为什么std：map不能接受指针作为键值？

Question

I am trying to realize a jni function putting a mutex object in a map . When compiling, I get the error "...because jni pass a pointer" and I canmot convert in the argument.

Why a std:map cannot accept a pointer as key-value?

The instruction which gives the error is this:

std::pair<_jobject , boost::fibers::recursive_mutex> v = 
   std::pair<_jobject , boost::fibers::recursive_mutex>(*ref, *new boost::fibers::recursive_mutex());

#include <jni.h>
#include <map>
#include "ext_concurrent_fiber_FiberLock.h"
#include <boost/fiber/recursive_mutex.hpp>
std::map<_jobject, boost::fibers::recursive_mutex> mutexes;
boost::fibers::recursive_mutex lock;
// ‘std::pair<_jobject*, boost::fibers::recursive_mutex>::
//pair(_jobject&, boost::fibers::recursive_mutex&)’
// mutexes.insert(std::pair<jobject , boost::fibers::recursive_mutex>((_jobject&)ref, *new boost::fibers::recursive_mutex()));

JNIEXPORT jboolean JNICALL Java_ext_concurrent_fiber_FiberLock_setLockIfNotPresent
  (JNIEnv * env, jclass clazz, jobject ref){
  lock.lock();
  std::map<_jobject, boost::fibers::recursive_mutex>::iterator i =mutexes.find(*ref);

    if(i == mutexes.end())
    {

        std::pair<_jobject , boost::fibers::recursive_mutex> v
        =std::pair<_jobject , boost::fibers::recursive_mutex>(*ref, *new boost::fibers::recursive_mutex());
        mutexes.insert(v);
        lock.unlock();
        return true;
    }
   lock.unlock();
   return false;
  }

Below you can see the log:

 jni:
         [echo] Generating JNI headers
         [exec] mkdir -p target/c++
         [exec] g++ -c -Wall -fPIC -I/usr/lib/jvm/java-8-oracle/include/ -I/usr/lib/jvm/java-8-oracle/include/linux -I/usr/include/boost_1_66_0  -o target/c++/ext_concurrent_fiber_NativeFiber.o src/main/c++/ext_concurrent_fiber_NativeFiber.cpp
         [exec] mkdir -p target/c++
         [exec] g++ -c -Wall -fPIC -I/usr/lib/jvm/java-8-oracle/include/ -I/usr/lib/jvm/java-8-oracle/include/linux -I/usr/include/boost_1_66_0  -o target/c++/ext_concurrent_fiber_FiberLock.o src/main/c++/ext_concurrent_fiber_FiberLock.cpp
         [exec] src/main/c++/ext_concurrent_fiber_FiberLock.cpp: In function ‘jboolean Java_ext_concurrent_fiber_FiberLock_setLockIfNotPresent(JNIEnv*, jclass, jobject)’:
         [exec] src/main/c++/ext_concurrent_fiber_FiberLock.cpp:21:105: error: no matching function for call to ‘std::pair<_jobject, boost::fibers::recursive_mutex>::pair(_jobject&, boost::fibers::recursive_mutex&)’
         [exec]         =std::pair<_jobject , boost::fibers::recursive_mutex>(*ref, *new boost::fibers::recursive_mutex());
         [exec]                                                                                                          ^
         [exec] In file included from /usr/include/c++/6/bits/stl_algobase.h:64:0,
         [exec]                  from /usr/include/c++/6/bits/stl_tree.h:63,
         [exec]                  from /usr/include/c++/6/map:60,
         [exec]                  from src/main/c++/ext_concurrent_fiber_FiberLock.cpp:3:
         [exec] /usr/include/c++/6/bits/stl_pair.h:423:9: note: candidate: template<class ... _Args1, long unsigned int ..._Indexes1, class ... _Args2, long unsigned int ..._Indexes2> std::pair<_T1, _T2>::pair(std::tuple<_Args1 ...>&, std::tuple<_Args2 ...>&, std::_Index_tuple<_Indexes1 ...>, std::_Index_tuple<_Indexes2 ...>)
         [exec]          pair(tuple<_Args1...>&, tuple<_Args2...>&,
         [exec]          ^~~~
         [exec] /usr/include/c++/6/bits/stl_pair.h:423:9: note:   template argument deduction/substitution failed:
         [exec] src/main/c++/ext_concurrent_fiber_FiberLock.cpp:21:105: note:   ‘_jobject’ is not derived from ‘std::tuple<_Args1 ...>’
         [exec]         =std::pair<_jobject , boost::fibers::recursive_mutex>(*ref, *new boost::fibers::recursive_mutex());
         [exec]                                                                                                          ^
         [exec] In file included from /usr/include/c++/6/bits/stl_algobase.h:64:0,
         [exec]                  from /usr/include/c++/6/bits/stl_tree.h:63,
         [exec]                  from /usr/include/c++/6/map:60,
         [exec]                  from src/main/c++/ext_concurrent_fiber_FiberLock.cpp:3:
         [exec] /usr/include/c++/6/bits/stl_pair.h:356:9: note: candidate: template<class ... _Args1, class ... _Args2> std::pair<_T1, _T2>::pair(std::piecewise_construct_t, std::tuple<_Args1 ...>, std::tuple<_Args2 ...>)
         [exec]          pair(piecewise_construct_t, tuple<_Args1...>, tuple<_Args2...>);
         [exec]          ^~~~
         [exec] /usr/include/c++/6/bits/stl_pair.h:356:9: note:   template argument deduction/substitution failed:
         [exec] src/main/c++/ext_concurrent_fiber_FiberLock.cpp:21:105: note:   ‘boost::fibers::recursive_mutex’ is not derived from ‘std::tuple<_Args1 ...>’
         [exec]         =std::pair<_jobject , boost::fibers::recursive_mutex>(*ref, *new boost::fibers::recursive_mutex());
         [exec]                                                                                                          ^

Answer 1

Your issue has nothing to do with pointers. The problem is that the object types you're trying to store in your map are not copyable, so you can't construct them and then copy them into the map. Instead, you can tell the map to construct them directly in place:

std::map<jobject, boost::fibers::recursive_mutex> mutexes;
boost::fibers::recursive_mutex lock;

JNIEXPORT jboolean JNICALL
Java_ext_concurrent_fiber_FiberLock_setLockIfNotPresent(JNIEnv* env, jclass clazz, jobject ref)
{
    std::lock_guard<boost::fibers::recursive_mutex> lck(lock);

    auto i = mutexes.find(ref);
    if(i == mutexes.end())
    {
        mutexes.emplace(std::piecewise_construct,
                        std::forward_as_tuple(ref),
                        std::tuple<>{});
        return true;
    }
    return false;
}

Here, I've used std::map::emplace along with std::pair 's std::piecewise_construct constructor to tell the map to construct its elements in place. That way, the mutex won't need to be copied into the map.