Custom transferable-ownership classes

mrts <>
Mon, 16 Aug 2010 14:11:32 -0700 (PDT)
First, a bit of high-level context:

I'm using JNI in a C++ project. Each JNI resource
should be accessed via a reference to the resource
object inside the Java virtual machine (JVM)
environment. That reference needs to be released
with when the resource is no longer needed.

E.g. to create a string, one would call

  jstring str_ref = env_->NewStringUTF("foo");

where env_ is the pointer to the JVM environment,
and to release the resources of the string,


This is a classic case of RAII. A smart "handle"
that would release the reference in it's
destructor would be a perfect, exception-safe
casing for the naked JVM references.

So far, so good.

In the spirit of proper encapsulation, all of the
JNI machinery is designed to be hidden behind a
high-level wrapper class, JNIWrapper. This is a
singleton class that owns all the JVM-related
resources, including the JVM environment pointer.

New resources should be created according to the
source/sink idiom [1], hiding the private
environment pointer:

JVMRef<jstring> JNIWrapper::newString(const char* str)
    // env_ is a private member of the JNIWrapper
    // instance
    JVMRef<jstring> ret(env_, env_->NewStringUTF(str));
    return ret;

where JVMRef should be an auto_ptr like
transferable-ownership class that needs a custom
deleter that has access to the environment. (Note
that I cannot use C++0X's std::unique_ptr.)

Here's a simple, auto_ptr-like first stab at

template <typename T>
class JVMRef {
    JVMRef(JNIEnv *env, T ref) :
        env_(env), ref_(ref) { }

    JVMRef(JVMRef& other) :
        env_(other.get_env()), ref_(other.release()) { }

    JVMRef& operator=(JVMRef &other)
    { reset(other.release()); return *this; }

    ~JVMRef() { reset(NULL); }

    T get()
    { return ref_; }

    JNIEnv *get_env()
    { return env_; }

    T release()
    { T ret = ref_; ref_ = NULL; return ret; }

    void reset(T other = NULL) {
        if (ref_ != other) {
            ref_ = other;

    JNIEnv *env_;
    T ref_;

The "source" function is given in
JNIWrapper::newString() above. Attempts to
use it in a "sink" function to construct a new
JVMRef to a Java string as follows:

    // jni is the instance of JNIWrapper
    JVMRef<jstring> jfoo(jni.newString("foo"));

fail as follows with g++ 4.4.1:


error: no matching function for call to

note: candidates are:
[with T = _jstring*]

JNI::JVMRef<T>::JVMRef(JNIEnv*, T)
[with T = _jstring*]


That's probably because the temporary that's created
when the "source" returns can't be used by reference.

How should I amend JVMRef to make it appease
the compiler? (Btw, I'm aware of Boost's shared_ptr that
supports deleters, but it isn't a neat fit.)


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