Thread: Can you compile the following template function code?

Hello everyone,


Here is the code and related compile error information. I am using Visual Studio 2008. Not sure whether you can compile using your compiler? Why Visual Studio 2008 can not deduce template parameter type?

Code:

#include <iostream>

using namespace std;

template <class T> void f(int a) {g (a);}

void g(int a)
{
	cout << a << endl;
}

int main()
{
	f (100); // can not compile, error C2783: 'void f(int)' : could not deduce template argument for 'T'
	// f<int> (100) can compile
	return 0;
}

thanks in advance,
George

Hi CornedBee,


Seems Bjarne is wrong? :-)

The code is (with small adaption) from his book, section C.13.8.3 Point of Instantiation Binding.

Originally Posted by CornedBee

How would it? It can only deduce template parameters that show up in the argument list, but you stealthily replaced T by int in the definition.

Actually, because g(a) is now no longer dependent, it shouldn't compile with an explicit parameter ...

Hi laserlight,


Seems f<int>(100) is the only way to call it. Why do you say the parameter is not used? It is used to pass to function g inside function f. Any comments?

Originally Posted by laserlight

In any case, it seems rather pointless to have a function template where the parameter is not used, so even if f<int>(100) is allowed, it is still pointless.

regards,
George

Thanks Elysia,


Question answered.

Originally Posted by Elysia

You are never using the type T anywhere in your template code. You specify a template with type T, but the only argument you have is int. Type T is never used anywhere.
The compiler cannot deduce the type for T since you never specify any argument for it.
The type T is unused. The only argument is int, which you pass in 100.
So the compiler has no idea whatsoever what type T is. And again, it's never used anywhere in the template code so it's redundant (does not need to be there), which is what everyone is telling you.

regards,
George

Thanks CornedBee,


The compile error and the "small" adaption gives me a good lesson.

Originally Posted by CornedBee

Guess what part is to blame for the compile error.
Yep, that's right, the small adaption. You replaced the T parameter with an int parameter.

regards,
George