How to use Structs in Templates as Parameters c++

Question

I saw a video about templates in c++ the other day and it had the following code:

template <typename T>
void someFunction(T num) {
   Print(num);
}

The "Print()" just prints out the var "num" in console. I basically understood how to use templates as parameters. However, I don't know how to use this with Structs, or if it's even possible. I mean something like this:

struct vec2 {
   float x, y;
};

struct vec3 {
   float x, y, z;
};

template <typename T>
void someFunction(T vec);

I want to check in the function whether "vec" is a vec2 or a vec3 and then use it differently. Like this:

if (vec==vec2) {
   //Print x and y
else if (vec==vec3) {
   //Print x, y and z
}

In the end my goal is simply to have a function that I can give 2 different structures to. I already heard about std::is_same and tried it but without any success.

Answer 1

This is actually possible using a type trait and if constexpr :

#include <iostream>
#include <type_traits>

struct vec2 {
   float x, y;
};

struct vec3 {
   float x, y, z;
};

template <typename T>
void someFunction(T vec)
{
   if constexpr (std::is_same_v<T, vec2>) {
      std::cout << vec.x << ' ' << vec.y << '\n';
   }
   else if constexpr (std::is_same_v<T, vec3>) {
      std::cout << vec.x << ' ' << vec.y << ' ' << vec.z << '\n';
   }
}

int main()
{
    vec2 v2 = {1, 2};
    vec3 v3 = {3, 4, 5};
    
    someFunction(v2);
    someFunction(v3);
}

(NB prior to C++17, you'll need std::is_same<T, vec2>::value .)

That being said, in your example, a simple set of function overloads would be more appropriate:

#include <iostream>

struct vec2 {
   float x, y;
};

struct vec3 {
   float x, y, z;
};

void someFunction(vec2 vec) {
   std::cout << vec.x << ' ' << vec.y << '\n';
}

void someFunction(vec3 vec) {
   std::cout << vec.x << ' ' << vec.y << ' ' << vec.z << '\n';
}

int main()
{
    vec2 v2 = {1, 2};
    vec3 v3 = {3, 4, 5};
    
    someFunction(v2);
    someFunction(v3);
}

In both cases, you may wish to consider taking the object by const reference instead of by value (which will copy it).

Answer 2

The answer from Asteroids With Wings gives two solutions, and the second of these is probably what you want. But the first is definitely not; you can achieve the same functionality simply by using function template specialisation:

#include <iostream>

struct vec2 {
   float x, y;
};

struct vec3 {
   float x, y, z;
};

template <typename T>
void someFunction(T vec) ;

template<>
void someFunction<>(vec2 vec)
{
   std::cout << vec.x << ' ' << vec.y << '\n';
}

template<>
void someFunction<>(vec3 vec)
{
   std::cout << vec.x << ' ' << vec.y << ' ' << vec.z << '\n';
}

int main()
{
    vec2 v2 = {1, 2};
    vec3 v3 = {3, 4, 5};
    
    someFunction(v2);
    someFunction(v3);
}