c++ - 为什么这不是一个 constexpr？

Question

#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。

Answer 1

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;