在CNI / C ++代码中实例化模板类

Question

I have been fooling around with CNI in order to interpolate Java and C++ code for the last few hours.

While I was looking the possibility to use the Java collections as a replacement for my old team missing libstd , I tried to create a java.util.Stack object and manipulate it.

However, the compiler ( gcc and gcj , in that probably the same) seems to play his cheap tricks upon my innocent mind:

# gcc -g -I. test.cc
test.cc: In function ‘int main(int, char**)’:
test.cc:24:3: error: ‘java::util::Stack’ is not a template
   Stack<Person> *stack = new Stack<Person>();
   ^
test.cc:24:30: error: ‘java::util::Stack’ is not a template
   Stack<Person> *stack = new Stack<Person>();

(the Person object is well defined java class)

Only when I removed the type argument specification, It let me use the data-structure, as if the type argument was chosen to be java.lang.Object . The CNI docs seems to mention nothing about it at all! no words about templates indeed.

Does anyone know about using templates in CNI context? is it supported? I searched google and stackoverflow for answers, but couldn't come with the slightest clue. Any help will be appreciated.

In addition, I would like to know if i'm using a gcc 2.95.2 (is there a gcj for that gcc version?)

I am working on Virtual Boxed Ubunto 12 over windows 10 host. More importantly gcj and gcc versions stands on 4.8.4 java --version yields 1.5 and libgcj 4.8.4

Answer 1

Not a CNI user, but the reason for this one seems to be obvious enough.

Generic specification java.util.Stack<T> in Java is only a compile time constraint that checks that you add objects of type compatible with T. Internally, java.util.Stack has only single implementation which accepts all types of Object s (actually, references to all types of objects). You may even trick compiler into addding not an instance of T by using unsafe conversions. So, java.util.Stack<String> and java.util.Stack<Date> are the same internally.

C++ is different. Instantiation of the same template with different arguments creates different incompatible types. For example, std::stack<int> , std::stack<int*> and std::stack<char> are totally different implementations and have different code. C++ template mechanism is a superset of java generics.

If, for example, you wanted to get Java functionality in C++, you would write std::stack<void*> even if you wanted to store objects of some particular type T. Type conversion to T has to be done manually.

CNI does the same thing. It instantiates the only one implementation of java.util.Stack which can accept all types of objects.