Why does replacing an argument with its value cause an error? - c++

Question

I'm fairly new to C++, and I wanted to test what I know about it by implementing a mediocre List class.
List.hpp:
#ifndef LIST_HPP
#define LIST_HPP
// Only supports int lists up to 100 items for now.
class List
{
public:
List(int arr[], int length);
void append(int item);
void print();
void remove();
int len();
List operator+(List obj);
protected:
private:
int *list = new int[100];
int size = 0;
};
#endif
List.cpp:
#include <iostream>
#include "List.hpp"
List::List(int arr[], int length)
: size(length)
{
for (int i = 0; i < length; i++)
{
list[i] = arr[i];
}
}
void List::append(int item)
{
list[size] = item;
size++;
}
void List::print()
{
for (int i = 0; i < size; i++)
{
if (i == 0)
{
std::cout << '[';
}
std::cout << list[i];
if (i == size - 1)
{
std::cout << ']' << std::endl;
}
else
{
std::cout << ", ";
}
}
}
void List::remove()
{
if (size == 0) {
std::cout << "Error: list is empty" << std::endl;
return;
}
else {
for (int i = 0; i < size - 1; i++) {
list[i] = list[i + 1];
}
size--;
}
}
int List::len()
{
return size;
}
List List::operator+(List obj)
{
List res({}, 0);
for (int i = 0; i < this->size; i++)
res.append((this->list)[i]);
for (int i = 0; i < obj.size; i++)
res.append((obj.list)[i]);
res.size = this->size + obj.size;
return res;
}
However, upon initializing List mylist({1, 2, 3}, 3);, the compiler says,
main.cpp: In function 'int main()':
main.cpp:9:29: error: no matching function for call to 'List::List(<brace-enclosed initializer list>, int)'
List mylist({1, 2, 3}, 3);
^
In file included from main.cpp:3:0:
List.hpp:9:9: note: candidate: List::List(int*, int)
List(int arr[], int length);
^~~~
List.hpp:9:9: note: no known conversion for argument 1 from '<brace-enclosed initializer list>' to 'int*'
List.hpp:6:7: note: candidate: constexpr List::List(const List&)
class List
^~~~
List.hpp:6:7: note: candidate expects 1 argument, 2 provided
List.hpp:6:7: note: candidate: constexpr List::List(List&&)
List.hpp:6:7: note: candidate expects 1 argument, 2 provided
But when I intialize mylist using
int myarr[3] = {1, 2, 3};
List mylist(myarr, 3);
it works perfectly. Why is this so?

Answer 1

In List mylist({1, 2, 3}, 3);, {1, 2, 3} isn't an array, its a std::initializer_list.