From 03683e1ff8584c197aca06d973bb366982d4d6c5 Mon Sep 17 00:00:00 2001
From: shahoian <ruben.shahoyan@cern.ch>
Date: Mon, 3 Jun 2024 13:52:45 +0200
Subject: [PATCH] Do not throw on merging non-initialized flat histors

---
 DataFormats/common/src/FlatHisto1D.cxx | 5 +++--
 DataFormats/common/src/FlatHisto2D.cxx | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/DataFormats/common/src/FlatHisto1D.cxx b/DataFormats/common/src/FlatHisto1D.cxx
index 1313da72da8b7..8e04c2c58614e 100644
--- a/DataFormats/common/src/FlatHisto1D.cxx
+++ b/DataFormats/common/src/FlatHisto1D.cxx
@@ -40,9 +40,10 @@ FlatHisto1D<T>& FlatHisto1D<T>::operator=(const FlatHisto1D<T>& rhs)
   if (this == &rhs) {
     return *this;
   }
-  if (!rhs.canFill()) {
+  if (!rhs.canFill() && rhs.getNBins()) { // was initialized
     throw std::runtime_error("trying to copy read-only histogram");
-  } else {
+  }
+  if (rhs.getNBins()) {
     mContainer = rhs.mContainer;
     init(gsl::span<const T>(mContainer.data(), mContainer.size()));
   }
diff --git a/DataFormats/common/src/FlatHisto2D.cxx b/DataFormats/common/src/FlatHisto2D.cxx
index b06241ff8d193..aa064eee10bca 100644
--- a/DataFormats/common/src/FlatHisto2D.cxx
+++ b/DataFormats/common/src/FlatHisto2D.cxx
@@ -43,9 +43,10 @@ FlatHisto2D<T>& FlatHisto2D<T>::operator=(const FlatHisto2D<T>& rhs)
   if (this == &rhs) {
     return *this;
   }
-  if (!rhs.canFill()) {
+  if (!rhs.canFill() && rhs.getNBins()) { // was initialized
     throw std::runtime_error("trying to copy read-only histogram");
-  } else {
+  }
+  if (rhs.getNBins()) {
     mContainer = rhs.mContainer;
     init(gsl::span<const T>(mContainer.data(), mContainer.size()));
   }