Built-in comma operator

1、是在学习cppreference Expression SFINAE的时候，发现的Built-in comma operator。

2、需要注意的是：Built-in comma operator的left-hand argument是Discarded-value expressions 。

cppreference Built-in comma operator

The comma in various comma-separated lists, such as function argument lists (f(a, b, c)) and initializer lists int a[] = {1,2,3}, is not the comma operator. If the comma operator needs to be used in such contexts, it has to be parenthesized: f(a, (n++, n+b), c).

NOTE: 从上面的描述来看，其实C++语言并没有对comma operator作出形式化的定义。

上段的最后一句话需要结合下面的例子来进行说明：

#include <iostream>

void f(int a, int b, int c)
{
  std::cout << "a:" << a << std::endl;
  std::cout << "b:" << b << std::endl;
  std::cout << "c:" << c << std::endl;
}
int main()
{
  int n = 0;
  int a = 1;
  int b = 2;
  int c = 3;
  f(a, (n++, n + b), c);
}
// g++ test.cpp

(n++, n + b)就是中的,就是comma operator ，上述程序所展示的是：comma operator的first argument会被evaluated。

上述程序的输出如下：

a:1
b:3
c:3