Bug: Clang compilation error due to const-correctness in image.h

[dkar-sto]

Summary

When compiling code that uses nvblox with clang, a build error occurs in nvblox/sensors/internal/impl/image_impl.h. The error, cannot assign to return value because function 'operator()' returns a const value, is triggered because MaskedImageView::setMasked() attempts to modify a value returned by a const operator(). This issue is not present when compiling with gcc, indicating stricter const-correctness enforcement by clang.

Build Environment

• Compiler: clang++ (LLVM version 18)
• C++ Standard: C++20
• Operating System: Linux

Steps to Reproduce

To reproduce the error, you can use the following minimal example. This code snippet isolates the problematic behavior by creating a MaskedImageView and calling setMasked(), which triggers the const-correctness error with clang.

1. Create a C++ source file

Create a file named repro.cpp:

#include <nvblox/sensors/image.h>

int main(int argc, char** argv) {
  // Dummy data
  constexpr int rows = 10;
  constexpr int cols = 10;
  float image_data[rows * cols] = {0.0f};
  unsigned char mask_data[rows * cols] = {0};

  // Create ImageViews (non-owning wrappers)
  nvblox::ImageView<float> image_view(rows, cols, image_data);
  nvblox::ImageView<const unsigned char> mask_view(rows, cols, mask_data);

  // Create a MaskedImageView
  nvblox::MaskedImageView<float> masked_image_view(image_view, mask_view);

  // This call fails to compile with clang because mask_view.operator() is const
  // and returns a const reference, which can't be assigned to.
  masked_image_view.setMasked(5, 5);

  return 0;
}

2. Create a CMakeLists.txt

Create a CMakeLists.txt file in the same directory:

cmake_minimum_required(VERSION 3.16)
project(nvblox_repro LANGUAGES CXX CUDA)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

# Find the dependencies for nvblox
find_package(glog REQUIRED)
find_package(gflags REQUIRED)
find_package(stdgpu REQUIRED)
find_package(CUDA REQUIRED)
find_package(nvblox REQUIRED)

add_executable(repro repro.cpp)

target_link_libraries(repro PRIVATE nvblox::nvblox_lib)

3. Configure and Build with Clang

Configure and build the project using clang as the compiler. This will fail at the build step.

export CC=/usr/bin/clang
export CXX=/usr/bin/clang++

cmake -B build
cmake --build build

The build will fail with the error described in the log below.

Build Error Log

Here is the relevant output from the build failure:

[ 50%] Building CXX object CMakeFiles/repro.dir/repro.cpp.o
In file included from /home/dkar/deps/nvblox/repro_test/repro.cpp:1:
In file included from /usr/local/include/nvblox/sensors/image.h:518:
/usr/local/include/nvblox/sensors/internal/impl/image_impl.h:265:27: error: cannot assign to return value because function 'operator()' returns a const value
  265 |   mask_(row_idx, col_idx) = image::kMaskedValue;
      |   ~~~~~~~~~~~~~~~~~~~~~~~ ^
/usr/local/include/nvblox/sensors/image.h:132:30: note: function 'operator()' which returns const-qualified type 'ElementType &' (aka 'const unsigned char &') declared here
  132 |   __host__ __device__ inline ElementType& operator()(const int row_idx,
      |                              ^~~~~~~~~~~~
1 error generated.
gmake[2]: *** [CMakeFiles/repro.dir/build.make:79: CMakeFiles/repro.dir/repro.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:87: CMakeFiles/repro.dir/all] Error 2
gmake: *** [Makefile:91: all] Error 2

Code Analysis

The error occurs in nvblox/sensors/internal/impl/image_impl.h at line 265, within the MaskedImageView<ElementType>::setMasked function:

// nvblox/sensors/internal/impl/image_impl.h:265
mask_(row_idx, col_idx) = image::kMaskedValue;

The mask_ member is an ImageView<unsigned char>. Its operator() is defined in nvblox/sensors/image.h as a const method that returns a const reference, which prevents modification of the underlying data.

// nvblox/sensors/image.h:~132 (inside ImageView)
__host__ __device__ inline const ElementType& operator()(const int row_idx,
                                                      const int col_idx) const {
  return get(row_idx, col_idx);
}

To allow modification, a non-const overload of operator() is probably needed in the ImageView class.

Workaround

A temporary workaround that resolves the immediate compilation issue is to use const_cast at the call site in image_impl.h:

// nvblox/sensors/internal/impl/image_impl.h:265
const_cast<unsigned char&>(mask_(row_idx, col_idx)) = image::kMaskedValue;

While this works, adding the non-const overload is the more correct and robust solution.

[FabianReister]

I can confirm the same behavior with GCC 14. While GCC 13 works without issues, the problem also occurs in the following environment:

• Compiler: g++-14 (14.2.0)
• C++ Standard: -std=gnu++17, -std=gnu++20 and -std=gnu++23
• Operating System: Ubuntu 24.04.3 LTS
• nvblox version: tested tag: v0.0.8 and tag: v0.0.9rc1

[nikolaseu]

Yeah. This is not even building