使类中的函数指针依赖于初始化值

Posted 2023-03-28

【中文标题】使类中的函数指针依赖于初始化值

【英文标题】：make function pointer in class dependent on initialized value

【发布时间】：2021-08-30 13:06:39

【问题描述】：

我想创建一个对象，并在初始化期间选择一个函数来执行一些计算。对于N 阶的多项式，必须调用某个函数，定义为someFunN。现在我可以用函数指针来做到这一点。我通过构造函数中的一个巨大的 if 块来做到这一点，

if (order == 2)
    SolveFun = &someFunPoly2;
else if (order == 3)
    SolveFun = &someFunPoly3;
// etc...
else
    SolveFun = &someFunPoly50;

但由于我的功能要到约 50 个订单，所以写起来有点乏味。还有其他方法可以定义someFunN 并在Polynomial 的初始化期间分配此函数吗？

someFunN的内容是由Matlab中的代码生成脚本生成的，还是可以修改的。

#include <iostream>

using namespace std;
struct vec;
class Polynomial;
double someFunPoly2(Polynomial *Poly, vec Pt, vec Ur);
double someFunPoly3(Polynomial *Poly, vec Pt, vec Ur);
double someFunPoly10(Polynomial *Poly, vec Pt, vec Ur); 

struct vec

    double x, y;
    vec(double x_, double y_) : x(x_), y(y_) 
;

class Polynomial

public:
    int order, n;
    double *p;
    double (*SolveFun)(Polynomial *, vec, vec);
    Polynomial(int order_)
    
        order = order_;
        n = order + 1;
        p = new double[n];
        for (int i = 0; i < n; i++)
            p[i] = 0.0;

        if (order == 2)
            SolveFun = &someFunPoly2;
        else if (order == 3)
            SolveFun = &someFunPoly3;
        else
            SolveFun = &someFunPoly10;
        // more and more cases...
    
;

double someFunPoly2(Polynomial *Poly, vec Pt, vec Ur)

    // some calculations for a poly of order 2
    cout << "using Poly with order " << Poly->order << " and this is someFunPoly2" << endl;
    return 2;


double someFunPoly3(Polynomial *Poly, vec Pt, vec Ur)

    // some calculations for a poly of order 2
    cout << "using Poly with order " << Poly->order << " and this is someFunPoly3" << endl;
    return 3;


double someFunPoly10(Polynomial *Poly, vec Pt, vec Ur)

    // some calculations for a poly of order 10
    cout << "using Poly with order " << Poly->order << " and this is someFunPoly10" << endl;
    return 10;


int main()

    vec P = vec(1.0, 2.0);
    vec U = vec(0.3, 0.5);
    Polynomial *Poly2 = new Polynomial(2);
    Polynomial *Poly10 = new Polynomial(10);

    cout << Poly2->SolveFun(Poly2, P, U) << endl;
    cout << Poly10->SolveFun(Poly10, P, U) << endl;

    return 0;

【问题讨论】：

指向函数的指针数组？

@S.M.哦，太棒了，这是个好主意！

【参考方案1】：

您可能正在寻找查找表：

#include <iostream>

void say_hello()  std::cout << "Hello!\n";
void say_bye()    std::cout << "Bye!\n";
void say_thanks() std::cout << "Thanks!\n";

int main(void)

    int n = /*something 0-2*/;
    void (*says[])() = say_hello, say_bye, say_thanks;
    void (*speech)() = says[n];
    speech();

如果 n 是 0、1 或 2，那么它将分别打印 Hello!、Bye! 或 Thanks!。