Add GetAttachedEnv function. (#50)

Author: Igor Tarasov

Makefile

@@ -2,10 +2,15 @@ VARIANT ?= android
 BUILD := build/$(VARIANT)
 
 COMPILER := $(shell CXX="${CXX}" misc/compiler.sh)
+COMPILER_MAJOR_VERSION := $(shell CXX="${CXX}" misc/compiler-major-version.sh)
+
 ifeq ($(COMPILER), clang)
-CXXFLAGS_WARNINGS := -Weverything -Wno-c++98-compat -Wno-c++98-compat-pedantic -Wno-exit-time-destructors
+	CXXFLAGS_WARNINGS := -Weverything -Wno-c++98-compat -Wno-c++98-compat-pedantic -Wno-exit-time-destructors
+	ifeq ($(shell test $(COMPILER_MAJOR_VERSION) -gt 4; echo $$?),0)
+		CXXFLAGS_WARNINGS += -Wno-unused-template
+	endif
 else ifeq ($(COMPILER), gcc)
-CXXFLAGS_WARNINGS := -Wall -Wextra -pedantic -Wno-unused-but-set-variable
+	CXXFLAGS_WARNINGS := -Wall -Wextra -pedantic -Wno-unused-but-set-variable
 endif
 
 CXXFLAGS := $(CXXFLAGS) --std=c++14 -fPIC -Iinclude $(CXXFLAGS_WARNINGS) -Werror

include/jni/functions.hpp

@@ -628,23 +628,28 @@ namespace jni
        }
 
 
+    namespace {
+      // Some implementations type the parameter as JNIEnv**, others as void**.
+      // See https://bugs.openjdk.java.net/browse/JDK-6569899
+      struct JNIEnvCast
+        {
+          using FunVoid = jint (JavaVM::*)(void**, void*);
+          using FunEnv = jint (JavaVM::*)(JNIEnv**, void*);
+
+          template <typename Fun, typename = std::enable_if_t<std::is_same<Fun, FunVoid>::value>>
+          void** operator()(JNIEnv** env, Fun) noexcept {
+            return reinterpret_cast<void**>(env);
+          }
+
+          template <typename Fun, typename = std::enable_if_t<std::is_same<Fun, FunEnv>::value>>
+          JNIEnv** operator()(JNIEnv** env, Fun) noexcept {
+            return env;
+          }
+        };
+    }
+
     inline UniqueEnv AttachCurrentThread(JavaVM& vm)
        {
-        // Some implementations type the parameter as JNIEnv**, others as void**.
-        // See https://bugs.openjdk.java.net/browse/JDK-6569899
-        struct JNIEnvCast
-          {
-           void** operator()(JNIEnv** env, jint (JavaVM::*)(void**, void*))
-             {
-              return reinterpret_cast<void**>(env);
-             }
-
-           JNIEnv** operator()(JNIEnv** env, jint (JavaVM::*)(JNIEnv**, void*))
-             {
-              return env;
-             }
-          };
-
         JNIEnv* result;
         CheckErrorCode(vm.AttachCurrentThread(JNIEnvCast()(&result, &JavaVM::AttachCurrentThread), nullptr));
         return UniqueEnv(result, JNIEnvDeleter(vm));
@@ -662,4 +667,18 @@ namespace jni
         CheckErrorCode(vm.GetEnv(reinterpret_cast<void**>(&env), Unwrap(version)));
         return *env;
        }
+
+    inline JNIEnv& GetAttachedEnv(JavaVM& vm, version version = jni_version_1_1)
+       {
+        JNIEnv* env = nullptr;
+        auto code = vm.GetEnv(reinterpret_cast<void**>(&env), Unwrap(version));
+
+        if (code == JNI_EDETACHED) {
+            CheckErrorCode(vm.AttachCurrentThread(JNIEnvCast()(&env, &JavaVM::AttachCurrentThread), nullptr));
+        } else {
+            CheckErrorCode(code);
+        }
+
+        return *env;
+       }
    }

misc/compiler-major-version.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+compiler=$(${CXX} -v 2>&1)
+
+case $compiler in
+    *clang*|*gcc*)
+        version=`${CXX} --version | grep -o '\([0-9]\)\+.\([0-9]\)\+.\([0-9]\)\+'`
+        echo ${version::1}
+        ;;
+    *)
+        echo
+        unknown
+        ;;
+esac