Struct problem in C++

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I came across a c++ code on for_each() that i didnt understand. Here is the code:

C++

struct Class2
{
    void operator() (int a)
    {
        cout << a * 3 << " ";
    }
} ob1;

int main() {
  int arr[5] = { 1, 5, 2, 4, 3 };
  cout << "Multiple of 3 of elements are : ";
    for_each(arr, arr + 5, ob1);
}

What i dont understand is the bit where it says obj1; at the end of the struct. How does this work? And also how is for_each() able to take obj1 as a parameter?
Can someone please answer this question.

What I have tried:

I have tried finding more information, but i dont know what this is called

Posted 18-Sep-21 20:22pm

iwanttoaskquestions

Updated 19-Sep-21 16:49pm

Greg Utas

v2

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Richard MacCutchan · Answer 1 · 2021-09-18T21:10:00

Solution 1

As described at std::for_each - cppreference.com[^], the third parameter is a unary function. In the case above, the ob1 reference implies a call to the operator() function, so the result will be something like:

Multiple of 3 of elements are : 3 15 6 12 9

Posted 18-Sep-21 21:10pm

Richard MacCutchan

Updated 18-Sep-21 21:58pm

v2

Greg Utas · Answer 2 · 2021-09-19T01:26:00

Solution 2

To answer the first part of your question, when something appears between the closing } and ; of a class/struct/union definition, it creates an instance of that class/struct/union with the specified name. It's the equivalent of

C++

struct Class2 {...};
Class2 ob1;

for those who are obsessed with typing fewer characters.

Posted 19-Sep-21 1:26am

Greg Utas

Comments

Richard MacCutchan 19-Sep-21 9:30am

Yes, I forgot to answer that part.

Joe Woodbury · Answer 3 · 2021-09-19T16:49:00

Solution 3

They're called predicates and you don't need to instantiate the struct to use them.

Posted 19-Sep-21 16:49pm

Joe Woodbury

v2