带有

每个操作系统都有其 UI 和特性，因此我们打算在这方面为每个平台编写特定的代码.另一方面，所有可以共享的逻辑代码、业务规则和东西我们都打算使用 C++ 编写，因此我们可以将相同的代码编译到每个平台.

在图中，您可以看到最底层的 C++ 层.所有共享代码都在此段中.最高层是常规的Obj-C/Java/Kotlin代码，这里没有消息，难的是中间层.

iOS端的中间层很简单；您只需将项目配置为使用 Obj-c 的变体进行构建，称为 Objective-C++ 就是这样，你可以访问 C++ 代码.

Android 方面的事情变得更加困难，Android 上的 Java 和 Kotlin 这两种语言都在 Java 虚拟机下运行.所以访问 C++ 代码的唯一方法是使用 JNI，请花点时间阅读 JNI 的基础知识.幸运的是，今天的 Android Studio IDE 在 JNI 方面有了很大的改进，并且在您编辑代码时会向您展示很多问题.

分步代码

我们的示例是一个简单的应用程序，您可以将文本发送到 CPP，然后它将该文本转换为其他内容并返回.想法是，iOS 将发送Obj-C"，Android 将发送各自语言的Java"，CPP 代码将创建如下文本cpp 向 问好><<收到的文本>>".

共享 CPP 代码

首先，我们将创建共享 CPP 代码，这样做我们有一个简单的头文件，其中包含接收所需文本的方法声明:

#include <iostream>const char *concatenateMyStringWithCppString(const char *myString);

以及 CPP 实施:

#include <string.h>#include "Core.h"const char *CPP_BASE_STRING = "cpp 向 %s 打招呼";const char *concatenateMyStringWithCppString(const char *myString) {char *concatenatedString = new char[strlen(CPP_BASE_STRING) + strlen(myString)];sprintf(连接字符串，CPP_BASE_STRING，myString)；返回连接字符串；}

Unix

一个有趣的好处是，我们还可以在 Linux 和 Mac 以及其他 Unix 系统上使用相同的代码.这种可能性特别有用，因为我们可以更快地测试我们的共享代码，所以我们将创建一个 Main.cpp，如下所示从我们的机器上执行它并查看共享代码是否正常工作.

#include <iostream>#include <字符串>#include "../CPP/Core.h"int main() {std::string textFromCppCore = concatenateMyStringWithCppString("Unix");std::cout <<textFromCppCore <<'
';返回0；}

要构建代码，您需要执行:

$ g++ Main.cpp Core.cpp -o main$ ./maincpp 向 Unix 打招呼

iOS

是时候在移动端实现了.只要 iOS 有一个简单的集成，我们就从它开始.我们的 iOS 应用是一个典型的 Obj-c 应用，只有一个区别；这些文件是 .mm 而不是 .m.即它是一个 Obj-C++ 应用程序，而不是一个 Obj-C 应用程序.

为了更好的组织，我们创建 CoreWrapper.mm 如下:

#import "CoreWrapper.h"@implementation CoreWrapper+ (NSString*) 连接MyStringWithCppString:(NSString*)myString {const char *utfString = [myString UTF8String];const char *textFromCppCore = concatenateMyStringWithCppString(utfString);NSString *objcString = [NSString stringWithUTF8String:textFromCppCore];返回对象字符串；}@结尾

该类负责将 CPP 类型和调用转换为 Obj-C 类型和调用.一旦你可以在 Obj-C 上调用任何你想要的文件上的 CPP 代码，这不是强制性的，但它有助于保持组织，并且在你的包装文件之外你维护一个完整的 Obj-C 样式的代码，只有包装文件变成 CPP 样式.

一旦您的包装器连接到 CPP 代码，您就可以将其用作标准 Obj-C 代码，例如视图控制器"

#import "ViewController.h"#import "CoreWrapper.h"@interface 视图控制器 ()@property (weak, nonatomic) IBOutlet UILabel *label;@结尾@implementation 视图控制器- (void)viewDidLoad {[超级视图DidLoad];NSString* textFromCppCore = [CoreWrapper concatenateMyStringWithCppString:@"Obj-C++"];[_label setText:textFromCppCore];}@结尾

看看应用的外观:

安卓

现在是集成 Android 的时候了.Android 使用 Gradle 作为构建系统，而对于 C/C++ 代码，它使用 CMake.所以我们需要做的第一件事就是在 gradle 文件上配置 CMake:

<代码>android {...外部原生构建 {制作{路径CMakeLists.txt"}}...默认配置 {外部原生构建 {制作{cppFlags "-std=c++14"}}...}

第二步是添加CMakeLists.txt文件:

cmake_minimum_required(版本 3.4.1)包含目录(../../CPP/)添加库(本机库共享src/main/cpp/native-lib.cpp../../CPP/Core.h../../CPP/Core.cpp)查找库(日志库日志)目标链接库(本机库${日志库})

CMake 文件是您需要添加将在项目中使用的 CPP 文件和头文件夹的位置，在我们的示例中，我们正在添加 CPP 文件夹和 Core.h/.cpp文件.要了解有关 C/C++ 配置的更多信息，请

Android with NDK has support to C/C++ code and iOS with Objective-C++ has support too, so how can I write applications with native C/C++ code shared between Android and iOS?

解决方案

Update.

This answer is quite popular even four years after I write it, in this four years a lot of things has changed, so I decided to update my answer to fit better our current reality. The answer idea does not change; the implementation has changed a little. My English also has changed, it has improved a lot, so the answer is more understandable to everyone now.

Please take a look at the repo so you can download and run the code I'll show below.

The Answer

Before I show the code, please take a lot on the following diagram.

Each OS has its UI and peculiarities, so we intend to write specific code to each platform in this regard. In other hands, all logic code, business rules, and things that can be shared we intend to write using C++, so we can compile the same code to each platform.

In the diagram, you can see the C++ layer at the lowest level. All shared code is in this segment. The highest level is regular Obj-C / Java / Kotlin code, no news here, the hard part is the middle layer.

The middle layer to iOS side is simple; you only need to configure your project to build using a variant of Obj-c know as Objective-C++ and it is all, you have access to C++ code.

The thing became harder on the Android side, both languages, Java and Kotlin, on Android, run under a Java Virtual Machine. So the only way to access C++ code is using JNI, please take time to read the basics of JNI. Fortunately, today's Android Studio IDE has vast improvements on JNI side, and a lot of problems are shown to you while you edit your code.

The code by steps

Our sample is a simple app that you send a text to CPP, and it converts that text to something else and returns it. The idea is, iOS will send "Obj-C" and Android will send "Java" from their respective languages, and the CPP code will create a text as a follow "cpp says hello to << text received >>".

Shared CPP code

First of all, we are going to create the shared CPP code, doing it we have a simple header file with the method declaration that receives the desired text:

#include <iostream>

const char *concatenateMyStringWithCppString(const char *myString);

And the CPP implementation:

#include <string.h>
#include "Core.h"

const char *CPP_BASE_STRING = "cpp says hello to %s";

const char *concatenateMyStringWithCppString(const char *myString) {
    char *concatenatedString = new char[strlen(CPP_BASE_STRING) + strlen(myString)];
    sprintf(concatenatedString, CPP_BASE_STRING, myString);
    return concatenatedString;
}

Unix

An interesting bonus is, we can also use the same code for Linux and Mac as well as other Unix systems. This possibility is especially useful because we can test our shared code faster, so we are going to create a Main.cpp as follow to execute it from our machine and see if the shared code is working.

#include <iostream>
#include <string>
#include "../CPP/Core.h"

int main() {
  std::string textFromCppCore = concatenateMyStringWithCppString("Unix");
  std::cout << textFromCppCore << '
';
  return 0;
}

To build the code, you need to execute:

$ g++ Main.cpp Core.cpp -o main
$ ./main 
cpp says hello to Unix

iOS

It is time to implement on the mobile side. As far as iOS has a simple integration we are starting with it. Our iOS app is a typical Obj-c app with only one difference; the files are .mm and not .m. i.e. It is an Obj-C++ app, not an Obj-C app.

To a better organization, we create the CoreWrapper.mm as follow:

#import "CoreWrapper.h"

@implementation CoreWrapper

+ (NSString*) concatenateMyStringWithCppString:(NSString*)myString {
    const char *utfString = [myString UTF8String];
    const char *textFromCppCore = concatenateMyStringWithCppString(utfString);
    NSString *objcString = [NSString stringWithUTF8String:textFromCppCore];
    return objcString;
}

@end

This class has the responsibility to convert CPP types and calls to Obj-C types and calls. It is not mandatory once you can call CPP code on any file you want on Obj-C, but it helps to keep the organisation, and outside your wrapper files you maintain a complete Obj-C styled code, only the wrappers file become CPP styled.

Once your wrapper is connected to the CPP code, you can use it as a standard Obj-C code, e.g. ViewController"

#import "ViewController.h"
#import "CoreWrapper.h"

@interface ViewController ()

@property (weak, nonatomic) IBOutlet UILabel *label;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    NSString* textFromCppCore = [CoreWrapper concatenateMyStringWithCppString:@"Obj-C++"];
    [_label setText:textFromCppCore];
}

@end

Take a look of how the app looks:

Android

Now it is time for Android integration. Android uses Gradle as the build system, and to C/C++ code it uses CMake. So the first thing we need to do is to configure the CMake on gradle file:

android {
...
externalNativeBuild {
    cmake {
        path "CMakeLists.txt"
    }
}
...
defaultConfig {
    externalNativeBuild {
        cmake {
            cppFlags "-std=c++14"
        }
    }
...
}

And the second step is to add the CMakeLists.txt file:

cmake_minimum_required(VERSION 3.4.1)

include_directories (
    ../../CPP/
)

add_library(
    native-lib
    SHARED
    src/main/cpp/native-lib.cpp
    ../../CPP/Core.h
    ../../CPP/Core.cpp
)

find_library(
    log-lib
    log
)

target_link_libraries(
    native-lib
    ${log-lib}
)

The CMake file is where you need to add the CPP files and header folders you will use on the project, on our example, we are adding the CPP folder and the Core.h/.cpp files. To know more about C/C++ configuration please read it.

Now the core code is part of our app it is time to create the bridge, to make the things more simple and organized we create a specific class named CoreWrapper to be our wrapper between JVM and CPP:

public class CoreWrapper {

    public native String concatenateMyStringWithCppString(String myString);

    static {
        System.loadLibrary("native-lib");
    }

}

Note this class has a native method and loads a native library named native-lib. This library is the one we create, in the end, the CPP code will become a shared object .so File embed in our APK, and the loadLibrary will load it. Finally, when you call the native method, the JVM will delegate the call to the loaded library.

Now the most strange part of Android integration is the JNI; We need a cpp file as follow, in our case "native-lib.cpp":

extern "C" {

JNIEXPORT jstring JNICALL Java_ademar_androidioscppexample_CoreWrapper_concatenateMyStringWithCppString(JNIEnv *env, jobject /* this */, jstring myString) {
    const char *utfString = env->GetStringUTFChars(myString, 0);
    const char *textFromCppCore = concatenateMyStringWithCppString(utfString);
    jstring javaString = env->NewStringUTF(textFromCppCore);
    return javaString;
}

}

The first thing you will notice is the extern "C" this part is necessary to JNI work correctly with our CPP code and method linkages. You will also see some symbols JNI uses to works with JVM as JNIEXPORT and JNICALL. To you understand the meaning of those things, it is necessary to take a time and read it, for this tutorial purposes just consider these things as boilerplate.

One significant thing and usually the root of a lot of problems is the name of the method; it needs to follow the pattern "Java_package_class_method". Currently, Android studio has excellent support for it so it can generate this boilerplate automatically and show to you when it is correct or not named. On our example our method is named "Java_ademar_androidioscppexample_CoreWrapper_concatenateMyStringWithCppString" it is because "ademar.androidioscppexample" is our package, so we replace the "." by "_", CoreWrapper is the class where we are linking the native method and "concatenateMyStringWithCppString" is the method name itself.

As we have the method correctly declared it is time to analyze the arguments, the first parameter is a pointer of JNIEnv it is the way we have access to JNI stuff, it is crucial to we make our conversions as you will see soon. The second is a jobject it is the instance of the object you had used to call this method. You can think it as the java "this", on our example we do not need to use it, but we still need to declare it. After this jobject, we are going to receive the arguments of the method. Because our method has only one argument - a String "myString", we have only a "jstring" with the same name. Also notice that our return type is also a jstring. It is because our Java method returns a String, for more information about Java/JNI types please read it.

The final step is to convert the JNI types to the types we use on CPP side. On our example, we are transforming the jstring to a const char * sending it converted to the CPP, getting the result and converting back to jstring. As all other steps on JNI, it is not hard; it is only boilerplated, all the work is done by the JNIEnv* argument we receive when we call the GetStringUTFChars and NewStringUTF. After it our code is ready to run on Android devices, lets take a look.