C++标准中一些鲜为人知的特性

1. trigraph三字符组
据说是为了照顾旧式键盘，还是为了键盘坏了，或者是使用非ASCII字符编码的语言输入方便，设计了一些三元字符组，两个问号加一个符号在预处理时替换为一个字符。MSDN这么解释

The source character set of C source programs is contained within the 7-bit ASCII character set but is a superset of the ISO 646-1983 Invariant Code Set. Trigraph sequences allow C programs to be written using only the ISO (International Standards Organization) Invariant Code Set.

这些三字符组有

Trigraph replacement	Trigraph replacement	Trigraph replacement
??= #	??( [	??< {
??/ /	??) ]	??> }
??’ ^	??! |	??- ~

2. alternative tokens
下面几对符号等价

Alternative Primary	Alternative Primary	alternative Primary
<% {	and &&	and_eq &=
%> }	bitor |	or_eq |=
<: [	or ||	xor_eq ^=
:> ]	xor ^	not !
%: #	compl ~	not_eq !=
%:%: ##	bitand &

于是下面的程序就是合法的C++程序，而且很标准：

??=include

int suM(int a, int b)
<%
int arr<:2:>;
arr<:0:> = a,
arr<:1 ??/
:> = b;
return arr??(0:> + arr??(1];
%>

int main(int argc, char* argv[])
??<
bool bv = false;
std::cout<<"Program name:"< if( ( bv ??! 1 and 1) not_eq false){
int iv = 1333;
std::cout<<!--??-iv<<std::endl; <br /--> }
std::cout<<"3+4="< return 0;
??>

用GCC编译的时候加上-trigraphs 选项即可，VC默认编译不通过，可能需要编译选项，暂时还没找到