Is it possible to access (read only) the variables captured by a lambda?

Question

Is it possible to access (read only) the variables captured by a lambda?

This doesn't work:

std::function<double  (const double)> plus (const double a) {
    return [a] (const double b) -> double {
        return a+b;
    };
}

auto plus5 = plus(5);
cout << plus5.a << endl;

Answer 1

auto plus( double a ) {
  using R = struct {
    double a;
    double operator()(double b)const{return b+a;}
  };
  return R{std::move(a)};
}

live example.

Please note that a std::function is not a lambda, and lambda is not a std::function. They work with each other, but using one term to refer to the other is the opposite of helpful.

Answer 2

This is not how a lambda should be used.

The interface of a lambda is its function signature. Its captures should be considered an implementation detail and not be visible to the user.

If you want explicit access to the captures, write your own function object and expose the respective data members accordingly:

struct MyPlus {
    double a;
    MyPlus(double x) : a(x) {}
    double operator()(const double b)
    {
        return a+b;
    }
};

auto plus5 = MyPlus(5);
std::cout << plus5.a;