Calling template function from inner class - c++

Question

I have a class like this to call a function depending on the type. I try to compile it, but have error error C2059 syntax error : 'template'
class A
{
call_1()
{
B<type> b;
b.template say(i);
}
template<class T>
struct B
{
template <typename T, typename I>
T say(I i) {
return word;
}
};
template<>
struct B<void>
{
template <typename T, typename I>
void say(I i) {
/**/
}
};
}
What am I doing wrong?

Answer 1

First, let's rewrite your example into something that is readable and is closer to being compilable, and we also print "1" or "2" in say() to know which function gets called:
#include <iostream>
using type = int;
class A {
void call_1() {
B<type> b;
int i = 0;
b.template say(i);
}
template<class T>
struct B
{
template <typename T, typename I>
T say(I i) {
std::cout << "1\n";
return T();
}
};
template<>
struct B<void>
{
template <typename T, typename I>
void say(I i) {
std::cout << "2\n";
}
};
};
OK, so first, you are trying to specialize B inside of A. This is not allowed, so let't move it outside of A:
using type = int;
class A {
void call_1() {
B<type> b;
int i = 0;
b.template say(i);
}
template<class T>
struct B
{
template <typename T, typename I>
T say(I i) {
std::cout << "1\n";
return T();
}
};
};
template<>
struct A::B<void>
{
template <typename T, typename I>
void say(I i) {
std::cout << "2\n";
}
};
Next up, you are using the same template parameter (T) in both B and say(). You don't need to repeat T, so let's delete it:
using type = int;
class A {
void call_1() {
B<type> b;
int i = 0;
b.template say(i);
}
template<class T>
struct B
{
template <typename I>
T say(I i) {
std::cout << "1\n";
return T();
}
};
};
template<>
struct A::B<void>
{
template <typename I>
void say(I i) {
std::cout << "2\n";
}
};
Finally, call_1() cannot be defined before the specialization of A::B, so we need to move it outside too:
using type = int;
class A {
void call_1();
template<class T>
struct B
{
template <typename I>
T say(I i) {
std::cout << "1\n";
return T();
}
};
};
template<>
struct A::B<void>
{
template <typename I>
void say(I i) {
std::cout << "2\n";
}
};
void A::call_1() {
B<type> b;
int i = 0;
b.template say(i);
}
This should now compile and do what you want. Calling call_1() will print 1. If you change the type from int to void:
using type = void;
it will print 2.