Fix -Wundefined-var-template warning (#963)

This avoids specializing CSingleton<T>::m_instance for each type, and
instead just defines it once in the header. This is allowed by the
standard, multiple definitions are merged in the same way that inline
functions are.

src/app/app.cpp

@@ -61,9 +61,6 @@
 #include <getopt.h>
 #include <localename.h>
 
-
-template<> CApplication* CSingleton<CApplication>::m_instance = nullptr;
-
 char CApplication::m_languageLocale[] = { 0 };
 
 

src/app/input.cpp

@@ -32,9 +32,6 @@
 #include <boost/lexical_cast.hpp>
 #include <SDL_system.h>
 
-
-template<> CInput* CSingleton<CInput>::m_instance = nullptr;
-
 CInput::CInput()
     : m_keyPresses()
 {

src/app/pathman.cpp

@@ -37,8 +37,6 @@
 #include <boost/algorithm/string.hpp>
 #include <boost/filesystem.hpp>
 
-template<> CPathManager* CSingleton<CPathManager>::m_instance = nullptr;
-
 CPathManager::CPathManager(CSystemUtils* systemUtils)
     : m_systemUtils(systemUtils)
 {

src/common/config_file.cpp

@@ -34,9 +34,6 @@
 #include <boost/property_tree/ini_parser.hpp>
 #include <boost/regex.hpp>
 
-
-template<> CConfigFile* CSingleton<CConfigFile>::m_instance = nullptr;
-
 namespace bp = boost::property_tree;
 
 CConfigFile::CConfigFile()

src/common/logger.cpp

@@ -22,10 +22,6 @@
 
 #include <stdio.h>
 
-
-template<> CLogger* CSingleton<CLogger>::m_instance = nullptr;
-
-
 CLogger::CLogger()
 {
     #if DEV_BUILD

src/common/settings.cpp

@@ -32,8 +32,6 @@
 
 #include "sound/sound.h"
 
-template<> CSettings* CSingleton<CSettings>::m_instance = nullptr;
-
 CSettings::CSettings()
 {
     m_tooltips       = true;

src/common/singleton.h

@@ -73,3 +73,4 @@ private:
     CSingleton(const CSingleton<T> &) = delete;
 };
 
+template <typename T> T* CSingleton<T>::m_instance = nullptr;

src/graphics/engine/engine.cpp

@@ -64,8 +64,6 @@
 #include <SDL_surface.h>
 #include <SDL_thread.h>
 
-template<> Gfx::CEngine* CSingleton<Gfx::CEngine>::m_instance = nullptr;
-
 // Graphics module namespace
 namespace Gfx
 {

src/level/robotmain.cpp

@@ -125,9 +125,6 @@ const Gfx::Color COLOR_REF_ALIEN = Gfx::Color(135.0f/256.0f, 170.0f/256.0f,  13.
 const Gfx::Color COLOR_REF_GREEN = Gfx::Color(135.0f/256.0f, 170.0f/256.0f,  13.0f/256.0f);  // green
 const Gfx::Color COLOR_REF_WATER = Gfx::Color( 25.0f/256.0f, 255.0f/256.0f, 240.0f/256.0f);  // cyan
 
-
-template<> CRobotMain* CSingleton<CRobotMain>::m_instance = nullptr;
-
 //! Constructor of robot application
 CRobotMain::CRobotMain()
 {

src/object/object_manager.cpp

@@ -36,10 +36,6 @@
 
 #include <algorithm>
 
-
-template<> CObjectManager* CSingleton<CObjectManager>::m_instance = nullptr;
-
-
 CObjectManager::CObjectManager(Gfx::CEngine* engine,
                                Gfx::CTerrain* terrain,
                                Gfx::COldModelManager* oldModelManager,