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POReference-type conversion operators: asking for trouble?
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copied!<p>When I compile the following code using <code>g++</code></p> <pre><code>class A {}; void foo(A&) {} int main() { foo(A()); return 0; } </code></pre> <p>I get the following error messages:</p> <pre><code>> g++ test.cpp -o test test.cpp: In function ‘int main()’: test.cpp:10: error: invalid initialization of non-const reference of type ‘A&’ from a temporary of type ‘A’ test.cpp:6: error: in passing argument 1 of ‘void foo(A&)’ </code></pre> <p>After some reflection, these errors make plenty of sense to me. <code>A()</code> is just a temporary value, not an assignable location on the stack, so it wouldn't seem to have an address. If it doesn't have an address, then I can't hold a reference to it. Okay, fine.</p> <p>But wait! If I add the following conversion operator to the class <code>A</code></p> <pre><code>class A { public: operator A&() { return *this; } }; </code></pre> <p>then all is well! My question is whether this even remotely safe. What exactly does <code>this</code> point to when <code>A()</code> is constructed as a temporary value?</p> <p>I am given some confidence by the fact that</p> <pre><code>void foo(const A&) {} </code></pre> <p>can accept temporary values according to <code>g++</code> and all other compilers I've used. The <code>const</code> keyword can always be cast away, so it would surprise me if there were any actual semantic differences between a <code>const A&</code> parameter and an <code>A&</code> parameter. So I guess that's another way of asking my question: why is a <code>const</code> reference to a temporary value considered safe by the compiler whereas a non-<code>const</code> reference is not?</p>

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