I created two simple functions which get template parameters and an empty struct defining a type:
//S<T>::type results in T&
template <class T>
struct S
{
typedef typename T& type;
};
//Example 1: get one parameter by reference and return it by value
template <class A>
A
temp(typename S<A>::type a1)
{
return a1;
}
//Example 2: get two parameters by reference, perform the sum and return it
template <class A, class B>
B
temp2(typename S<A>::type a1, B a2)//typename struct S<B>::type a2)
{
return a1 + a2;
}
The argument type is applied to the struct S to get the reference. I call them with some integer values but the compiler is unable to deduce the arguments:
int main()
{
char c=6;
int d=7;
int res = temp(c);
int res2 = temp2(d,7);
}
Error 1 error C2783: 'A temp(S::type)' : could not deduce template argument for 'A'
Error 2 error C2783: 'B temp2(S::type,B)' : could not deduce template argument for 'A'
Why is this happening? Is it that hard to see that the template arguments are char and int values?
Just as first note, typename name is used when you mention a dependent name. So you don't need it here.
template <class T>
struct S
{
typedef T& type;
};
Regarding the template instantiation, the problem is that typename S<A>::type
characterizes a nondeduced context for A. When a template parameter is used only in a nondeduced context (the case for A in your functions) it's not taken into consideration for template argument deduction. The details are at section 14.8.2.4 of the C++ Standard (2003).
To make your call work, you need to explicitly specify the type:
temp<char>(c);