How to overload + operator for arrays in C++?

Question

For a C++ exercise for school I need to overload the + operator to concatenate 2 arrays. And I need to do that in a generic class.

for ex:

a1 = {1, 3, 4}
a2 = {2, 3, 5, 6}
=> a = {1, 3, 4, 2, 3, 5, 6}

And I have this code but it's not working.

#include <iostream>
#include <array>

using namespace std;

template <class T>
T operator+(const T& a1, const T& a2)
{
  T a;
  for (typename T::size_type i = 0; i < a1.size() + a2.size; i++) {
      if (i < a1.size()) {
        a[i] = a1[i];
      } else {
          a[i] = a2[i];
      }
  }
  return a;
}

int main()
{
  array<int,3> a1 = { 1, 3, 4 };
  array<int,4> a2 = { 2, 3, 5, 6 };
  array<int,7> a3 = a1 + a2;

  for (int i = 0; i < 5; i++)
    cout << a1[i] << '+' << a2[i] << '=' << a3[i] << ' ';

  cout << endl;
  return 0;
}

This code doesn't work, this is what I receive in the console:

main.cpp:32:24: error: no match for ‘operator+’ (operand types are ‘std::array’ and ‘std::array’)
   array<int,7> a3 = a1 + a2;
                     ~~~^~~~
main.cpp:15:3: note: candidate: template T operator+(const T&, const T&)
 T operator+(const T& a1, const T& a2)

What should I do to resolve the problem? And another question, does this code use generic class? Any feedback will be appreciated!

T operator+(const T& a1, const T& a2) is all the same Ts, but array<int,3>, <array<int,4> and array<int,4> are different types — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented May 31, 2021 at 12:43
Yes, your operator+ says that you can add a type to the same type and get the same type, but this is not what you try to do in your main function. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented May 31, 2021 at 12:44
I'm a Java developer -- Does this have any relevance to the C++ question you're asking? Are you attempting to make C++ code look like Java? — PaulMcKenzie
– PaulMcKenzie, Commented May 31, 2021 at 12:44
parts of main looks like you want elementwise + but the imlementation looks like you want to concatenate. What do you actually want? — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented May 31, 2021 at 12:46
And I need to do that in a generic class. -- I see no generic class. I see a standalone template function. — PaulMcKenzie
– PaulMcKenzie, Commented May 31, 2021 at 12:49

463035818_is_not_an_ai · Accepted Answer · 2021-05-31 12:58:46Z

2

T operator+(const T& a1, const T& a2) takes two Ts as parameter and returns a T. That is not what you want.

This is how you can concatenate two arrays by using operator+:

#include <iostream>
#include <array>

    
template <typename T,size_t M,size_t N>
std::array<T,M+N> operator+(const std::array<T,M>& a1, const std::array<T,N>& a2) {
  std::array<T,M+N> a;
  for (size_t i = 0; i < M; ++i) {
      a[i] = a1[i];
  }
  for (size_t i = 0; i < N; ++i) {
      a[i + M] = a2[i];
  }
  return a;
}

int main() {
    std::array<int,3> a1 = { 1, 3, 4 };
    std::array<int,4> a2 = { 2, 3, 5, 6 };
    std::array<int,7> a3 = a1 + a2;

    for (const auto& e : a3){
        std::cout << e << " ";
    } 

    std::cout << "\n";
}

Your code somehow mixes elementwise addition and concatenation. And there were some typos in the implementation of operator+ (eg size instead of size() and accessing a2 out of bounds).

And another question, does this code use generic class?

No. Your operator+ (and the one above) is a free function. std::array is a generic container and std::array<int,N> is a class.

edited May 31, 2021 at 12:58

answered May 31, 2021 at 12:50

463035818_is_not_an_ai

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3 Comments

elvis Over a year ago

Thank you for help! And how to convert it into a generic class?

463035818_is_not_an_ai Over a year ago

@elvis I dont know what that means. You have to ask your instructor. As I wrote, std::array is generic, it is a class template (it is not a class). std::array<int,3> is a class, but it isnt generic. "generic class" is not a common term in C++, at least not to me

463035818_is_not_an_ai Over a year ago

@elvis anyhow, nitpicking on wording aside, I really don't know how they expect you to use a class template for this

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