类

class与struct

为什么c++里面经常定义struct？

利用struct默认访问修饰符为public的特性，

属性

static成员

#include<iostream>

using std::cout;
using std::endl;

class A {
public:
    static long a;
    A() {
    }
};
long A::a = 233; //不初始化会报错

int main() {
    A aa;
    aa.a = 1;
    cout << aa.a << '\n';
    A bb;
    bb.a = 2;
    cout << aa.a << '\n';
}

继承

虚拟继承

在未加virtual时会出现这种情况

代码中需要指定调用哪个父类的方法，这样就很奇怪

而加了virtual后就不需要了

#include<iostream>

using std::cout;
using std::endl;

class Base {
protected:
    int value;
public:
    Base() {
        cout << "in Base" << endl;
    }
};

class DerivedA : virtual protected Base {
public:
    DerivedA() {
        cout << "in DerivedA" << endl;
    }
};

class DerivedB : virtual protected Base {
public:
    DerivedB() {
        cout << "in DerivedB" << endl;
    }
};

class MyClass : DerivedA, DerivedB {
public:
    MyClass() {
        cout << "in MyClass" << value << endl;
    }
};
int main() {
    MyClass myClass;

}

拷贝构造函数

就是一个构造函数，完了参数是同类型的对象，在使用等号赋值的时候会调用拷贝构造函数

#include<iostream>

using std::cout;
using std::endl;

struct A {
public:
    int a;
    A(int _a) {
        a = _a;
    }
    A(const A &obj) {
        cout << "调用拷贝构造函数" << '\n';
        a = obj.a + 1;
    }
};

int main() {
    A a1(233);
    A a2 = a1; //调用拷贝构造函数
    cout << a2.a;
}

技巧：禁止类copy

干掉copy 构造函数即可

class DBImpl : public DB {
 public:
  DBImpl(const DBOptions& options, const std::string& dbname,
         const bool seq_per_batch = false, const bool batch_per_txn = true,
         bool read_only = false);
  // No copying allowed
   DBImpl(const DBImpl&) = delete;
   void operator=(const DBImpl&) = delete;