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javac++gccgcj

instantiating a template class in CNI/C++ code

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 bejava.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

Solution

  • 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 Objects (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.