Make a const proxy.

arbor/util/piecewise.hpp

@@ -159,8 +159,8 @@ struct pw_element_proxy {
  extent(pw.extent(i)), value(pw.value(i)) {}
 
  operator pw_element<X>() const { return pw_element<X>{extent, value}; }
- operator X() const { return value; }
- pw_element_proxy& operator=(X x) {value = std::move(x); return *this; };
+ operator X&() const { return value; }
+ pw_element_proxy& operator=(X x) { value = std::move(x); return *this; };
 
  double lower_bound() const { return extent.first; }
  double upper_bound() const { return extent.second; }
@@ -169,6 +169,21 @@ struct pw_element_proxy {
  X& value;
 };
 
+template <typename X>
+struct pw_element_proxy<const X> {
+ pw_element_proxy(const pw_elements<X>& pw, pw_size_type i):
+ extent(pw.extent(i)), value(pw.value(i)) {}
+
+ operator pw_element<X>() const { return pw_element<X>{extent, value}; }
+ operator const X&() const { return value; }
+
+ double lower_bound() const { return extent.first; }
+ double upper_bound() const { return extent.second; }
+
+ const std::pair<double, double> extent;
+ const X& value;
+};
+
 // Compute indices into vertex set corresponding to elements that cover a point x:
 
 namespace {
@@ -238,12 +253,12 @@ struct pw_elements {
  const_iterator(): pw_(nullptr) {}
 
  using value_type = pw_element<X>;
- using pointer = const pointer_proxy<pw_element<X>>;
- using reference = pw_element<X>;
+ using pointer = const pointer_proxy<pw_element_proxy<const X>>;
+ using reference = pw_element_proxy<const X>;
 
- reference operator[](difference_type j) const { return (*pw_)[j+*c_]; }
- reference operator*() const { return (*pw_)[*c_]; }
- pointer operator->() const { return pointer{(*pw_)[*c_]}; }
+ reference operator[](difference_type j) const { return {*pw_, j + *c_}; }
+ reference operator*() const { return {*pw_, *c_}; }
+ pointer operator->() const { return pointer{reference{*pw_, *c_}}; }
 
  // (required for iterator_adaptor)
  counter<pw_size_type>& inner() { return c_; }