[英]Initializing a const int with a floating point literal
Example 例
int main()
{
const int i = 1.0; // Notice I am assigning a double to an int here
char a[i];
}
Question 题
Compiling the above code with g++ -O0 -Wall -pedantic -ansi -std=c++11
gives no errors (except for an unused variable). 使用
g++ -O0 -Wall -pedantic -ansi -std=c++11
编译上述代码不会产生任何错误(除了未使用的变量)。 However, if I remove -std=c++11
, I get the following warning: 但是,如果我删除
-std=c++11
,我会收到以下警告:
warning: ISO C++ forbids variable length array
警告:ISO C ++禁止可变长度数组
According to this SO question , I believe that in C++03, the code is invalid. 根据这个SO问题 ,我相信在C ++ 03中,代码无效。 However, can someone explain how the rule has changed in C++11?
但是,有人可以解释一下规则在C ++ 11中的变化吗?
(This question was a result of a previous question I answered.) (这个问题是我回答的上一个问题的结果。)
An array bound must be an integral constant expression, see 8.3.4 [dcl.array]/1 (same wording in C++03 and C++11): 数组绑定必须是一个整型常量表达式,参见8.3.4 [dcl.array] / 1(C ++ 03和C ++ 11中的相同措辞):
If the constant-expression (5.19) is present, it shall be an integral constant expression and its value shall be greater than zero.
如果存在常量表达式(5.19),则它应为整数常量表达式,其值应大于零。
In C++03 an integral constant expression cannot be initialized by a floating literal unless cast to integral type, see the last sentence of 5.19 [expr.const]/1: 在C ++ 03中,除非转换为整数类型,否则不能通过浮动文字初始化整数常量表达式,请参见5.19 [expr.const] / 1的最后一句:
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 template parameters of integral or enumeration types, andsizeof
expressions.整数常量表达式只能包含文字(2.13),枚举器,
const
变量或使用常量表达式(8.5)初始化的整数或枚举类型的静态数据成员,整数或枚举类型的非类型模板参数以及sizeof
表达式。 Floating literals (2.13.3) can appear only if they are cast to integral or enumeration types.浮动文字(2.13.3)只有在转换为整数或枚举类型时才会出现。
This means that in C++03 i
is not an integral constant expression, so cannot be used as an array bound. 这意味着在C ++ 03中
i
不是整数常量表达式,因此不能用作数组绑定。
GCC and Clang allow variable-length arrays as an extension to C++03, so it compiles with a non-constant bound, but you get a warning with -pedantic
. GCC和Clang允许可变长度数组作为C ++ 03的扩展,因此它使用非常量绑定进行编译,但是会出现
-pedantic
警告。 Changing the constant's initializer to cast it to integral type makes i
a valid integral constant expression: 更改常量的初始值设定项以将其强制转换为整型,使得
i
成为有效的积分常量表达式:
const int i = (int) 1.0;
With that change the array is no longer variable length and there is no warning even with -pedantic
. 通过该更改,数组不再是可变长度,即使使用
-pedantic
也没有警告。
In C++11 5.19 [expr.const]/3 says: 在C ++ 11 5.19 [expr.const] / 3中说:
A literal constant expression is a prvalue core constant expression of literal type, but not pointer type.
文字常量表达式是文字类型的prvalue核心常量表达式,但不是指针类型。 An integral constant expression is a literal constant expression of integral or unscoped enumeration type.
整数常量表达式是整数或未整数枚举类型的文字常量表达式。
The preceding (quite lengthy) paragraphs describe the rules for core constant expressions, but basically in C++11 the double initializer does not prevent i
being a core constant expression, even without a cast, so it is an integral constant expression and therefore a valid array bound, so no warning. 前面(相当冗长的)段落描述了核心常量表达式的规则,但基本上在C ++ 11中,双重初始化器并不会阻止
i
成为核心常量表达式,即使没有强制转换,因此它是一个整型常量表达式,因此有效的数组绑定,所以没有警告。
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