# "Multiple definition" error only for additional header file in Qt CMakeFile

**URL:** <https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640>\
**Category:** Code\
**Created:** [February 15, 2020, 10:15pm UTC](https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640 "2020-02-15T22:15:35Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![prex](https://discourse.cmake.org/user_avatar/discourse.cmake.org/prex/32/306_2.png) [@prex](https://discourse.cmake.org/u/prex)\
**Post date:** [February 15, 2020, 10:15pm UTC](https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640/1 "2020-02-15T22:15:35Z")

</div>

I’m building Qt 5 with CMake. This worked fine so far until I added an additional header file with some message definitions. The header file is called “messages.h” and contains `message_definitions::error_msgs_` string. I don’t explicitly include it in the CMakeFile.

Only this additional header file gives a “multiple definition” error. I’m clearly not including the header in any other file and it is included in the same way as the other three header files which don’t give any error. What is going on here?

The `messages.h.` header is included only in `qrosnode.h`. Overall I’m wondering why I get such an error only for `messages.h`.

Btw, the `cs` commands are macros from catkin\_simple: [https://github.com/catkin/catkin\_simple](https://github.com/catkin/catkin_simple)

CMakeLists:

```
cmake_minimum_required(VERSION 2.8.3)
project(nav_gui)

find_package(catkin_simple REQUIRED)
catkin_simple(ALL_DEPS_REQUIRED)

add_definitions(-std=c++11 -fPIC -Wall -Wextra)

##################
# QT ENVIRONMENT #
##################
#set(CMAKE_INCLUDE_CURRENT_DIR ON)
set(CMAKE_AUTOMOC ON)
#set(CMAKE_AUTOUIC ON)

find_package(Qt5 COMPONENTS Core Quick Gui QuickControls2 REQUIRED)

############
# SECTIONS #
############

file(GLOB QT_RESOURCES RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} resources/*.qrc)

QT5_ADD_RESOURCES(QT_RESOURCES_CPP ${QT_RESOURCES})

include_directories(
    ${Qt5Core_INCLUDE_DIRS}
    ${Qt5Quick_INCLUDE_DIRS}
    ${Qt5Gui_INCLUDE_DIRS}
    ${Qt5QuickControls2_INCLUDE_DIRS}
    )

#############
# LIBRARIES #
#############

#cs_add_library(${PROJECT_NAME}
# src/qros.cpp
# src/qrosnode.cpp
# src/imagewriter.cpp
#)

############
# BINARIES #
############
cs_add_executable(nav_gui_node 
                src/main.cpp 
                ${QT_RESOURCES_CPP} 
                ${QT_MOC_HPP}

                src/qros.cpp
                src/qrosnode.cpp
                src/imagewriter.cpp
                
                include/nav_gui/qros.h
                include/nav_gui/qrosnode.h
                include/nav_gui/imagewriter.h
                )

target_link_libraries(nav_gui_node
    Qt5::Core
    Qt5::Quick
    Qt5::Gui
    Qt5::QuickControls2
)

##########
# EXPORT #
##########
cs_install()
cs_export()

```

Error:

```
CMakeFiles/nav_gui_node.dir/src/qros.cpp.o:(.bss+0x0): multiple definition of `message_definitions::error_msgs_[abi:cxx11]'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o:(.bss+0x0): first defined here
CMakeFiles/nav_gui_node.dir/src/qrosnode.cpp.o:(.bss+0x0): multiple definition of `message_definitions::error_msgs_[abi:cxx11]'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o:(.bss+0x0): first defined here
CMakeFiles/nav_gui_node.dir/nav_gui_node_autogen/mocs_compilation.cpp.o:(.bss+0x0): multiple definition of `message_definitions::error_msgs_[abi:cxx11]'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o:(.bss+0x0): first defined here

```

Furthermore, why can’t I add `qros.cpp`, `qrosnode.cpp` and `imagewriter.cpp` as a library with `cs_add_library`? This is what I normally do with my classes. Here I get the error:

```
CMakeFiles/nav_gui_node.dir/src/main.cpp.o: In function `main':
main.cpp:(.text.startup+0xa0a): undefined reference to `QROS::finished()'
main.cpp:(.text.startup+0xa5e): undefined reference to `QROS::staticMetaObject'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o: In function `int qmlRegisterType<ImageWriter>(char const*, int, int, char const*)':
main.cpp:(.text._Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_[_Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_]+0x1c): undefined reference to `ImageWriter::staticMetaObject'
main.cpp:(.text._Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_[_Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_]+0x141): undefined reference to `ImageWriter::staticMetaObject'
main.cpp:(.text._Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_[_Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_]+0x16f): undefined reference to `ImageWriter::staticMetaObject'
main.cpp:(.text._Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_[_Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_]+0x363): undefined reference to `ImageWriter::staticMetaObject'
main.cpp:(.text._Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_[_Z15qmlRegisterTypeI11ImageWriterEiPKciiS2_]+0x3d2): undefined reference to `ImageWriter::staticMetaObject'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o: In function `QQmlPrivate::QQmlElement<ImageWriter>::~QQmlElement()':
main.cpp:(.text._ZN11QQmlPrivate11QQmlElementI11ImageWriterED2Ev[_ZN11QQmlPrivate11QQmlElementI11ImageWriterED5Ev]+0x24): undefined reference to `vtable for ImageWriter'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o: In function `non-virtual thunk to QQmlPrivate::QQmlElement<ImageWriter>::~QQmlElement()':
main.cpp:(.text._ZN11QQmlPrivate11QQmlElementI11ImageWriterED2Ev[_ZN11QQmlPrivate11QQmlElementI11ImageWriterED5Ev]+0x80): undefined reference to `vtable for ImageWriter'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o: In function `non-virtual thunk to QQmlPrivate::QQmlElement<ImageWriter>::~QQmlElement()':
main.cpp:(.text._ZN11QQmlPrivate11QQmlElementI11ImageWriterED0Ev[_ZN11QQmlPrivate11QQmlElementI11ImageWriterED5Ev]+0x30): undefined reference to `vtable for ImageWriter'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o: In function `QQmlPrivate::QQmlElement<ImageWriter>::~QQmlElement()':
main.cpp:(.text._ZN11QQmlPrivate11QQmlElementI11ImageWriterED0Ev[_ZN11QQmlPrivate11QQmlElementI11ImageWriterED5Ev]+0x94): undefined reference to `vtable for ImageWriter'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o:(.data.rel.ro._ZTIN11QQmlPrivate11QQmlElementI11ImageWriterEE[_ZTIN11QQmlPrivate11QQmlElementI11ImageWriterEE]+0x10): undefined reference to `typeinfo for ImageWriter'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o:(.data.rel.ro._ZTVN11QQmlPrivate11QQmlElementI11ImageWriterEE[_ZTVN11QQmlPrivate11QQmlElementI11ImageWriterEE]+0x10): undefined reference to `ImageWriter::metaObject() const'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o:(.data.rel.ro._ZTVN11QQmlPrivate11QQmlElementI11ImageWriterEE[_ZTVN11QQmlPrivate11QQmlElementI11ImageWriterEE]+0x18): undefined reference to `ImageWriter::qt_metacast(char const*)'
CMakeFiles/nav_gui_node.dir/src/main.cpp.o:(.data.rel.ro._ZTVN11QQmlPrivate11QQmlElementI11ImageWriterEE[_ZTVN11QQmlPrivate11QQmlElementI11ImageWriterEE]+0x20): undefined reference to `ImageWriter::qt_metacall(QMetaObject::Call, int, void**)'
CMakeFiles/nav_gui_node.dir/src/qros.cpp.o: In function `QROS::setPower(bool)':
qros.cpp:(.text+0x177): undefined reference to `QROS::powerChanged(bool)'
CMakeFiles/nav_gui_node.dir/src/qros.cpp.o: In function `QROS::resetLog()':
...

```

I hope someone can add some clarity here.

---

<div class="post-metadata">

**Author:** ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)\
**Post date:** [February 16, 2020, 1:19am UTC](https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640/2 "2020-02-16T01:19:19Z")

</div>

It looks like `QT_MOC_HPP` is never set. I expect that would fix the `staticMetaObject` and `vtable` errors.

As for the `error_msgs_` error, could you show its declaration? I suspect that it’s not getting the right linker rules applied to it.

---

<div class="post-metadata">

**Author:** ![prex](https://discourse.cmake.org/user_avatar/discourse.cmake.org/prex/32/306_2.png) [@prex](https://discourse.cmake.org/u/prex)\
**Post date:** [February 16, 2020, 12:43pm UTC](https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640/3 "2020-02-16T12:43:18Z")

</div>

> [@ben.boeckel](#):
>
> It looks like `QT_MOC_HPP` is never set. I expect that would fix the `staticMetaObject` and `vtable` errors.

MOC is relatively new to me. I’m reading about it right now. Not totally sure what you mean by QT\_MOC\_HPP. I do `set(CMAKE_AUTOMOC ON)`. It works with `add_executable` while also listing the header files. So what would be the difference with `add_library`?

> [@ben.boeckel](#):
>
> As for the `error_msgs_` error, could you show its declaration? I suspect that it’s not getting the right linker rules applied to it.

I’m not doing something different than with my other header files apart from defining the string in the header file directly. This is my complete header file `messages.h`:

```
#ifndef MESSAGES_H_
#define MESSAGES_H_

namespace message_definitions {

std::string error_msgs_[5] = {"a sub component is in a fault state",
                                      "sub system states are inconsistant",
                                      "MD 1 in a fault state",
                                      "MD 2 in a fault state",
                                      "MD 3 in a fault state"};                                  
}

#endif // MESSAGES_H_

```

---

<div class="post-metadata">

**Author:** ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)\
**Post date:** [February 17, 2020, 11:17am UTC](https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640/4 "2020-02-17T11:17:18Z")

</div>

> [@prex](#):
>
> It works with `add_executable` while also listing the header files. So what would be the difference with `add_library` ?

It’s the “while also listing the header files” that is different. You have a variable name `QT_MOC_HPP` that you’re adding to your sources, but it is an empty list.

> [@prex](#):
>
> I’m not doing something different than with my other header files apart from defining the string in the header file directly.

That’s the problem. Each of your source files sees this and embeds it since there’s no one source that has special instructions that “this is yours”, so the linker then says “you can’t define this multiple times”. You’ll need to move the definition into one place or make it `constexpr` or something to satisfy the ODR (one definition rule).

---

<div class="post-metadata">

**Author:** ![prex](https://discourse.cmake.org/user_avatar/discourse.cmake.org/prex/32/306_2.png) [@prex](https://discourse.cmake.org/u/prex)\
**Post date:** [February 17, 2020, 1:53pm UTC](https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640/5 "2020-02-17T13:53:28Z")

</div>

> [@ben.boeckel](#):
>
> That’s the problem. Each of your source files sees this and embeds it since there’s no one source that has special instructions that “this is yours”, so the linker then says “you can’t define this multiple times”. You’ll need to move the definition into one place or make it `constexpr` or something to satisfy the ODR (one definition rule).

Ok, in general I understand this. But this is also happening if I only include it in one source file? I added `static`, which solves this issue. Is this a valid solution?

> [@ben.boeckel](#):
>
> It’s the “while also listing the header files” that is different. You have a variable name `QT_MOC_HPP` that you’re adding to your sources, but it is an empty list.

Ooops, you’re right. But why do I even need to add any header files in `add_executable`? I was not doing this in any other packages. And even if I add the header files to `add_library`, it still gets me the undefined reference error.

---

<div class="post-metadata">

**Author:** ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)\
**Post date:** [February 17, 2020, 2:26pm UTC](https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640/6 "2020-02-17T14:26:51Z")

</div>

> [@prex](#):
>
> I added `static` , which solves this issue. Is this a valid solution?

Each TU will get its own copy of the string list. This is fine as long as you don’t require that the address of each error string is the same between TUs.

> [@prex](#):
>
> But why do I even need to add any header files in `add_executable` ?

If you don’t list the headers, automoc doesn’t know which files to scan for `Q_OBJECT` and friends.

> [@prex](#):
>
> And even if I add the header files to `add_library` , it still gets me the undefined reference error.

I suspect some other headers are likely missing in that case. Depends on the exact errors.

---

<div class="post-metadata">

**Author:** ![prex](https://discourse.cmake.org/user_avatar/discourse.cmake.org/prex/32/306_2.png) [@prex](https://discourse.cmake.org/u/prex)\
**Post date:** [February 17, 2020, 2:38pm UTC](https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640/7 "2020-02-17T14:38:17Z")

</div>

> [@ben.boeckel](#):
>
> I suspect some other headers are likely missing in that case. Depends on the exact errors.

Between all the tries, I forgot to include the library again. Now it seems to work fine. Thanks!

> [@ben.boeckel](#):
>
> If you don’t list the headers, automoc doesn’t know which files to scan for `Q_OBJECT` and friends.

So the list of header files is a moc related thing and not necessary in non-Qt building, right?

---

<div class="post-metadata">

**Author:** ![ben.boeckel](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/b/ea5d25/32.png) [@ben.boeckel](https://discourse.cmake.org/u/ben.boeckel)\
**Post date:** [February 17, 2020, 2:41pm UTC](https://discourse.cmake.org/t/multiple-definition-error-only-for-additional-header-file-in-qt-cmakefile/640/8 "2020-02-17T14:41:05Z")

</div>

> [@prex](#):
>
> So the list of header files is a moc related thing and not necessary in non-Qt building, right?

Yes, but I recommend listing them anyways; they’ll show up for IDE users and I personally find it better to be explicit in build systems.