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I do not understand why the following code compiles on GCC 8.0:
decltype(auto) foo(int&& r) {
return r;
}
In foo, the declaration type of r is int&&, and so the return type of foo is also int&&. But r itself is an lvalue, and an lvalue cannot bind to an rvalue reference.
Am I missing something?
207
An lvalue reference can bind to an lvalue, but not to an rvalue.
“l-value” refers to a memory location that identifies an object. “r-value” refers to the data value that is stored at some address in memory. References in C++ are nothing but the alternative to the already existing variable.
Lvalues and rvalues are fundamental to C++ expressions. Put simply, an lvalue is an object reference and an rvalue is a value. The difference between lvalues and rvalues plays a role in the writing and understanding of expressions.
An lvalue refers to an object that persists beyond a single expression. An rvalue is a temporary value that does not persist beyond the expression that uses it.
According to [dcl.spec.auto]/5, the return type is deduced as if the return statement's operand was the operand of decltype. And [dcl.type.simple]/(4.2) clearly states that, as the operand is not parenthesized, the type of the entity is the type yielded by decltype, that is, int&&. And indeed, r is an lvalue ([expr.prim.id.unqual]).
Fortunately, this has been discovered and filed as bug 64892 two years ago. (I wonder why no one could find the time to fix this?)
67
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