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c++referencec++14language-lawyerconstexpr
M.M
M.M

Reputation: 141633

constexpr reference to non-const object

Is it permitted to declare a non-const reference as constexpr? Example code:

int x = 1;
constexpr int& r = x;

This is accepted by gcc and clang (I tried several current and past versions of both, back to C++11, and all accepted it). However I think it should not be accepted because C++14 [dcl.constexpr/9] says:

if a constexpr specifier is used in a reference declaration, every full- expression that appears in its initializer shall be a constant expression

and x is not a constant expression.

The language in the latest C++17 draft of [dcl.constexpr] changed and doesn't even mention constexpr references explicitly any more, I can't make head nor tail of what it is trying to say about them.

Upvotes: 5

Views: 393

Answers (1)

T.C.
T.C.

Reputation: 137414

Assuming that x has static storage duration, the lvalue expression x is a perfectly valid constant expression.

If you use x in a context that requires a prvalue, which causes the lvalue-to-rvalue conversion to be applied to it, then the resulting prvalue expression - call it TO_RVALUE(x) - would not be a constant expression, for obvious reasons. But in the case of reference binding, there is no such conversion.

Upvotes: 4

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