fix: support sparse storage with accumulators (#421)

Signed-off-by: Henry Schreiner <henryfs@princeton.edu>

Author: henryiii

include/boost/histogram/storage_adaptor.hpp

@@ -171,7 +171,8 @@ struct map_impl : T {
       if (it != static_cast<T*>(map)->end()) {
         it->second += u;
       } else {
-        map->emplace(idx, u);
+        auto pair = map->emplace(idx, value_type{});
+        pair.first->second += u;
       }
       return *this;
     }
@@ -183,7 +184,8 @@ struct map_impl : T {
       if (it != static_cast<T*>(map)->end()) {
         it->second -= u;
       } else {
-        map->emplace(idx, -u);
+        auto pair = map->emplace(idx, value_type{});
+        pair.first->second -= u;
       }
       return *this;
     }

test/CMakeLists.txt

@@ -102,6 +102,7 @@ boost_test(TYPE run SOURCES unlimited_storage_test.cpp)
 boost_test(TYPE run SOURCES tools_test.cpp)
 boost_test(TYPE run SOURCES issue_327_test.cpp)
 boost_test(TYPE run SOURCES issue_353_test.cpp)
+boost_test(TYPE run SOURCES issue_416_test.cpp)
 boost_test(TYPE run SOURCES accumulators_fraction_test.cpp)
 boost_test(TYPE run SOURCES utility_binomial_proportion_interval_test.cpp)
 boost_test(TYPE run SOURCES utility_wald_interval_test.cpp)

test/Jamfile

@@ -104,6 +104,7 @@ alias cxx14 :
     [ run tools_test.cpp ]
     [ run issue_327_test.cpp ]
     [ run issue_353_test.cpp ]
+    [ run issue_416_test.cpp ]
     [ run histogram_ostream_test.cpp : : :
       <testing.launcher>"set LANG=UTF" ]
     [ run histogram_ostream_ascii_test.cpp : : :

test/issue_416_test.cpp

@@ -0,0 +1,31 @@
+// Copyright 2026 Hans Dembinski
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/core/lightweight_test.hpp>
+#include <boost/histogram.hpp>
+#include <boost/histogram/algorithm/sum.hpp>
+#include <unordered_map>
+#include <vector>
+#include "throw_exception.hpp"
+
+namespace bh = boost::histogram;
+
+int main() {
+  using cell_t = bh::accumulators::weighted_sum<double>;
+
+  auto h = bh::make_histogram_with(std::unordered_map<std::size_t, cell_t>(),
+                                   bh::axis::regular<>(10, 0.0, 1.0));
+
+  const std::vector<double> values = {0.2, 0.3};
+  const std::vector<double> weights = {1.0, 1.0};
+  h.fill(values, bh::weight(weights));
+
+  const auto s = bh::algorithm::sum(h);
+  BOOST_TEST_EQ(s.value(), 2.0);
+  BOOST_TEST_EQ(s.variance(), 2.0);
+
+  return boost::report_errors();
+}