Re: Inherited constructors (templates)

From:

Juha Nieminen <nospam@thanks.invalid>

Newsgroups:

comp.lang.c++

Date:

Thu, 21 May 2009 10:05:43 GMT

Message-ID:

<XH9Rl.61$4e1.15@read4.inet.fi>

Johannes Bauer wrote:

Hello group,

I'm having a problem with inheritance and I've run out of ideas. I'm
trying to encpsulate threads in a nice(tm) way. Therefore I declared a
template class:

template <typename ArgumentType, typename ReturnType> class Thread

with a purely virtual method

virtual ReturnType Action() = 0;

of which my actual threads should inherit from:

class AddingThread : public Thread<int*, int> {
    public:
        int Action() {
            int sum = 0;
            for (unsigned int i = 0; i < 10; i++) {
                sum += Data[i];
            }
            return sum;
        }
};

Now I want to call that thing just like

int foo[10];
memset(foo, 0, sizeof(foo));
foo[5] = 9;

AddingThread t1(foo);
int result = t1.join();

The problem I get is:

threadtest.cpp: In function int main():
threadtest.cpp:23: error: no matching function for call to
AddingThread::AddingThread(int [10])
threadtest.cpp:6: note: candidates are: AddingThread::AddingThread()
threadtest.cpp:6: note: AddingThread::AddingThread(const
AddingThread&)

I can't really understand what exactly is the problem you are seeing.
Your AddingThread class very clearly has no constructor which takes an
int pointer as parameter, which is exactly what the compiler is telling
you. The template part has nothing to do with this. Obviously the exact
same thing would happen even if the base class is not a template.

I can solve it, of course, by doing:

AddingThread(int *x) : Thread<int*, int>(x) { }

Where the "Thread" constructor does all the magic (i.e. actually
creating the thread and calling the trampoline to kick off "Action").
However, I do not want this redundant constructor delegation - is it
possible to tell the compiler to always use only the parent's
constructor of identical prototype if no child constructor is available?

In the next C++ standard it will be possible to do something like
that. In the current standard, no.