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

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;

Thanks for your help.