一个类中的任何static静态成员对象都表示一个单件
有且仅有一个对象被创建
想第1次调用某个函数时创建一个日志文件
函数返回了那个日志文件的引用
头文件完成任务
//: C10:LogFile.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#ifndef LOGFILE_H
#define LOGFILE_H
#include <fstream>
std::ofstream& logfile();
#endif // LOGFILE_H ///:~函数的实现必须不是内联的 must not be inlined
意味着整个函数包括在其中定义的静态对象
在任何包含它的翻译单元中都被复制
//: C10:LogFile.cpp {O}
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#include "LogFile.h"
std::ofstream& logfile() {
static std::ofstream log("Logfile.log");
return log;
} ///:~现在log对象不被初始化
直到函数logfile()第1次调用才被初始化
如果创建一个函数
//: C10:UseLog1.h
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#ifndef USELOG1_H
#define USELOG1_H
void f();
#endif // USELOG1_H ///:~在函数的实现中使用logfile()
//: C10:UseLog1.cpp {O}
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
#include "UseLog1.h"
#include "LogFile.h"
void f() {
logfile() << __FILE__ << std::endl;
} ///:~在另一个文件中再次使用logfile()
//: C10:UseLog2.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
//{L} LogFile UseLog1
#include "UseLog1.h"
#include "LogFile.h"
using namespace std;
void g() {
logfile() << __FILE__ << endl;
}
int main() {
f();
g();
getchar();
} ///:~无输出
首次调用函数f()
对象log才被创建
很容易将在一个成员函数内部的静态对象的创建与单件类结合在一起
//: C10:SingletonPattern2.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Meyers' Singleton.
#include <iostream>
using namespace std;
class Singleton {
int i;
Singleton(int x) : i(x) { }
void operator=(Singleton&);
Singleton(const Singleton&);
public:
static Singleton& instance() {
static Singleton s(47);
return s;
}
int getValue() { return i; }
void setValue(int x) { i = x; }
};
int main() {
Singleton& s = Singleton::instance();
cout << s.getValue() << endl;
Singleton& s2 = Singleton::instance();
s2.setValue(9);
cout << s.getValue() << endl;
getchar();
} ///:~输出
47
9
如果两个文件彼此依赖
就会产生有趣的情况
//: C10:FunctionStaticSingleton.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
class Singleton1 {
Singleton1() {}
public:
static Singleton1& ref() {
static Singleton1 single;
return single;
}
};
class Singleton2 {
Singleton1& s1;
Singleton2(Singleton1& s) : s1(s) {}
public:
static Singleton2& ref() {
static Singleton2 single(Singleton1::ref());
return single;
}
Singleton1& f() { return s1; }
};
int main() {
Singleton1& s1 = Singleton2::ref().f();
} ///:~无输出
调用Singleton2::ref()时
导致其唯一的Singleton2对象被创建
对象的创建过程中
Singleton1::ref()被调用
导致唯一的Singleton1对象被创建
单件的另外一种变体采用将一个对象 单件属性 Singleton-ness
从其实现中分离出来
//: C10:CuriousSingleton.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Separates a class from its Singleton-ness (almost).
#include <iostream>
using namespace std;
template<class T> class Singleton {
Singleton(const Singleton&);
Singleton& operator=(const Singleton&);
protected:
Singleton() {}
virtual ~Singleton() {}
public:
static T& instance() {
static T theInstance;
return theInstance;
}
};
// A sample class to be made into a Singleton
class MyClass : public Singleton<MyClass> {
int x;
protected:
friend class Singleton<MyClass>;
MyClass() { x = 0; }
public:
void setValue(int n) { x = n; }
int getValue() const { return x; }
};
int main() {
MyClass& m = MyClass::instance();
cout << m.getValue() << endl;
m.setValue(1);
cout << m.getValue() << endl;
getchar();
} ///:~
输出
0
1
MyClass通过3个步骤产生一个单件
1 声明其构造函数为私有或保护的
2 声明内Singleton<MyClass>为友元
3 从Singleton<MyClass>派生出MyClass