Android and cpp

Android and C++Joan Puig Sanz

About me

@joanpuigsanz

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What we will see?

1. Apps for all the platforms 2. C++ 11 3. Libraries 4. Java, C++ and JNI 5. Final thoughts

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The same app running in all the platforms4

Most common solutions?

Xamarin

PhoneGap Adobe Air

Titanium6

✦ Not native UI

✦ Slow performance

✦ Custom components.

✦ Depend on a company

✦ Poor user experience

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Smooth

UI

UX

Good user experience

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An other old solution…C++

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Why C++?

Cross platform language

Better performance

Mature language

Lots of libraries

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Combining C++ with Android

Native UI and UX

JNI

C++ core

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C++ 11 features

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C++11 Features :: Auto

The compiler deducing the typeauto i = 42; // i is an intauto l = 42LL; // l is an long longauto p = new Foo(); // p is a foo*

std::map<std::string, std::vector<int>> map;for(auto it = begin(map); it != end(map); ++it) {}

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C++11 Features :: Range-based for loops

Iterate collections with foreachstd::vector<int> v;v.push_back(1);v.push_back(2);v.push_back(3);

for (auto value : v) { std::cout << value << std::endl;}

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C++11 Features :: Smart pointersunique_ptr: Ownership of a memory resource it is not shared, but it can be transferred to another unique_ptr

shared_ptr: Ownership of a memory resource should be shared

weak_ptr: Holds a reference to an object managed by a shared_ptr, but does not contribute to the reference count; it is used to break dependency cycles.

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C++11 Features :: Lambdas

Powerful feature borrowed from functional programming.

You can use lambdas wherever a function object or a functor or a std::function is expected

std::function<int(int)> lfib = [&lfib](int n) { return n < 2 ? 1 : lfib(n-1) + lfib(n-2);};

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C++11 Features :: More

Final and overridenon-member begin() and end()Initializer listsObject construction improvementUnrestricted unionsUser-defined literalsstatic_assertStrongly-typed enums…

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Complementing C++Libraries

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Complementing C++C++11 standard library is missing utilities:

HTTP Requests

XML and JSON

Big numbers

Security

Math20

C++ libs :: BoostSupports:

Licence:

AS IS

Big community

Lots of modules21

C++ libs :: JuceSupports:

Licence:

Core: AS IS

Other modules: GPL and commercial licence

Very well written code

Easy to use

No WP (for now)

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C++ libs :: QtSupports:

Licence:

GPL and commercial licence

Big community

Lots of modules23

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Putting stuff together JNI

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Calling JNI from JavaDeclare native methods

Implement C method

private native void doSomethingNative (String str);

JNIEXPORT void Java_com_example_MyClass_doSomethingNative (JNIEnv *env, jobject obj, jstring s){ const char* const utf8 = env->GetStringUTFChars (s, nullptr); CharPointer_UTF8 utf8CP (utf8); String cppString (utf8CP); env->ReleaseStringUTFChars (s, utf8);

// Do Something}

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Calling JNI from JavaJNIEXPORT void Java_com_example_MyClass_doSomethingNative

(JNIEnv *env,jobject obj,jstring s)

Required macro to support Windows

Java + package_name + className + methodName

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Calling C from JavaJNIEXPORT void Java_com_example_MyClass_doSomethingNative(JNIEnv *env,

jobject obj,

jstring s)

Structure to access all the JNI functionsJava object that calls the native function

Java method arguments

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Local and global referencesLocal reference will be managed by java

A global reference is managed by the developer

Max number

of java

references

= 512

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Calling Java from C public String myMethod (int[] array, boolean enabled)

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Calling Java from C

JNIEnv* env = getEnv();

jclass myJavaClass = env->GetObjectClass (myJavaObject);jmethodID myJavaMethod = env->GetMethodID(myJavaClass , ”methodName", "([I;Z)Ljava/lang/String");

jstring jresult = (jstring) env->CallObjectMethod (myJavaObject, myJavaMethod , myIntArray, myBoolean);

// Do something

env->DeleteLocalRef (jresult);env->DeleteLocalRef (myJavaClass);

public String myMethod (int[] array, boolean enabled)

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Calling Java from C


jclass myJavaClass = env->GetObjectClass(myJavaObject);jmethodID myJavaMethod = env->GetMethodID(myJavaClass , ”methodName", "([I;Z)Ljava/lang/String");


// Do something



Must not be NULL This is the java object that has the method

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Calling Java from C


jclass myJavaClass = env->GetObjectClass(myJavaObject);jmethodID myJavaMethod = env->GetMethodID(myJavaClass, ”myMethod", "([I;Z)Ljava/lang/String");


// Do something



Method signature

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// Do something


Calling Java from C public String myMethod (int[] array, boolean enabled)

Method signature

Type Signature Java TypeZ BooleanB byteC charS shortI intJ longF floatD double

L class ; full qualified class[type; type[]

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Calling Java from C




// Do something



Method signature

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Calling Java from C


jclass myJavaClass = env->GetObjectClass(myJavaObject);jmethodID myJavaMethod = env->GetMethodID(myJavaClass , ”myMethod", "([I;Z)Ljava/lang/String");


// Do something



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Calling Java from C


jclass myJavaClass = env->GetObjectClass(myJavaObject);jmethodID myJavaMethod = env->GetMethodID(myJavaClass , ”myMethod", "([I;Z)Ljava/lang/String");


// Do something



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Java and C++ object binding

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Java (OO)

C

C++ (OO)


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Java (OO)

C

C++ (OO)


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Foo Java implementationpublic class Foo { native void destroyCppInstanceNative(double ref); native long newCppInstanceNative(); native String getStringNative(double ref); private double _ref = 0; public Foo() { _ref = newCppInstanceNative(); } public String getString() { return getStringNative(_ref); } public void destroy() { destroyCppInstanceNative(_ref); }} 40

Foo Java implementationJNIEXPORT long Java_com_example_Foo_newCppInstanceNative(JNIEnv *env, jobject obj) { Foo* newFoo = new Foo(); return (double)(newFoo);}

JNIEXPORT jstring Java_com_example_Foo_getStringNative(JNIEnv *env, jobject obj, double ref) { Foo* foo = (Foo*)(ref); jstring jStringResult = env->NewStringUTF (foo->getString()); return jStringResult;}

JNIEXPORT void Java_com_example_Foo_destroyCppInstanceNative(JNIEnv *env, jobject obj, double ref) { Foo* foo = (Foo*)(ref); delete foo;} 41

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Some advice to live happier with C++

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Some adviceCode quality! (Code conventions, code reviews, etc.)

C++ is not a read only code, don’t put the blame on it

Remember to initialise all the values in the constructor

Use unity builds to speed up compilation time

JNI could be painful. Check out djinni to generate cross-language type declarations and interface bindings.

dropbox/djinni 43

Final thoughtsPros

Same core and with native UI/UX per platform Better performance Faster cross platform development Easier maintenance

Cons Need to learn a new language Android apps will be bigger (code compiled for different architectures)

Can be fixed distributing specific apk per each architecture

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?@joanpuigsanz

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Android and cpp

Software

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