error: undefined reference to 'oboe::AudioStreamBuilder::openStream(oboe::AudioStream**)'

Question

I'm trying to use Google Oboe from my Android NDK application. When I try to use oboe::AudioStreamBuilder from native-lib.cpp all is working fine. But when I try to use oboe::AudioStreamBuilder from a class then I get the error message "error: undefined reference to 'oboe::AudioStreamBuilder::openStream(oboe::AudioStream**)'" when I rebuild the project.

Here is native-lib.cpp that is ok :

#include <jni.h>
#include <string>
#include <android/log.h>
#include "OboeAudioRecorder.h"

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_oboeaudiorecorder_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}


extern "C" JNIEXPORT jboolean JNICALL
Java_com_example_oboeaudiorecorder_MainActivity_recordAudio(
        JNIEnv * env,
        jobject MainActivity
) {
    oboe::AudioStreamBuilder builder;
    builder.setDirection(oboe::Direction::Input);
    builder.setPerformanceMode(oboe::PerformanceMode::LowLatency);
    builder.setDeviceId(0);

    oboe::AudioStream *stream;
    oboe::Result r = builder.openStream(&stream);
    if (r != oboe::Result::OK) {
        return false;
    }

    return true;
}

And here is the class that cannot compile :

#include <oboe/Oboe.h>
#include <oboe/AudioStreamBuilder.h>
#include "OboeAudioRecorder.h"

OboeAudioRecorder::OboeAudioRecorder() {

    oboe::AudioStreamBuilder builder;
    builder.setDirection(oboe::Direction::Input);
    builder.setPerformanceMode(oboe::PerformanceMode::LowLatency);
    builder.setDeviceId(0);

    oboe::AudioStream *stream;
    oboe::Result r = builder.openStream(&stream);
    if (r != oboe::Result::OK) {
        return;
    }

}

The CMakeLists.txt file :

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.4.1)

# <Begin> OBOE SPECIFIC
# Set the path to the Oboe directory.
set (OBOE_DIR "../../../../../oboe")

# Add the Oboe library as a subdirectory in your project.
# add_subdirectory tells CMake to look in this directory to
# compile oboe source files using oboe's CMake file.
# ./oboe specifies where the compiled binaries will be stored
add_subdirectory (${OBOE_DIR} ./oboe)

# Specify the path to the Oboe header files.
# This allows targets compiled with this CMake (application code)
# to see public Oboe headers, in order to access its API.
include_directories (${OBOE_DIR}/include)
# <End> OBOE SPECIFIC

# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.

add_library( # Sets the name of the library.
             native-lib
             # Sets the library as a shared library.
             SHARED
             # Provides a relative path to your source file(s).
             native-lib.cpp )

add_library(
        OboeAudioRecorder
        SHARED
        OboeAudioRecorder.cpp
)

# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.

find_library( # Sets the name of the path variable.
              log-lib
              # Specifies the name of the NDK library that
              # you want CMake to locate.
              log )

# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.

target_link_libraries( # Specifies the target library.
                       native-lib oboe OboeAudioRecorder
                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

Answer 1

In your CMakeLists.txt change:

add_library( # Sets the name of the library.
             native-lib
             # Sets the library as a shared library.
             SHARED
             # Provides a relative path to your source file(s).
             native-lib.cpp )

to

add_library( # Sets the name of the library.
             native-lib
             # Sets the library as a shared library.
             SHARED
             # Provides a relative path to your source file(s).
             native-lib.cpp
             OboeAudioRecorder.cpp
 )

And remove all other references to OboeAudioRecorder.

Why does this work?

With the following lines you're attempting to build another library called OboeAudioRecorder:

add_library(
        OboeAudioRecorder
        SHARED
        OboeAudioRecorder.cpp
)

This library is dependent on oboe but you don't specify that anywhere, you only specify the dependencies for the native-lib library:

target_link_libraries( # Specifies the target library.
                       native-lib oboe OboeAudioRecorder
                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

That's why you get error: undefined reference when linking the OboeAudioRecorder library.

You could add a 2nd target_link_libraries(OboeAudioRecorder oboe) line but my guess is you don't want 2 separate libraries, you just want a single library which is dependent on oboe.