C 如果SEQ为空，则不要为每个扩展BOOST_PP_SEQ

如果参数列表为空

#define key_evaluate(r, key, sig) key |= 1 << sig;
#define getKey(...)\
({\
      ComponentKey key = 0;\
      BOOST_PP_SEQ_FOR_EACH(key_evaluate, key, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))
      key;\
})
    
int main(void)
{
     getKey();
}

我不知道

我知道

• 宏通常是有问题的
• 大括号组表达式是非标准的

看起来您正试图将多个积分SIG组合到一个密钥掩码中，因此制作一个示例可能如下所示：

enum SigKeys {
    key0 = 1,
    key1 = 2,
    key2 = 4,
    key3 = 8,
    key4 = 16,
};

#include <iostream>
int main(void) {
    std::cout << "key1, key2: " << (key1 + key2) << " == " << getKey(1, 2) << "\n";
    std::cout << "key2, key4: " << (key2 + key4) << " == " << getKey(4, 2) << "\n";
}

重拍，没有宏 假设您的编译器达到了标准，您只需将其编写为：

template <typename... T>
constexpr ComponentKey getKey(T&&... sig) {
    return (0 | ... | (1 << sig));
}

#include <iostream>
int main(void) {
    std::cout << "key1, key2: " << (key1 + key2) << " == " << getKey(1, 2) << "\n";
    std::cout << "key2, key4: " << (key2 + key4) << " == " << getKey(4, 2) << "\n";
}

key1, key2: 6 == 6
key2, key4: 20 == 20

template <typename... T>
constexpr ComponentKey getKey(T&&... sig) {
    return (0 | ... | (1 << sig));
}

ComponentKey getKey(std::initializer_list<int> sig) {
    return std::accumulate(sig.begin(), sig.end(), 0,
            [](int r, int s) { return r | (1 << s); });
}

ComponentKey getKey(std::initializer_list<int> sig) {
    ComponentKey r = 0;
    for (auto& s : sig)
        r |= 1 << s;
    return r;
}