y. When x = x = x1××× xn, Its characteristics are: the front area and the definition area overlap; Unit price:; 0 so for any domain, as long as x satisfies c? 6? 1δ& lt； X & ltC+δ, established. Real function or virtual function Real function refers to a function whose domain and value domain are real numbers. One of the characteristics of real functions is that they can draw pictures on coordinates. Virtual function is an important concept in object-oriented programming. When inheriting from the parent class, the virtual function and the inherited function have the same signature. However, in the process of running, the running system will automatically choose the appropriate specific implementation to run according to the type of object. Virtual function is the basic means to realize polymorphism by object-oriented programming.