c++ - 迈耶斯单例范围

Question

在下面的程序中，注册表单例似乎没有在对静态函数的调用中保持不变。这种方法有什么问题？

#include <iostream>
#include <string>
#include <unordered_map>

using namespace std;

class Test {
typedef unordered_map<string,string> Registry;
public:
    static Registry &registry() {
        static Registry reg;
        return reg;
    }

    static void put(string key, string val) {
        Registry reg = Test::registry();
        reg[key] = val;
    }

    static string get(string key) {
        Registry reg = Test::registry();
        return reg[key];
    }
};

int main() {
    Test::put("a", "apple");
    Test::put("b", "banana");
    cout << Test::get("a") << endl;
    cout << Test::get("b") << endl;
    return 0;
}

Answer 1

您正确地返回了对您的单例的引用，但是当您使用它时，您正在获取一个拷贝。违规行如下:

Registry reg = Test::registry();

要解决此问题，请将其修改为:

Registry & reg = Test::registry();

为防止这种情况发生，您可以通过删除复制构造函数和赋值运算符来阻止编译器允许复制:

class Registry : public unordered_map<string,string>
{
    public:
        Registry() {}
        Registry( const Registry & ) = delete;
        Registry & operator=( const Registry & ) = delete;
};