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1.前言

本篇目的：通过模板实现单例模式，帮助读者理解模板和单例模式。

2.Android使用模板类单例模式例子

<1>.实例化

frameworks/av/media/libaaudio/src/binding/AAudioBinderClient.cpp

ANDROID_SINGLETON_STATIC_INSTANCE(AAudioBinderClient);

AAudioBinderClient::AAudioBinderClient() : AAudioServiceInterface(), Singleton<AAudioBinderClient>() {
    gKeepBinderClient = this; // so this singleton won't get deleted
    mAAudioClient = new AAudioClient(this);
    ALOGV("%s - this = %p, created mAAudioClient = %p", __func__, this, mAAudioClient.get());
}

AAudioBinderClient继承自AAudioServiceInterface和Singleton类。
而Singleton为模板类单例模式。

<2>.android源码使用例子

frameworks/av/media/libaaudio/src/core/AudioStreamBuilder.cpp

stream = new AudioStreamInternalCapture(AAudioBinderClient::getInstance(), false);

调用：AAudioBinderClient::getInstance()获取AAudioBinderClient类实例化对象。

3.实现模板类单例模式

#include <iostream>
#include <stdlib.h>

using namespace std;

template <typename TYPE>
class Singleton{
public:
  static TYPE& getInstance() {
    TYPE* instance = sInstance;
    if (instance == nullptr) {
      instance = new TYPE();
      sInstance = instance;
    }
    return *instance;
  }

protected:
  Singleton() {
    printf("%s(), line = %d\n",__FUNCTION__,__LINE__);
  }
  ~Singleton() {
    printf("%s(), line = %d\n",__FUNCTION__,__LINE__);
  }

private:
  Singleton(const Singleton&);
  Singleton& operator = (const Singleton&);
  static TYPE* sInstance;
};

#define SINGLETON_STATIC_INSTANCE(TYPE)				\
  template<> TYPE* Singleton< TYPE >::sInstance(nullptr);	\
  template class Singleton< TYPE >;

class/*struct*/ SingleTest : public Singleton<SingleTest> {
public:
  SingleTest(){
    printf("%s(), line = %d\n",__FUNCTION__,__LINE__);
  }
  ~SingleTest(){
    printf("%s(), line = %d\n",__FUNCTION__,__LINE__);
  }
  unsigned int contents;
};

SINGLETON_STATIC_INSTANCE(SingleTest);

int main(){

  //1.单例模式
  SingleTest *s1 = &SingleTest::getInstance();
  SingleTest *s2 = &SingleTest::getInstance();
  if (s1 == s2) {
    cout << "s1 == s2" << endl;
  } else {
    cout << "s1 != s2" << endl;
  }
  s1->contents = 100;
  printf("s1->contents = %d\n",s1->contents);
  //销毁一次,因为只有一个实例
  delete s1;

  //2.一般构造
  SingleTest *s3 = new SingleTest();
  s3->contents = 200;
  printf("s3->contents = %d\n",s3->contents);
  delete s3;
}