C ++外部函数，带有指向函数作为参数的指针，在带有成员函数的类内部使用

问题描述

是C ++的新手.假设我有一堂课:

Fairly new to C++. Suppose I have a class:

class A
{
private:
    double m_x, m_y;
public:
    A(double x, double y): m_x {x}
    {
        m_y = extF(m_x, y, *intF);
    }

    double intF(double x) { return 2*x; }
};

它利用了在其他地方定义的外部全局函数:

And it makes use of an external global function, defined elsewhere:

double extF(double x, double y, std::function<double(double)> f)
{
    if (x*y < 0)
        return f(x);
    else
        return f(y);
}

配方是假的.这不会编译.我尝试了简单的intF，A::*intF，&A::intF，甚至是一些非正统的组合，但这只是猜测.问题在于，类A并不是唯一一个使用全局外部函数的类，它应该可以在运行时成为用户的选择.搜索显示了一些答案，表明不可能像这样的成员函数创建指针，因为它需要实例化(?)，但是我没有找到解决方案.能做到吗?如果是，怎么办?

Formulas are bogus. This does not compile. I tried simple intF, A::*intF, &A::intF, even some unorthodox combinations, but that's just guessing. The problem is that class A is not the only one which makes use of the global external function and it's something that should be able to be a user choice at runtime. Searches revealed some answers saying it's not possible to make a pointer to a member function like this because it needs instantiation(?), but I found no solutions. Can this be done? If yes, how?

另一个问题:如果成员函数为const double f(...) const，如何完成指向成员函数的指针?

Additional question: how can the pointer to member function be done if the member function is const double f(...) const?

推荐答案

一个变种就是使用lambda:

One variant is just to use lambda:

class A
{
private:
    double m_x, m_y;
public:
    A(double x, double y): m_x {x}
    {
         m_y = extF(m_x, y, [&](double d){ return intF(d);});
    }

    double intF(double x) { return 2*x; }
};

另一种变体是使用lambda和 std::mem_fn (省略您课程的其余代码):

Another variant is to use lambda and std::mem_fn (omitting the rest code of your class):

A(double x, double y): m_x {x}
{
    auto fn = std::mem_fn(&A::intF);
    m_y = extF(m_x, y, [&](double d) {return fn(this, d);});
}

最后，如果您绑定，则可能会摆脱lambda成员函数指针的对象参数:

And finally you may get rid of lambdas if you bind the object parameter of member function pointer:

A(double x, double y): m_x {x}
{
    auto fn1 = std::bind(std::mem_fn(&A::intF), this, std::placeholders::_1);
    m_y = extF(m_x, y, fn1);
}

所有这些都可以与常量成员函数一起使用.

All these also work with constant member functions.

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