在C ++中使用std :: bind和std :: function时出错 [英] Error using std::bind and std::function in C++

问题描述

我尝试在多元函数上尝试使用牛顿方法的代码段，并使用了std::bind和std::function.但是我陷入了错误

I attempt to try my snippet of Newton's method on a multivariate function and used std::bind and std::function. But I was stuck on an error

错误:从'std :: _ Bind_helper& ;, int> :: type {aka转换 std :: __ Bind，int))(double，double， double)>}'转换为非标量类型'std :: function' 请求

error: conversion from 'std::_Bind_helper&, int>::type {aka std::_Bind, int))(double, double, double)>}' to non-scalar type 'std::function' requested

此错误消息是什么意思，我该如何修复当前代码?

What does this error message mean and how should I fix my current code?

#include <iostream>
#include<functional>
#include<cmath>

double newton(std::function<double(double)> F, std::function<double(double)> f,
              double x=0, int maxiter=1000, double epsilon=0.001)
{
    int n = 0;
    while((n < maxiter) && (fabs(F(x)) > epsilon))
    {
        x = x - F(x) / f(x);
        n++;
    }
    return x;
}

// I'd like to fix x and z at 1 and 2 and find root for y
double ftest(double x, double y, double z) 
{
    return x * x + (y * y - 2 * y - 4) + z * z;
}

// Partial derivative of ftest with regards to y
double ftest1(double y) 
{
    return 2 * y - 2;
}

int main()
{
    using namespace std::placeholders;
    std::function<double(double)> F = std::bind(ftest, 1, _2, 2);
    std::function<double(double)> f = ftest1;
    std::cout << newton(F, f);
    return 0;
}

推荐答案

此处的问题:

std::function<double(double)> F = std::bind(ftest, 1, _2, 2);

是F是一个接受类型为double的单个参数的函数，但是绑定表达式涉及_2-指的是传递给bind()返回的函数对象的第二个参数.即 second 参数.基本上，您将大致构造此功能对象:

is that F is a function that takes a single argument of type double, but your bind expression involves _2 - which refers to the second argument passed to the function object that bind() returns. That is, the second argument. Basically, you're constructing this function object, roughly:

struct {
    template <class T, class U>
    auto operator()(T, U arg) {
        return ftest(1, arg, 2);
    }
};

该对象有两个参数. std::function<double(double)>不允许这样做-它要求您的可调用项允许单个参数.

That object takes two arguments. std::function<double(double)> doesn't allow for that - it requires that your callable allow for a single argument.

简单的解决方法是修复占位符:

The simple fix is to fix the placeholder:

std::function<double(double)> F = std::bind(ftest, 1, _1, 2);

或者更好的是，根本不使用bind()而是使用lambda:

or, better, just don't use bind() at all and prefer a lambda:

std::function<double(double)> F = [](double y) { return ftest(1, y, 2); }

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