g ++:数组边界不是整数常量 [英] g++: array bound is not an integer constant
问题描述
const double rotationStep = 0.001;
const int N = 2 * int(M_PI / rotationStep)+ 3;
static unsigned int counts [N];
g ++
给出错误:
数组绑定不是一个整数常量,在»»«标记
我正在使用 g ++
/ gcc
版本4.6.1
有人可以告诉我为什么 g ++
会抱怨这个表达式吗?
从2003年的ISO C ++标准来看,这不是一个整型常量表达式。标准引用第5.19节:
lockquote
整型常量表达式只能涉及文字(2.13),
枚举数, const
变量或整型或
的枚举类型的静态数据成员用常量表达式(8.5),
初始化的枚举类型非类型tem-积分或枚举类型的板参数,以及
sizeof
表达式。浮动文字(2.13.3)只有在
被转换为整数或枚举类型时才会出现。
您可以更改这:
const double rotationStep = 0.001;
const int N = 2 * int(M_PI / rotationStep)+ 3; b
const int inverseRotationStep = 1000;
const int N = 2 * int(M_PI)* inverseRotationStep + 3;
(假设 M_PI
是在某处定义的;它没有在标准中指定,但它是一个常见的扩展。)
2011 ISO C ++标准稍微放松了一点。 5.19p3(引用N3337草案)说:
一个整型常量表达式是一个字面常量表达式$
I think 2 * int b $ b $或$ unsignedd枚举类型。 (M_PI / rotationStep)+ 3
,因此 N
符合新规则,但您的编译器可能尚未实现它们。
With the code,
const double rotationStep = 0.001;
const int N = 2*int(M_PI/rotationStep) + 3;
static unsigned int counts[N];
g++
gives the error:
array bound is not an integer constant before »]« token
I am using g++
/gcc
version 4.6.1
Can anybody tell me why g++
complains about the expression?
As of the ISO C++ standard of 2003, that's not an integral constant-expression. Quoting section 5.19 of the standard:
An integral constant-expression can involve only literals (2.13), enumerators,
const
variables or static data members of integral or enumeration types initialized with constant expressions (8.5), non-type tem-plate parameters of integral or enumeration types, andsizeof
expressions. Floating literals (2.13.3) can appear only if they are cast to integral or enumeration types.
You could change this:
const double rotationStep = 0.001;
const int N = 2*int(M_PI/rotationStep) + 3;
to this:
const int inverseRotationStep = 1000;
const int N = 2*int(M_PI)*inverseRotationStep + 3;
(That's assuming M_PI
is defined somewhere; it's not specified in the standard, but it's a common extension.)
The 2011 ISO C++ standard loosens this up a bit. 5.19p3 (quoting the N3337 draft) says:
An integral constant expression is a literal constant expression of integral or unscoped enumeration type.
I think 2*int(M_PI/rotationStep) + 3
, and therefore N
, qualifies under the new rules, but it's likely your compiler doesn't yet implement them.
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