c++学习2

• C++多态

• #include <iostream>
  #include <iomanip>
  using namespace std;
  
  class Base {
  public:
      virtual void f() { cout << "base" << endl; }
  };
  class Derived : public Base{
  public:
      void f() { cout << "derived" << endl; }
  };
  class Base2 {
  public:
      virtual void g() { cout << "base2 process g" << endl; } // dynamic_cast only work under polymorphic settings
  };
  class Derived2 : public Base2{
  public:
      void f() { cout << "derived2" << endl; }
  };
  
  int main() {
      /*
      Base *b = new Derived;
      b->f(); //base
  
      Base *b = new Derived;
      Derived *a = static_cast<Derived *>(b);
      a->f(); //derived
  
      Base *b = new Derived;
      b->f(); //derived, derived overwrite base in virtual table
      Derived *a = dynamic_cast<Derived *>(b);
      a->f(); //derived, same with b->f()
   */
      Base2 *b = new Derived2;
      //b->f(); // compile error
      Derived2 *a = dynamic_cast<Derived2 *>(b);
      a->f(); //derived2, same with b->f()
      return 0;
  }

• 可调用函数, 匿名函数

• auto f = [](int a, int b){return a + b;};
  cout << f(1,2) << endl; //3
  cout << [](int a, int b){return a + b;}(1,2) << endl; //3
  int a = 0; int b = 0; int c = 0;
  
  //cout << [=](int a, int b){c = 1; return a + b;}(1,2) << "	" << c << endl;
  // error: cannot assign to a variable captured by copy in a non-mutable lambda
  //cout << [](int a, int b){int c = 1; return a + b;}(1,2) << "	" << c << a << endl; //3 	 0
  
  cout << [&](int a, int b){c = 1; return a + b;}(1,2) << "	" << c << "	" << a << endl; //3 	 1
  cout << [&c](int a, int b){c = 1; return a + b;}(1,2) << "	" << c << endl; //3 	 1
  

C++ 宏下的 # 和 ##
• #的功能是将其后面的宏参数进行字符串化操作
• ##被称为连接符（concatenator）

• /// Macro for registering a layer type.
  /// Example: REGISTER_LAYER(crf_error, CRFDecodingErrorLayer);
  #define REGISTER_LAYER(__type_name, __class_name) 
    static InitFunction __reg_type_##__type_name(   
        []() { Layer::registrar_.registerClass<__class_name>(#__type_name); })
  

• C++explicit关键字详解
  • 　　http://www.cnblogs.com/ymy124/p/3632634.html
  • C++中的explicit关键字只能用于修饰只有一个参数的类构造函数, 它的作用是表明该构造函数是显示的, 而非隐式的, 跟它相对应的另一个关键字是implicit, 意思是隐藏的,类构造函数默认情况下即声明为implicit(隐式). explicit关键字的作用就是防止类构造函数的隐式自动转换. explicit关键字只对有一个参数的类构造函数有效, 如果类构造函数参数大于或等于两个时, 是不会产生隐式转换的, 所以explicit关键字也就无效了。 但是 当除了第一个参数以外的其他参数都有默认值的时候, explicit关键字依然有效, 此时, 当调用构造函数时只传入一个参数, 等效于只有一个参数的类构造函数