C++进阶--Named Parameter Idiom

//############################################################################
/* Named Parameter Idiom */

/* 主要解决的问题
C++函数只支持位置参数，不支持像Python那样的命名参数
*/

class OpenFile {
public:
   OpenFile(string filename, bool readonly=true, bool appendWhenWriting=false,
            int blockSize=256, bool unbuffered=true, bool exclusiveAccess=false);
}

//像下面的函数调用，我得记得每个参数的意思和位置，非常不方便，可读性差，而且不灵活
OpenFile pf = OpenFile("foo.txt", true, false, 1024, true, true);

// 理想的情况是像下面这样的:
OpenFile pf = OpenFile(.filename("foo.txt"), .blockSize(1024) );

/* 解决方法 1 */
class OpenFile {
public:
  OpenFile(std::string const& filename);
  OpenFile& readonly(bool ro)  { readonly_ = ro; return *this; }
  OpenFile& createIfNotExist(bool c) { createIfNotExist_ = c; return *this; }
  OpenFile& blockSize(unsigned nbytes) { blockSize_ = nbytes; return *this; }
  ...
};

OpenFile f = OpenFile("foo.txt")
           .blockSize(1024)
           .createIfNotExist(true)
           .appendWhenWriting(true)
           .unbuffered(false)
           .readonly(true)
           .exclusiveAccess(false);

OpenFile f = OpenFile("foo.txt").blockSize(1024);

/* 问题:
 * 如果是非成员函数呢?
 */

/* 方法 2: 使用类型*/

void setBirthDate(int month, int day, int year);

setBirthDate(3, 1, 2012);  // 1月3日还是3月1日？

//定义结构体类型
struct Day {
   explicit Day(int d):val(d){}  //设置为explicit，不能隐式转换
   int val;
}
struct Month {
   explicit Month(int d):val(d){}  //设置为explicit，不能隐式转换
   int val;
}
struct Year {
   explicit Year(int d):val(d){}  //设置为explicit，不能隐式转换
   int val;
}
void setBirthDate(Month m, Day d, Year y);

setBirthDate(Month(3), Day(1), Year(2010)); //正确
setBirthDate(3, 1, 2010);  // 编译不过，很难使用出错

//############################################################################
/* Template Specialization for STL
 *
 * Specialize the standard library templates' behavior for our class
 *
 * std:: is a special namespace where we are not allowed to alter its contents
 * But we can specialize them for our types
 */

class collar;

class dog {
   collar* pCollar;
	dog(string name = "Bob") {pCollar = new collar(); cout << name << " is born." << endl; }
}

int main() {
   dog dog1("Henry");
   dog dog2("Boq");
   std::swap(dog1, dog2);
}