C ++中有上下阵定义的制约？ [英] C++ Array Definition with Lower and Upper Bound?

问题描述

我女儿的第12届标准C ++教科书说，

My daughter's 12th standard C++ textbook says that

数组的符号可以（也）给出如下：数组名
  [下限L，上限C]

the notation of an array can (also) be given as follows: Array name [lower bound L, upper bound U]

这对我是一个惊喜。我知道帕斯卡尔有这个符号，但是C ++？从来没有见过这点。我在她的prescribed编译器（古代的Turbo C ++ 4.5）写了一个快速程序，并且不支持它。没有任何发现这个语法大师Stanley Lippman的书。互联网搜索没有抛出了这个。或者，也许我没有正确地搜索？

This was a surprise for me. I know Pascal has this notation, but C++? Had never seen this earlier. I wrote a quick program in her prescribed compiler (the ancient Turbo C++ 4.5), and that does not support it. Did not find this syntax in Stanley Lippman's book either. Internet search did not throw up this. Or maybe I didn't search correctly?

那么，是不是一个有效的声明？

So, is it a valid declaration?

推荐答案

这是无效的，从的草案的C ++标准部分 8.3.4 的阵列的声明必须是这种形式：

This is not valid, from the draft C++ standard section 8.3.4 Arrays the declaration must be of this form:

D1 [常量-EX pressionopt]属性说明符seqopt

D1 [ constant-expressionopt] attribute-specifier-seqopt

和我们可以从部分 5.19 的恒前pressions 的语法为恒前pression 是：

and we can from section 5.19 Constant expressions the grammar for constant expression is:

constant-expression:
   conditional-expression

这个语法并不能让我们获取到的逗号操作符的要么做这样的事情：

This grammar does not allow us to get to the comma operator either to do something like this:

int a[ 1, 2 ] ;
        ^

正如其他人暗示，因为有没有路径的逗号操作符的距离的条件-EX pression 的。如果你加括号虽然我们可以获取到的逗号操作符的自的条件-EX pression 的让我们去的主要-EX pression 的这会让我们（）所以下面是有效的：

as others have implied since there is no path to comma operator from conditional-expression. Although if you add parenthesis we can get to the comma operator since conditional-expression allows us to get to primary-expression which gets us () so the following would be valid:

int a[ (1, 2) ] ;
       ^   ^

请注意，在C ++ 03你明确地在一个恒定的前pression使用逗号操作被禁止的。

Note, in C++03 you were explicitly forbidden from using the comma operator in a constant expression.

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