为什么这不是一个 constexpr?
Why is this not a constexpr?
#include <iostream>
union gc_bits {
size_t value;
struct {
size_t arena : 2;
} bits;
constexpr gc_bits(size_t value_) : value(value_) {
}
};
static constexpr size_t get_max_arenas() {
return gc_bits(~0ULL).bits.arena;
}
size_t current_colour[get_max_arenas()]; // error
int main() {
std::cout << get_max_arenas() << std::endl;
}
数组声明错误,因为 get_max_arenas 不是一个 constexpr。我不清楚为什么会这样。
稍微改写你的程序:
static constexpr auto gma = get_max_arenas();
size_t current_colour[gma]; // error
给出 Clang 错误:
read of member 'bits' of union with active member 'value' is not
allowed in a constant expression
出现此错误的原因是构造函数设置了 value
,然后您尝试读取 bits
。正如@gurka 所评论的那样,这是不允许的。
Standard 引用:
[expr.const]
2 A conditional-expression e is a core constant expression unless the
evaluation of e, following the rules of the abstract machine (1.9),
would evaluate one of the following expressions:
(2.8) — an lvalue-to-rvalue conversion (4.1) or modification (5.18,
5.2.6, 5.3.2) that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;
#include <iostream>
union gc_bits {
size_t value;
struct {
size_t arena : 2;
} bits;
constexpr gc_bits(size_t value_) : value(value_) {
}
};
static constexpr size_t get_max_arenas() {
return gc_bits(~0ULL).bits.arena;
}
size_t current_colour[get_max_arenas()]; // error
int main() {
std::cout << get_max_arenas() << std::endl;
}
数组声明错误,因为 get_max_arenas 不是一个 constexpr。我不清楚为什么会这样。
稍微改写你的程序:
static constexpr auto gma = get_max_arenas();
size_t current_colour[gma]; // error
给出 Clang 错误:
read of member 'bits' of union with active member 'value' is not allowed in a constant expression
出现此错误的原因是构造函数设置了 value
,然后您尝试读取 bits
。正如@gurka 所评论的那样,这是不允许的。
Standard 引用:
[expr.const]
2 A conditional-expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine (1.9), would evaluate one of the following expressions:
(2.8) — an lvalue-to-rvalue conversion (4.1) or modification (5.18, 5.2.6, 5.3.2) that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;