Android and cpp

Android and C++
Joan Puig Sanz

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

Text
The same app running in all the platforms
4

Most common solutions?
PhoneGap Adobe Air
Xamarin
Titanium
6

✦ Not native UI
✦ Slow performance
✦ Custom components.
✦ Depend on a company
✦ Poor user experience
8

Good user experience
Smooth
UI
UX
9

Text
An other old solution…
C++ 10

Why C++?
Cross platform
language
Better performance
Mature language
Lots of libraries
11

Combining C++ with Android
Native UI and UX
JNI
C++ core
12

C++11 Features :: Auto
The compiler deducing the type
auto i = 42; // i is an int
auto l = 42LL; // l is an long long
auto p = new Foo(); // p is a foo*
std::map<std::string, std::vector<int>> map;
for(auto it = begin(map); it != end(map); ++it)
{}
14

C++11 Features ::
Range-based for loops
Iterate collections with foreach
std::vector<int> v;
v.push_back(1);
v.push_back(2);
v.push_back(3);
for (auto value : v) {
std::cout << value << std::endl;
}
15

C++11 Features ::
Smart pointers
unique_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.
16

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);
};
17

C++11 Features :: More
Final and override
non-member begin() and end()
Initializer lists
Object construction improvement
Unrestricted unions
User-defined literals
static_assert
Strongly-typed enums
…
18

Text
Complementing C++
Libraries
19

Complementing C++
C++11 standard library is
missing utilities:
HTTP Requests
XML and JSON
Big numbers
Security
Math
20

C++ libs :: Boost
Supports:
Licence:
AS IS
Big community
Lots of modules
21

C++ libs :: Juce
Supports:
No WP (for now)
Licence:
Core: AS IS
Other modules: GPL and commercial licence
Very well written code
Easy to use
22

C++ libs :: Qt
Supports:
Licence:
GPL and commercial licence
Big community
Lots of modules
23

Text
Putting stuff together
JNI
24

Calling JNI from Java
Declare native methods
private native void doSomethingNative (String str);
Implement C method
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
}
25

Calling JNI from Java
JNIEXPORT void Java_com_example_MyClass_doSomethingNative
(JNIEnv *env,
jobject obj,
jstring s)
Required macro to support Windows
Java + package_name +
className + methodName
26

Calling C from Java
JNIEXPORT void Java_com_example_MyClass_doSomethingNative
(JNIEnv *env,
jobject obj,
jstring s)
Structure to access all the JNI functions
Java object that calls the native function
Java method arguments
27

Local and global references
Local reference will be managed by java
A global reference is managed by the
developer
Max number
of java
references
= 512
28

Calling Java from C
public String myMethod (int[] array, boolean enabled)
29

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);
30

Calling Java from C
jclass myJavaClass = env->GetObjectClass(myJavaObject);
”methodName", "([I;Z)Ljava/lang/String");
// Do something
Must not be NULL
This is the java object that has the method
31

Calling Java from C
jmethodID myJavaMethod = env->GetMethodID(myJavaClass,
”myMethod", "([I;Z)Ljava/lang/String");
// Do something
env->DeleteLocalRef Method signature
(jresult);
32

Calling Java from C
// Do something
(jresult);
Type Signature Java Type
Z Boolean
B byte
C char
S short
I int
J long
F float
D double
L class ; full qualified
[type; tcylpases[]
33

Calling Java from C
// Do something
(jresult);
34

Calling Java from C
// Do something
35

Calling Java from C
// Do something
36

Java and C++ object binding
37

Java (OO)
C
C++ (OO)
38

Java (OO)
C
C++ (OO)
39

Foo Java implementation
public 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 implementation
JNIEXPORT 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

Text
Some advice to live happier with C++
42

Some advice
Code 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 thoughts
Pros
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 44

Android and cpp

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