Run time choice of method using std::bind. Problem with access to variable-CodePudding

I need to choice the particular method of the class at the run time. To do it I use std::bind. Below is an example demonstrating what I do:

#include<iostream>
#include<functional>
class bc {
public:
    bc(int, double);
    std::function<double(double)> f; //pointer to function which will be binded with f1 or f2 
    void setVal(double v) {val = v;} //method allowing to change val

    double val;  //some variable used by f1 and f2
    double f1(double v) {return 2*v*val;} 
    double f2(double v) {return 3*v*val;}
};

bc::bc(int fid, double v)    //depends on fid the f1 or f2 is chosen for binding
{
    using namespace std::placeholders;
    this->val = v;
    if(fid == 1)
        f = std::bind(&bc::f1, *this, _1);
    else
        f = std::bind(&bc::f2, *this, _1);
}

So depends on the value of fid given to the constructor the necessary implementation (f1 or f2) is chosen. Then in the main:

int main()
{
    bc my(1, 2.0);
    std::cout << my.f(1) << std::endl; //1
    my.setVal(5.0);
    std::cout << my.f(1) << std::endl; //2
    return 0;
}

The first output from string //1 is as expected: 4 . But the second output (//2) is also 4, while it should be 10, because the value of val should be changed to 5 by my.setVal(5.0).

I expect that something like a copy of the class was made at the stage of binding, and the change of the val by my.setVal(5.0) have no effect on this "copy".

How can I solve this problem? Or may be there is a better way to make run time choice between several implementations of some function.

CodePudding user response：

Do not dereference this before passing it to std::bind:

    if(fid == 1)
        f = std::bind(&bc::f1, this, _1);
    else
        f = std::bind(&bc::f2, this, _1);

By default std::bind stores copies of the arguments, so you ended up with a copy of bc in it, and changing the val field of the original has no effect. Alternatively, you can use std::reference_wrapper to achieve the same effect:

    if(fid == 1)
        f = std::bind(&bc::f1, std::ref(*this), _1);
    else
        f = std::bind(&bc::f2, std::ref(*this), _1);

CodePudding user response：

*this causes a copy of the current object to be bound, use this or std::ref(*this) instead.