Where is const mandated in the Standard when binding an lvalue reference to an rvalue?
I'm reading the part of the Standard about initializing an lvalue reference from an rvalue (https://eel.is/c++draft/dcl.init.ref#5.3 ). I know a reference to const is required in this scenario, but I cannot find out where the const is mandated in the Standard text. Take the first initialization in Example 5 of the webpage as an example with const removed:
Here I purposely remove the const qualifier in the declaration of rca2. Next I will show my analysis according to the Standard text. Here the initializer f() is an rvalue, so we jump to (5.3). “cv1 T1” is "A" while “cv2 T2” is "B". I think “cv1 T1” is reference-compatible with “cv2 T2” because a pointer to B can be converted to a pointer to A via a standard conversion. As a result, conditions in (5.3.1) are all satisfied. Converted initializer f() is a prvalue, whose type is B (T4 in the Standard text (5.3)). Now “cv1 T4” is B, from which we apply the temporary materialization conversion, resulting in a glvalue whose type is still B. Then the lvalue reference rca2 is bound to this glvalue result of type B.
There must be somewhere I made a mistake in the above analysis because an error is issued if I omit the const qualifier. But I don't know where it is that const is mandated. Please point out that to me. Thank you.
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Here I purposely remove the const qualifier in the declaration of rca2. Next I will show my analysis according to the Standard text. Here the initializer f() is an rvalue, so we jump to (5.3). “cv1 T1” is "A" while “cv2 T2” is "B". I think “cv1 T1” is reference-compatible with “cv2 T2” because a pointer to B can be converted to a pointer to A via a standard conversion. As a result, conditions in (5.3.1) are all satisfied. Converted initializer f() is a prvalue, whose type is B (T4 in the Standard text (5.3)). Now “cv1 T4” is B, from which we apply the temporary materialization conversion, resulting in a glvalue whose type is still B. Then the lvalue reference rca2 is bound to this glvalue result of type B.
There must be somewhere I made a mistake in the above analysis because an error is issued if I omit the const qualifier. But I don't know where it is that const is mandated. Please point out that to me. Thank you.
I don't think I've ever actually read the standard in over 30 years of using c++!
Put simply you can't initialise a ref from a rvalue as the rvalue doesn't 'exist' past the statement (rvalue life expires). However with a const ref, rvalue 'life' is extended to the life of the const ref.
Put simply you can't initialise a ref from a rvalue as the rvalue doesn't 'exist' past the statement (rvalue life expires). However with a const ref, rvalue 'life' is extended to the life of the const ref.
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