#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constexpr.我不清楚为什么会这样.
解决方法
稍微改写一下你的程序：
static constexpr auto gma = get_max_arenas(); size_t current_colour[gma]; // error
给出了Clang错误：
read of member ‘bits’ of union with active member ‘value’ is not
allowed in a constant expression
您收到此错误的原因是构造函数设置了值,然后您尝试读取位.正如@gurka所评论的那样,这是不允许的.
Standard报价：
[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;
