c++ - error: ‘X’ does not name a type X in template functions

Question

I have the following code:

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

template< typename X>   
X unary(X x)
{
    return x*10;
}

X binary( X x,X y)
{
    return x+y;
}

int main()
{
    vector<int> v1{1,2,3,4,5,6};
    vector<int> v2(v1);

    vector<int>::iterator i;

    for(i=v2.begin();i<v2.end();i++)
        cout<<*i<<endl;

    cout<<"unary "<<unary<int>(2)<<endl;
    cout<<"binary "<<binary<int>(2,7);
}

However, it does not compile, and instead I get the following error message:

transform.cpp:12:1: error: ‘X’ does not name a type
X binary( X x,X y)
transform.cpp: In function ‘int main()’:
transform.cpp:28:19: error: expected primary-expression before template’

Which appears on the following line:

cout<<"binary "<< binary<int,int>(2,7);

Why does X name a type for unary, but not binary?

Answer 1

The 'T' in the template is just a placeholder for the type used in the function, the class or the entity it is associated with. And the scope of the template parameter ends with the scope of this entity.

See here: When does a template end?

You have to write another template for binary(X x, X y) as follows:

template< typename X>   
X unary(X x)
{
  return x*10;
}

template< typename X> 
X binary( X x,X y)
{
    return x+y;
}