Where is disable_if in C++0x?

问题

Boost has both enable_if and disable_if, but C++0x seems to be missing the latter. Why was it left out? Are there meta-programming facilities in C++0x that allow me to build disable_if in terms of enable_if?

---

Oh, I just noticed that std::enable_if is basically boost::enable_if_c, and that there is no such thing as boost::enable_if in C++0x.

回答1:

At the risk of seeming stupid, just do !expression instead of expression in the bool template parameter in enable_if to make it behave like a disable_if? Of course if that idea works, you could just expand on it to write a class with disable_if-like behavior?

Ok, I believe you could implement disable_if like this:

template <bool B, typename T = void>
struct disable_if {
    typedef T type;
};

template <typename T>
struct disable_if<true,T> {
};

来源：https://stackoverflow.com/questions/3111045/where-is-disable-if-in-c0x