Why does all the sudden 2 become a different number

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I have this function:

C++

void Add(T item) { // template <class T> is assigned at class level
  array.push_back(item);
} //THIS WORKS FINE

void AddRange(T* collection) {
  for (int i = 0; i < (sizeof(collection)/sizeof(collection[0])); i++) {
    Add(collection[i];
  }
} //THIS DOES NOT WORK

List<int> intList = List<int>();
int arr[] = { 1, 2, 34, 5 };
intList.AddRange(arr);
PrintI(intList[0]); //prints "1". Like it should
PrintI(intList[1]); //prints random numbers, examples include "16318656" and "15925440"

But, however this works:

C++

for (int i = 0; i < (sizeof(arr)/sizeof(arr[0])); i++) {
  intList.Add(arr[i]);
}

I assume it has something to do with the template <class t="">, how do i fix it. And how does the template screw it up?

What I have tried:

I have tried many things, obviously i tried moving the code to the main function which works, and many things. Also the main function is in main.cpp, while the rest is in arrayFuncs.h

Posted 21-Nov-21 9:02am

Member 14769677

Updated 21-Nov-21 9:23am

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Rick York · Accepted Answer · 2021-11-21T09:23:00

The problem is in this line :

C++

for (int i = 0; i < (sizeof(collection)/sizeof(collection[0])); i++)

because it will iterate only once. Add an output statement there to see for yourself how many items are added.

C++

void AddRange(T* collection)
{
    for (int i = 0; i < (sizeof(collection)/sizeof(collection[0])); i++)
    {
        Add( collection[i] );
        std::cout << "added item " << i + 1 << std::endl;
    }
}

The reason the second one works is that sizeof(arr) gives the result you expect because the compiler knows it is an array. collection is just a pointer so in the previous one and not an array. If you pass the count into the Add method then it can work as you expect.

C++

void AddRange( T* collection, size_t count )
{
    for( size_t i = 0; i < count; i++ )
    {
        Add( collection[i] );
        std::cout << "added item " << i + 1 << std::endl;
    }
}

Visual C++ has a built in macro for that expression : _countof Macro | Microsoft Docs[^]