extract_double.h

#pragma once

#include <stdexcept>

#include "drake/common/nice_type_name.h"

namespace drake {

/// Converts a ScalarType value to a double, failing at runtime (not compile
/// time) if the type cannot be converted to a double.
///
/// This function is useful for writing ScalarType-generic code that (1) can
/// reasonably discard any supplemental scalar data, e.g., the derivatives of an
/// AutoDiffScalar, and (2) is reasonable to fail at runtime if the extraction
/// fails.
///
/// The default implementation throws an exception.  ScalarTypes that can hold a
/// numeric value must overload this method to provide an appropriate
/// extraction.  An overload for `double` is already provided.
///
/// See autodiff_overloads.h to use this with Eigen's AutoDiffScalar.
/// See symbolic_expression.h to use this with symbolic::Expression.
template <typename T>
double ExtractDoubleOrThrow(const T&) {
  throw std::runtime_error(NiceTypeName::Get<T>() +
                           " cannot be converted to a double");
}

/// Returns @p scalar as a double.  Never throws.
inline double ExtractDoubleOrThrow(double scalar) { return scalar; }

}  // namespace drake