Question

With the following macro:

#define ASSERT_IF_TEMP(expr) static_assert(?, "Is temporary!");

What should I put for question mark?

Answer 1

First we should clarify: What do you mean by “temporary”?

Many people mean different things when they say temporary. Technically, int() is not a temporary, but most people will include them into their own meaning of that term. Technically, given std::string s;, then move(s) isn’t a temporary either, but you may want to treat it as one with your macro.

The first kind of “temporaries” I mentioned above are really “prvalue expressions”. Those are the std::string("foo") or int() kind of things, but not the move(s) and also (for sure) not the s kind of things. The decltype operator yields a non-reference type for the first kind of “temporaries” I talked about above. For the second kind, move(s), which are xvalues, it will yield an rvalue reference. And for the “non-temporaries”, i.e the s cases, it will yield an lvalue reference.

So to summarize, I will define three precise macros, and you can choose from them

#define IS_LVALUE(...) std::is_lvalue_reference<decltype((__VA_ARGS__))>::value
#define IS_XVALUE(...) std::is_rvalue_reference<decltype((__VA_ARGS__))>::value
#define IS_PRVALUE(...) !std::is_reference<decltype((__VA_ARGS__))>::value