Template parameter Deduction

[Neelesh]

Hi all,
I had some confusion about deduction of non-type template parameters
for function templates :

template <class T, int i> void foo (T, int p = i) ;

void bar()
{
foo<int, 10>(40,40); // OK, T = int, i = 10
foo(40,40); // ERROR, T = int but i cannot be deduced.
}

However here I have another example :

template <class T, int i> void lookup ( Buffer<T,i> b);

void bar(Buffer<int, 128> b)
{
lookup(b,100); // No error, can infer T = int, i = 128
}

So it seems that when 'i' gets inferred as a "side effect" of type
matching, the compiler accepts that. However, it doesnot try to infer
it explicitly if required.

What does the standard say about this?

 

[Greg]

Hi all,
I had some confusion about deduction of non-type template parameters
for function templates :

template <class T, int i> void foo (T, int p = i) ;

void bar()
{
foo<int, 10>(40,40); // OK, T = int, i = 10
foo(40,40); // ERROR, T = int but i cannot be deduced.
}

The compiler cannot deduce non-type parameters for function templates
since there is no requirement that the parameters be compile-time
constants. Moreover, non-type template parameters are not all that
useful. Why call one routine when the parameter is 40 and a different
routine when it is 20? Aren't parameters supposed to be variables?
Can't one routine accept both a 20 or a 40 as a parameter?

Even if the routines should be distinct, they need not be function
templates. One function could be named Function20 and the other
Function40. Non-type function templates provide no more abstraction
than parameter-accepting functions, but can easily add significantly
more bloat to an application.

However here I have another example :

template <class T, int i> void lookup ( Buffer<T,i> b);

void bar(Buffer<int, 128> b)
{
lookup(b,100); // No error, can infer T = int, i = 128
}

So it seems that when 'i' gets inferred as a "side effect" of type
matching, the compiler accepts that. However, it doesnot try to infer
it explicitly if required.

The compiler deduces that the type Buffer<int, 128> - so the 128 is
deduced from the type of the parameter. You want the compiler to deduce
40 from the value of the parameter - a completely different
proposition.

Greg

 

[Neelesh]

The compiler deduces that the type Buffer<int, 128> - so the 128 is
deduced from the type of the parameter. You want the compiler to deduce
40 from the value of the parameter - a completely different
proposition.

Great. That last sentence just clears my confusion. Thanks a lot.

 

[John Carson]

The compiler cannot deduce non-type parameters for function templates
since there is no requirement that the parameters be compile-time
constants.

That is news to me. See if you can get this to compile

template<int i>
void foo()
{
int x=i;
}
int a = 5;

int main()
{
foo<a>();
}

 

[John Carson]

That is news to me. See if you can get this to compile

template<int i>
void foo()
{
int x=i;
}
int a = 5;

int main()
{
foo<a>();
}

Or perhaps I misunderstood you and you meant that in

template <class T, int i> void foo (T, int p = i) ;

the ordinary (not template) p parameter of foo doesn't have to be a compile
time constant.