if a parameter has a default argument, all the parameters that follow it must also have default arguments.
those least likely to use a default value appear first and those most likely to use a default appear last. (因为顺着可以忽略，倒着没法忽略)

Default Argument Declarations

最好就一次，但你要真来多次，那么请遵守：已有的default不能变¹，可以增加default

string screen(int, int, char = ' ');
string screen(int, int, char = '*');  // error: redeclaration
string screen(int = 24, int = 80, char);  // ok: add default arug

看最后一个好像很奇怪，怎么感觉违反了第一条？其实不是，这里沿用了之前的default

Default Argument Initializers

其实已有的default也可以变，只要用跟function decalration处在同一个scope的变量来default就行

int wd = 80;
char def = ' ';
int ht();
string screen(int = ht(), int = wd, char = def);
 
string window = screen();  // calls screen(ht(), 80, ' ')
 
void f2(){
	def = '*';    // changes the default
	sz wd = 100;  // hide
	window = screen();   // calls screen(ht(), 80, '*')
}