Re: Odd error I can't seem to recreate
On 04.11.2011 19:52, Noah Roberts wrote:
In a significant code base I'm running into this problem:
This works:
test_service service;
std::string result = job->run_query(service);
This does not.:
std::string result = job->run_query(test_service());
At this point the service object has been destroyed.
And perhaps the query is running in the background?
Oh, I see below that the problem isn't the obvious asynchronous one that
I thought it was.
Or perhaps the potential problem I mentioned above comes in addition.
OK then, on to the real problem...
the run_query function expects a const reference to test_service's
base class:
struct location_job
{
virtual ~location_job() {}
virtual std::string run_query(location_service const& service) const
= 0;
};
The error I get from g++4.1.2 is:
pipe_processor_test.cpp:142: error: no matching function for call to
?test_service::test_service(test_service)?
../inc/test_service.h:27: note: candidates are:
test_service::test_service(bool)
../inc/test_service.h:7: note:
test_service::test_service(test_service&)
Well, first note that the formal argument is of type `location_service
const&`, while the actual argument is of type `test_service`.
Assumming that `test_service` is derived from `location_service`, or is
the same class.
Then apparently g++ abides by C++98 rules that require an accessible
copy constructor, because C++98 allowed the compiler to introduce any
number of temporaries. This was fixed in C++03. But with C++98 rules, a
copy constructor must be accessible and the result is "as if" it's used,
when you pass a temporary to T const&.
And the test_service copy constructor has non-const formal argument,
test_service&
Hence it cannot be used for copying the actual argument.
The bool constructor is made by me, but the non-const copy constructor
is the compiler's. Line 7 is the opening brace for the class.
It may have been generated due to presence of e.g. std::auto_ptr as member.
Any data member or base class that has T(T&) forces that.
It couldn't (logically) be any other way.
I can't make any sense of this. Does anyone have ANY idea what might
be going on so I can try to narrow it down and create a minimal
example to get better help? The attempts I've made so far work
fine.
Yes, see above.
Under what conditions in which the first could work might the second
not when the parameter is const ref? I can't think of any.
Thanks for any help that can be provided. I know it's a reach.
Cheers & hth.,
- Alf