error: ‘foo’ is not captured Lambda function and boost integration

Question

Im triyng to perform a numerical integral using Boost Library quadrature. I did it using trapezoidal like this

double integrand(double xp, double yp, double x, double y, double a, double b) {
    double eps;
    if(x - xp == 0 && y - yp == 0)
        eps = 1e-6;
    else
        eps = 0.0;

    return std::log(std::hypot(x - xp + eps, y - yp + eps)) * w(xp, yp);
}
double integrate(double x, double y, double a, double b) {
    auto inner_integral = [m, n, a, b](double yp) {
        auto f = [z, m, n, a, b](double xp) { return integrand(xp, yp, x, y, a, b); };
        return boost::math::quadrature::trapezoidal(f, 0.0, a, 1e-8);
    };
    return boost::math::quadrature::trapezoidal(inner_integral, 0.0, b, 1e-8);
}

This works fine but because we have an indetermination close to log(0) it takes a lot of time, so I searched for an alternative. In Boost, they have some integration methods for this kind of situation, so I decided to try tanh_sinh method. I implemented it like this following the given example.

double Integrate(double x, double y, double a, double b) {
    boost::math::quadrature::tanh_sinh<double> integrator;
    auto inner_integral = [x, y, a, b](double yp) {
        auto f = [yp, x, y, a, b](double xp) { return integrand(xp, yp, x, y, a, b); };
        return integrator.integrate(f, 0.0, a);
    };
    return integrator.integrate(inner_integral, 0.0, b);
}

The problem is that when I compile it says error: ‘integrator’ is not captured followed with note: the lambda has no capture-default and a huge error.

What am I doing wrong? Why doesn't work?

Answer 1

The problem is exactly what it says. Your lambda isn't capturing integrator. Change [x,y,a,b] to [x,y,a,b,integrator] to capture by value or [x,y,a,b,&integrator] to capture by reference.

Alternatively you can capture everything by value with [=] or everything by reference with [&].

See lambda expressions for syntax.