Function Templates

[Oliver M.]

Well, I think I am stuck:

template<int TSIZE>
struct A
{
	int get_size() const { return TSIZE; };
	char buffer[TSIZE];
};

template<int TSIZE>
void Foo()
{
	char buffer[TSIZE];
	std::cout << TSIZE << std::endl;
}

int main(int argc, char* argv[])
{
	A<12> a12;
	std::cout << a12.get_size() << std::endl; // outputs 12, ok

	A<5> a5;
	std::cout << a5.get_size() << std::endl; // outputs 5, ok

	Foo<12>(); // outputs 5, WHY???
	Foo<5>(); // outputs 5, ok

	return 0;
}

Can somebody explain why Foo<12>(); outputs 5 and not - as I expected - 12?

[humptydumpty]

in your protgram if u comment last line Foo<5> you can see now u r getting F<12> value to 12 Only
But at the time when u uncomment follwong line you will see that due to last function first is also changing it's value if u write any third Function
like Foo<15>() u will see now all three having value of 15.in you First two call you are calling your structure member function get_size
so it's printing 12 & 5 properly but in your Function call both are taking same address space in memory so changing ones will affect the value of both
that's why when u r Printing individual value u will get right result but in case of 2 or more what is the last value that will replaced all previos value

[exterminator]

This is actually a bug with the VC++ 6.0 compiler that has been fixed in the version 7 and I guess you are working with this compiler. I tried your code with gcc and it works as expected. VC++ 6.0 seems to ignore the template arguments in this case and distinguishes the functions based on their function arguments. Here is a FAQ regarding this at C++ Templates FAQ. Look at FAQ 5. Hope this helps. Regards.

EDIT: You could try using std::string instead of char buffers or else you would need to make dynamic allocations using new() and then call delete to maintain char buffer.

[Oliver M.]

@humptydumpty
Thanks for your comment. I also realized this behavior, since I am playing around with this issue quite a few days. The question was *why* both function calls have the same address.

@exterminator
You guessed right... (I can not upgrade because my workgroup stick with it). I had the strong feeling that I wrote something like this before and it worked, but it was most probably at home on my VC7.1 compiler

Thanks for the link and for your suggestion, too. I actually do not need to reserve a buffer at all, it was just for demonstration. What I really wanted to do in the actual product is to use a member function as a template argument of a make_xxx function that creates a templated class object that in turn calls the passed member function later on. Everything worked fine without the make_xxx function, so I started a simple demo project to play around with it.

Now, I'll have to find a different solution... Thank you very much!