QGIS Build Failure: Ambiguous QByteArray Call

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Have you encountered a frustrating build failure while working with the latest QGIS master branch? Specifically, are you seeing the error message "call of overloaded (... QByteArray ...) is ambiguous"? If so, you're not alone! This article dives deep into this issue, explaining the root cause, potential solutions, and how it might relate to your system configuration.

Understanding the Issue

At the heart of the problem is an ambiguity in how the QByteArray class interacts with certain functions within the QGIS codebase, particularly in the context of the vector_tile.pb.h file. This file, generated by the Protocol Buffers (protobuf) compiler, defines the data structures used for vector tile encoding. The error arises when the compiler encounters multiple possible interpretations of a function call involving a QByteArray, leaving it unable to determine the correct one.

Let's break down the specific error messages to get a clearer picture:

In file included from /usr/local/src/QGIS/src/core/vectortile/qgsvectortilemvtencoder.h:23,
                 from /usr/local/src/QGIS/src/core/vectortile/qgsvectortilemvtencoder.cpp:16:
/usr/local/src/QGIS/build/src/core/vector_tile.pb.h: In instantiation of ‘void vector_tile::Tile_Layer::set_name(Arg_&&, Args_ ...) [with Arg_ = QByteArray; Args_ = {}]’:
/usr/local/src/QGIS/src/core/vectortile/qgsvectortilemvtencoder.cpp:206:22:   required from here
  206 |   tileLayer->set_name( layerName.toUtf8() );
      |   ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/local/src/QGIS/build/src/core/vector_tile.pb.h:2120:19: error: call of overloaded ‘Set(QByteArray, google::protobuf::Arena*)’ is ambiguous
 2120 |   _impl_.name_.Set(static_cast<Arg_&&>(arg), args..., GetArena());
      |   ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/src/QGIS/build/src/core/vector_tile.pb.h:2120:19: note: there are 6 candidates
In file included from /usr/local/src/QGIS/build/src/core/vector_tile.pb.h:22:
/usr/include/google/protobuf/arenastring.h:294:8: note: candidate 1: ‘void google::protobuf::internal::ArenaStringPtr::Set(absl::lts_20250814::string_view, google::protobuf::Arena*)’
  294 |   void Set(absl::string_view value, Arena* arena);
      |        ^~~
/usr/include/google/protobuf/arenastring.h:464:13: note: candidate 2: ‘void google::protobuf::internal::ArenaStringPtr::Set(const char*, google::protobuf::Arena*)’
  464 | inline void ArenaStringPtr::Set(const char* s, Arena* arena) {
      |             ^~~~~~~~~~~~~~

This snippet highlights the first instance of the error, occurring within the set_name function of the Tile_Layer class in vector_tile.pb.h. The error message indicates that the Set function, which is part of the protobuf library's string handling mechanism, has multiple overloads that could potentially accept a QByteArray. The compiler is unable to choose the correct overload, leading to the ambiguous call error. This ambiguity problem typically surfaces when you're dealing with string conversions between Qt's QByteArray and the string types used by Protocol Buffers. The next error message block further confirms the issue:

In file included from /usr/local/src/QGIS/build/src/core/vector_tile.pb.h:24:
/usr/include/google/protobuf/generated_message_util.h: In instantiation of ‘void google::protobuf::internal::AddToRepeatedPtrField(google::protobuf::RepeatedPtrField<std::__cxx11::basic_string<char> >&, Arg&&, Args ...) [with Arg = QByteArray; Args = {}]’:
/usr/local/src/QGIS/build/src/core/vector_tile.pb.h:2257:54:   required from ‘void vector_tile::Tile_Layer::add_keys(Arg_&&, Args_ ...) [with Arg_ = QByteArray; Args_ = {}]’
 2257 |   ::google::protobuf::internal::AddToRepeatedPtrField(*_internal_mutable_keys(),
      |   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
 2258 |                                ::std::forward<Arg_>(value),
      |                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 2259 |                                args... );
      |                                ~~~~~~~~~              
/usr/local/src/QGIS/src/core/vectortile/qgsvectortilemvtencoder.cpp:213:24:   required from here
  213 |     tileLayer->add_keys( fields[i].name().toUtf8() );
      |     ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/google/protobuf/generated_message_util.h:374:17: error: call of overloaded ‘AssignToString(std::__cxx11::basic_string<char>&, QByteArray)’ is ambiguous
  374 |   AssignToString(*dest.Add(), std::forward<Arg>(value), args...);
      |   ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/google/protobuf/generated_message_util.h:374:17: note: there are 6 candidates
/usr/include/google/protobuf/generated_message_util.h:351:13: note: candidate 1: ‘void google::protobuf::internal::AssignToString(std::string&, const char*, BytesTag)’
  351 | inline void AssignToString(std::string& dest, const char* value,
      |             ^~~~~~~~~~~~~~
/usr/include/google/protobuf/generated_message_util.h:363:13: note: candidate 2: ‘void google::protobuf::internal::AssignToString(std::string&, absl::lts_20250814::string_view, BytesTag)’
  363 | inline void AssignToString(std::string& dest, absl::string_view value,
      |             ^~~~~~~~~~~~~~

Here, the ambiguity occurs in the AssignToString function, which is used to add keys to a repeated field within the protobuf message. Again, the compiler struggles to determine the correct overload to use with the provided QByteArray.

The Qt 6.10 Connection

The original bug report highlights a crucial detail: this issue surfaced after upgrading Qt to version 6.10.0. This suggests that changes in Qt 6.10, particularly in how QByteArray interacts with other libraries, might be the trigger for this ambiguity. Qt 6 introduced significant changes, including stricter overload resolution rules and changes to string handling. These changes, while generally improving code quality and consistency, can sometimes expose ambiguities that were previously tolerated by older compilers or Qt versions. Essentially, what used to "just work" might now require more explicit type conversions or function calls to resolve the ambiguity.

Impact and Affected Code

The errors specifically point to code related to vector tile encoding (qgsvectortilemvtencoder.cpp) and the generated protobuf code (vector_tile.pb.h). This suggests that the issue is primarily affecting the part of QGIS responsible for creating and handling vector tiles, a crucial component for modern web mapping and data visualization. If you're working with vector tiles in QGIS, this build failure will prevent you from compiling the latest master branch.

Potential Causes and Solutions

So, what can be done to fix this? Here's a breakdown of potential causes and solution strategies:

  1. Qt 6.10 Incompatibilities: The most likely culprit is a change in Qt 6.10's handling of QByteArray or its interaction with protobuf. This could involve stricter type checking or changes in the available implicit conversions. Possible Solutions:
    • Explicit Type Conversions: The most direct approach is to explicitly convert the QByteArray to the expected type by the protobuf functions. This might involve using QString::fromStdString to convert to a standard C++ string or using the appropriate absl::string_view constructor.
    • Qt Bug: It's possible that this is a bug in Qt 6.10 itself. Checking the Qt bug tracker and reporting the issue there could lead to a fix in a future Qt release. If this is the case, QGIS developers can potentially work around the issue in the QGIS code while waiting for a Qt fix.
  2. Protobuf Version: While the bug report mentions protobuf 32.0.2, it's worth ensuring that you're using a compatible version of protobuf with both Qt 6.10 and the QGIS master branch. Older protobuf versions might have compatibility issues with newer Qt versions. Possible Solutions:
    • Update Protobuf: Try updating to the latest stable version of protobuf. Newer versions often include bug fixes and improved compatibility.
    • Check Compatibility: Consult the QGIS documentation or build instructions to determine the recommended protobuf version.
  3. Compiler Issues: In rare cases, the compiler itself might have trouble resolving the overloads, especially with complex code involving templates and multiple inheritance. Possible Solutions:
    • Try a Different Compiler: If possible, try building QGIS with a different compiler version (e.g., GCC, Clang) to see if the issue persists.
    • Compiler Flags: Experiment with different compiler flags related to overload resolution and template instantiation.
  4. QGIS Code Changes: It's always possible that a recent change in the QGIS codebase has introduced the ambiguity. Possible Solutions:
    • Revert Commits: If the issue appeared recently, try reverting to an older commit in the QGIS master branch to see if the build succeeds. This can help pinpoint the problematic commit.
    • QGIS Developers: The best course of action is to report the issue to the QGIS developers (as was done in the original bug report). They have the most in-depth knowledge of the codebase and can likely identify the root cause and implement a fix.

Practical Steps to Resolve the Issue

For those encountering this build failure, here's a step-by-step approach you can take:

  1. Confirm Your Setup:
    • Verify your Qt version (qmake --version).
    • Check your protobuf version (protoc --version).
    • Note the QGIS commit hash you're building (e.g., 63a8d7b).
  2. Try Explicit Conversions:
    • Locate the problematic code sections in qgsvectortilemvtencoder.cpp and vector_tile.pb.h (using the error messages as a guide).
    • Insert explicit type conversions (e.g., QString::fromStdString(layerName.toUtf8().constData())) to resolve the ambiguity. This is a trial-and-error process, as the correct conversion might vary depending on the specific context.
  3. Update Dependencies:
    • Update protobuf to the latest stable version.
    • If possible, try building with a slightly older Qt 6 version (e.g., 6.9 if available) to see if the issue is specific to 6.10.
  4. Report the Issue:
    • If you can't resolve the issue yourself, provide detailed information (Qt version, protobuf version, QGIS commit hash, error messages, steps to reproduce) in a bug report on the QGIS issue tracker.

Example of Explicit Conversion

Let's illustrate how an explicit conversion might look in practice. Consider this line from the original error message:

tileLayer->set_name( layerName.toUtf8() );

This line is causing the ambiguity because layerName.toUtf8() returns a QByteArray, which has multiple possible interpretations in the set_name function. To resolve this, we can explicitly convert the QByteArray to a standard C++ string:

tileLayer->set_name( QString::fromStdString(layerName.toUtf8().constData()).toStdString() );

This conversion chain does the following:

  1. layerName.toUtf8(): Converts the layerName (presumably a QString) to a QByteArray.
  2. .constData(): Returns a const char* pointer to the underlying data in the QByteArray.
  3. QString::fromStdString( ... ): Creates a QString from the const char*.
  4. toStdString(): converts the QString to a standard C++ string

Important: This is just an example, and the correct conversion might differ depending on the specific function and the expected input type.

Conclusion

The "call of overloaded (... QByteArray ...) is ambiguous" error in QGIS builds, particularly after upgrading to Qt 6.10, highlights the complexities of modern software development. It underscores the importance of understanding how different libraries interact and the potential for subtle changes in one library to impact another. By carefully analyzing the error messages, experimenting with explicit type conversions, and engaging with the QGIS developer community, you can overcome this build failure and continue contributing to this powerful open-source GIS platform. Remember, these build issues often pave the way for a more robust and well-defined code base in the long run. So, keep digging, keep reporting, and keep building!