std::mutex是noncopyable的结构，因此不存在拷贝构造函数，所以这里错误提示引用已经删除的函数。

错误示例代码如下：

解决方法：

     将包含std::mutex的类的拷贝构造函数和赋值操作符重载函数，自定义或者标记为delete.

例如:

class Account {
public:
	Account(int id_, double ba = 0.0) :id(id_), balance(ba){}

	void withdraw(double amount){
		balance -= amount;
	}

	void deposit(double amount){
		balance += amount;
	}
	void printInfo() const {
		std::cout << "Account id: " << id << " balance: " << balance << std::endl;
	}
	Account(const Account& other) = delete;
	Account& operator=(const Account& other) = delete;
	friend void transfer(Account& from, Account& to, double amount);
private:
	double balance;
	int id;
	std::mutex m;
};