问题描述
这可能是一个基本问题，但我现在看不到自己的回复.
This is maybe a basic question, but I cannot see the response by myself right now.
考虑以下代码:
template<bool b>
struct T {
static constexpr int value = (b ? 42 : 0);
};
template<bool b>
struct U {
enum { value = (b ? 42 : 0) };
};
int main() {
static_assert(T<true>::value == 42, "!");
static_assert(T<false>::value == 0, "!");
static_assert(U<true>::value == 42, "!");
static_assert(U<false>::value == 0, "!");
}
我习惯于使用像 T 这样的结构，但我不止一次看到像 U 这样的结构用于相同的目的(主要是特征定义).
I'm used to using structs like T, but more than once I've seen structs like U used for the same purpose (mostly traits definition).
据我所知，它们都是在编译时解决的，它们解决了几乎相同的问题，但在我看来 T 比 U 更具可读性代码>(好吧，我知道，我的个人意见).
As far as I can see, they are both resolved at compile time and they solve almost the same problem, but it seems to me that T is far more readable than U (well, I know, my personal opinion).
我的问题很简单:有什么技术原因可以证明一种解决方案比另一种更好吗?
更重要的是，是否有任何一种情况不是可行的解决方案?
My question is pretty simple: is there any technical reason for which one solution is better than the other one?
Even more, is there any case for which one of them is not a viable solution?
推荐答案
请注意，以下答案不适用于 C++ 17 及更高版本.
这样使用时，积分常数不会有明显差异.
There will be no noticeable difference for integral constants when used like this.
然而，enum 实际上更好，因为它是一个真正的命名常量.constexpr 整数常量是一个对象，例如可以被 ODR 使用 - 这会导致链接错误.
However, enum is actually better, because it is a true named constant. constexpr integral constant is an object which can be, for example, ODR-used - and that would result in linking errors.
#include <iostream>
struct T {
static constexpr int i = 42;
enum : int {x = 42};
};
void check(const int& z) {
std::cout << "Check: " << z << "
";
}
int main() {
// check(T::i); // Uncommenting this will lead to link error
check(T::x);
}
当check(T::i)被取消注释时，程序无法链接:
When check(T::i) is uncommented, the program can not be linked:
/tmp/ccZoETx7.o:在函数`main'中:ccc.cpp:(.text+0x45): 未定义引用 `T::i' collect2:错误:ld 返回 1 退出状态
/tmp/ccZoETx7.o: In function `main':ccc.cpp:(.text+0x45): undefined reference to `T::i'collect2: error:ldreturned1exit status
然而，真正的 enum 总是有效的.
However, the true enum always works.
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