C++ 模板歧义实例化

Question

我正在尝试使用模板进行一些插值，但出现 "ambiguous template instantiation" 错误。这是代码

// interpolation rules
enum InterRule {trap, rect, trapSum};

// Rectangle rule
template <int n, int k, InterRule rule, class Expr> struct Integration {
    static double integrate(double a, double b){
        return (b-a)/n * Expr::eval(a + (k-1)*(b-a)/n) + Integration<n, k - 1, rule, Expr>::integrate(a,b);
    }
};

// termination case
template <int n, InterRule rule, class Expr> struct Integration<n,0,rule,Expr> {
    static double integrate(double a, double b){
        return 0;
    }
};

// Trapezoidal rule
template <int n, int k, class Expr> struct Integration<n, k, trap, Expr> {
    static double integrate(double a, double b){
        return (b-a)/n * (Expr::eval(a)/2 + Integration<n,k-1,trapSum,Expr>::integrate(a,b) + Expr::eval(b)/2);
    }
};

// Trapezoidal sum
template <int n, int k, class Expr> struct Integration<n, k, trapSum, Expr> {
    static double integrate(double a, double b){
        return Expr::eval(a + k*(b-a)/n) + Integration<n,k-1,trapSum,Expr>::integrate(a,b);
    }
};

基本上，我正在尝试实施梯形法则以使其静态展开。但是，编译器似乎对使用 "termination case" 还是 "trapezoidal sum" 感到困惑。我做错了什么，有解决方法吗？我想强制它使用 "termination case" if k==0 而不管 InterRule 规则的类型。

编辑使其成为运行:

的附加代码

// the types of expressions (+,-,*, etc.)
enum ExprType { mul, divide, add, sub, constant};

// constant
template <ExprType eType, class Left, class Right, int coeff, int power> struct Expr {
    static double eval(double x){
        return coeff * std::pow(x, power);
    }
};


int main()
{

    double a = 1;
    double b = 2;

    // Expr defines the function f(x) = x
    Integration<50, 50, trap, Expr<constant,int,int,1,1>> inte2;
    std::cout << inte2.integrate(a,b) << std::endl;

    return 0;
}

Answer 1

您可以尝试通过添加额外的模板参数来消除歧义。

可能是

// .................................................VVVVVVVVVVV
template <int n, int k, InterRule rule, class Expr, bool = true>
struct Integration {
   // ...
};

false

明确的地面案例

template <int n, InterRule rule, class Expr>
struct Integration<n, 0, rule, Expr, false> { // <--- false !
    static double integrate(double a, double b){
        return 0;
    }
};

并在递归调用中添加正确的参数

    return (b-a)/n * Expr::eval(a + (k-1)*(b-a)/n)
       + Integration<n, k - 1, rule, Expr, (k>1)>::integrate(a,b);
    // ....................................^^^^^

下面是一个完整的编译示例

#include <cmath>
#include <iostream>

// interpolation rules
enum InterRule {trap, rect, trapSum};

// Rectangle rule
template <int n, int k, InterRule rule, class Expr, bool = true>
struct Integration {
    static double integrate(double a, double b){
        return (b-a)/n * Expr::eval(a + (k-1)*(b-a)/n)
           + Integration<n, k - 1, rule, Expr, (k>1)>::integrate(a,b);
    }
};

// termination case
template <int n, InterRule rule, class Expr>
struct Integration<n, 0, rule, Expr, false> {
    static double integrate(double a, double b){
        return 0;
    }
};

// Trapezoidal rule
template <int n, int k, class Expr>
struct Integration<n, k, trap, Expr> {
    static double integrate(double a, double b){
        return (b-a)/n * (Expr::eval(a)/2
           + Integration<n,k-1,trapSum,Expr,(k>1)>::integrate(a,b)
           + Expr::eval(b)/2);
    }
};

// Trapezoidal sum
template <int n, int k, class Expr>
struct Integration<n, k, trapSum, Expr> {
    static double integrate(double a, double b){
        return Expr::eval(a + k*(b-a)/n)
           + Integration<n,k-1,trapSum,Expr,(k>1)>::integrate(a,b);
    }
};

// the types of expressions (+,-,*, etc.)
enum ExprType { mul, divide, add, sub, constant};

// constant
template <ExprType eType, class Left, class Right, int coeff, int power> struct Expr {
    static double eval(double x){
        return coeff * std::pow(x, power);
    }
};


int main ()
 {

    double a = 1;
    double b = 2;

    // Expr defines the function f(x) = x
    Integration<50, 50, trap, Expr<constant,int,int,1,1>> inte2;
    std::cout << inte2.integrate(a,b) << std::endl;
 }

C++ 模板歧义实例化

C++ template ambiguous instantiation

c++

templates

template-specialization

template-meta-programming

variadic-templates