c++ - 使用 constexpr 编译时出错

Question

我正在尝试编译这段代码:

enum class Order : char
{
    Little,
    Big
};

constexpr Order get_order() {
    uint16_t x = 1;
    return *((uint8_t *) &x) == 0 ? Order::Big : Order::Little;
}

我用 -std=c++14 标志来做，但我得到了那个错误:

In function ‘constexpr byteorder::Order byteorder::get_order()’: /home/user/dev/c++/render/include/byteorder.h:19:1: error: body of constexpr function ‘constexpr byteorder::Order byteorder::get_order()’ not a return-statement

看起来像 c++11 !

如果 c++14 允许在 constexpr 函数中使用局部变量，那怎么可能呢？

Debian Jessie，gcc 4.9.2

Answer 1

clang 说:

test.cpp:9:17: error: constexpr function never produces a constant expression
    [-Winvalid-constexpr]
constexpr Order get_order() {
                ^
test.cpp:11:14: note: cast that performs the conversions of a reinterpret_cast
is not allowed in a constant expression
    return *((uint8_t *) &x) == 0 ? Order::Big : Order::Little;
             ^
1 error generated.