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    <p>It's intentional, 9.4.2/4 says:</p> <blockquote> <p>If a static data member is of const integral or const enumeration type, its declaration in the class definition can specify a constant-initializer which shall be an integral constant expression (5.19) In that case, the member can appear in integral constant expressions. The member shall still be defined in a namespace scope if it is used in the program</p> </blockquote> <p>When you pass the static data member by const reference, you "use" it, 3.2/2:</p> <blockquote> <p>An expression is potentially evaluated unless it appears where an integral constant expression is required (see 5.19), is the operand of the sizeof operator (5.3.3), or is the operand of the typeid operator and the expression does not designate an lvalue of polymorphic class type (5.2.8). An object or non-overloaded function is used if its name appears in a potentially-evaluated expression.</p> </blockquote> <p>So in fact, you "use" it when you pass it by value too, or in a <code>static_cast</code>. It's just that GCC has let you off the hook in one case but not the other.</p> <p>[Edit: gcc is applying the rules from C++0x drafts: "A variable or non-overloaded function whose name appears as a potentially-evaluated expression is odr-used unless it is an object that satisfies the requirements for appearing in a constant expression (5.19) and the lvalue-to-rvalue conversion (4.1) is immediately applied.". The static cast performs lvalue-rvalue conversion immediately, so in C++0x it's not "used".]</p> <p>The practical problem with the const reference is that <code>foo</code> is within its rights to take the address of its argument, and compare it for example with the address of the argument from another call, stored in a global. Since a static data member is a unique object, this means if you call <code>foo(kConst)</code> from two different TUs, then the address of the object passed must be the same in each case. AFAIK GCC can't arrange that unless the object is defined in one (and only one) TU.</p> <p>OK, so in this case <code>foo</code> is a template, hence the definition is visible in all TUs, so perhaps the compiler could in theory rule out the risk that it does anything with the address. But in general you certainly shouldn't be taking addresses of or references to non-existent objects ;-)</p>
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    1. COThanks for the example of taking the address of the reference. I thinks that's a real practical reason why the compiler doesn't do what I expect.
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    2. COFor complete conformance, I think you'd have to define something like `template <int N> int intvalue() { return N; }`. Then with `intvalue<kConst>`, `kConst` only appears in a context requiring an integral constant expression, and so is not used. But the function returns a temporary with the same value as `kConst`, and that can bind to a const reference. I'm not sure, though, there might be a simpler way to portably enforce that `kConst` is not used.
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