add unsafe_access and test it

include/boost/histogram/accumulators/mean.hpp

@@ -152,6 +152,8 @@ public:
   }
 
 private:
+  friend struct unsafe_access;
+
   internal_data_type data_{0, 0, 0};
 };
 

include/boost/histogram/unsafe_access.hpp

@@ -110,6 +110,15 @@ struct unsafe_access {
   static constexpr auto& storage_adaptor_impl(storage_adaptor<T>& storage) {
     return static_cast<typename storage_adaptor<T>::impl_type&>(storage);
   }
+
+  /**
+    Get internal data of accumulators::mean.
+    @param obj instance of accumulator.
+  */
+  template <class T>
+  static constexpr auto& accumulators_mean_impl(T&& m) {
+    return m.data_;
+  }
 };
 
 } // namespace histogram

test/accumulators_mean_test.cpp

@@ -7,6 +7,7 @@
 #include <boost/core/lightweight_test.hpp>
 #include <boost/histogram/accumulators/mean.hpp>
 #include <boost/histogram/accumulators/ostream.hpp>
+#include <boost/histogram/unsafe_access.hpp>
 #include <boost/histogram/weight.hpp>
 #include <sstream>
 #include "is_close.hpp"
@@ -110,5 +111,16 @@ int main() {
     BOOST_TEST_IS_CLOSE(a.variance(), b.variance(), 1e-3);
   }
 
+  // unsafe_access
+  {
+    m_t a;
+    a(1);
+    a(2);
+
+    BOOST_TEST_EQ(a.count(), 2);
+    unsafe_access::accumulators_mean(a).sum_ = 1;
+    BOOST_TEST_EQ(a.count(), 1);
+  }
+
   return boost::report_errors();
 }