Lambda capturing constexpr object
c++, c++11, constexpr, lambda
Solution
The second template-argument to `std::integral_constant< int, i >` is for a template-parameter of non-type form, specifically of integral or enumeration type (14.3.2p1 bullet 1) and so must be a converted constant expression of type `int`.
In a lambda-expression, implicit capture occurs when an entity is odr-used in the compound statement (5.1.2p11); use of a converted constant expression in an explicit template instantiation is not odr-use (3.2p3), so the first example is valid.
In the second example, I think gcc is incorrect to reject it; 5.1.2p17 says in a note that:
An id-expression that is not an odr-use refers to the original entity, never to a member of the closure type.
Although the paragraph as a whole is discussing capture by copy, there's no reason not to apply this rule to capture by reference as well. It's unsurprising that the standard is unclear on this; there's really no reason to capture an entity that can be used in a converted constant expression by reference.
Problem
GCC 4.7.2 compiles this: ``` constexpr int i = 5; []{ std::integral_constant< int, i >(); }; // nonstandard: i not captured ``` but not this: ``` constexpr int i = 5; [&i]{ std::integral_constant< int, i >(); }; // GCC says i not constexpr ``` The latter example appears correct to me, according to C++11 §5.1.2/15: An entity is captured by reference if it is implicitly or explicitly captured but not captured by copy. It is unspecified whether additional unnamed non-static data members are declared in the closure type for entities captured by reference. It seems the captured object `i` inside the lambda refers to the variable in the enclosing scope, which is `constexpr`, not merely a `const` reference. The standard explicitly says that the use of a by-value capture is transformed into a use of the corresponding member of the lambda object. And I think that 5.1.2 hints that my interpretation is correct. Is there anything that explicitly says that whether a capture by reference refers to the object in the enclosing scope or a reference?