What is decltype with two arguments?

问题:

Edit, in order to avoid confusion: decltype does not accept two arguments. See answers.

The following two structs can be used to check for the existance of a member function on a type T during compile-time:

// Non-templated helper struct:
struct _test_has_foo {
    template<class T>
    static auto test(T* p) -> decltype(p->foo(), std::true_type());

    template<class>
    static auto test(...) -> std::false_type;
};

// Templated actual struct:
template<class T>
struct has_foo : decltype(_test_has_foo::test<T>(0))
{};

I think the idea is to use SFINAE when checking for the existance of a member function, so in case p->foo() isn't valid, only the ellipses version of test, which returns the std::false_type is defined. Otherwise the first method is defined for T* and will return std::true_type. The actual "switch" happens in the second class, which inherits from the type returned by test. This seems clever and "lightweight" compared to different approaches with is_same and stuff like that.

The decltype with two arguments first looked surprising to me, as I thought it just gets the type of an expression. When I saw the code above, I thought it's something like "try to compile the expressions and always return the type of the second. Fail if the expressions fail to compile" (so hide this specialization; SFINAE).

But:

Then I thought I could use this method to write any "is valid expression" checker, as long as it depends on some type T. Example:

...
    template<class T>
    static auto test(T* p) -> decltype(bar(*p), std::true_type());
...

http://ideone.com/dJkLPF

This, so I thought, will return a std::true_type if and only if bar is defined accepting a T as the first parameter (or if T is convertible, etc...), i.e.: if bar(*p) would compile if it was written in some context where p is defined of type T*.

However, the modification above evaluates always to std::false_type. Why is this? I don't want to fix it with some complicated different code. I just want to know why it doesn't work as I expected it to. Clearly, decltype with two arguments works different than I thought. I couldn't find any documentation; it's only explained with one expression everywhere.

回答1:

It's an comma-separated list of expressions, the type is identical to the type of the last expression in the list. It's usually used to verify that the first expression is valid (compilable, think SFINAE), the second is used to specify that decltype should return in case the first expression is valid.

回答2:

decltype does not take two arguments. Simply, it can can have an expression as its argument, and the comma operator is one way of creating expressions. Per Paragraph 5.18/1:

[...] A pair of expressions separated by a comma is evaluated left-to-right; the left expression is a discarded-value expression (Clause 5). Every value computation and side effect associated with the left expression is sequenced before every value computation and side effect associated with the right expression. The type and value of the result are the type and value of the right operand; the result is of the same value category as its right operand, and is a bit-field if its right operand is a glvalue and a bit-field. If the value of the right operand is a temporary (12.2), the result is that temporary.

Therefore:

static_assert(std::is_same<decltype(42, 3.14), double>::value, "Will not fire");