C++: A syntax constraint when defining variables in the statements of if, while and switch

整理自 Thinking in C++。

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You can also define variables inside the control expressions of for loops and while loops, inside the conditional of an if statement, and inside the selector statement of a switch.

for (int i = 0; i < 10; ++i) 这种是最常见的。其实 if, while 和 switch 的括号里都可以定义变量，举个 switch 的例子：

switch(int i = cin.get()) {
	case 'A': cout << "Snap" << endl; break;
	case 'B': cout << "Crackle" << endl; break;
	case 'C': cout << "Pop" << endl; break;
	default: cout << "Not A, B or C!" << endl;
}

Although the example also shows variables defined within while, if, and switch statements, this kind of definition is much less common than those in for expressions, possibly because the syntax is so constrained. For example, you cannot have any parentheses. That is, you cannot say:

while((char c = cin.get()) != 'q') { // ERROR
	...
}

震惊了！这样一句看起来人畜无害的语句竟然是非法的！就是因为这个原因我才要贴这篇 blog。

去掉括号也不行，结合律不占优势……

话说我不明白为啥不能这么写……