Questions about the Dart memory policy

Question

Suppose you have the following code.

List<File> files = [file1, file2, file3, file4];

Each file is 10mb in size.

So how much memory will it use? It sounds simple, but it's confusing.

1. It will use 40mb. Because it had 4 10mb files.

2. I don't know exactly, but I'll use much less than 40mb.This is because memory is not used as much as the actual size of the file, but as much as the size of a pointer pointing to the memory address of the file.

1 or 2?

Answer 1

The currently accepted answer is incorrect. Dart is always pass-by-value and whether it is or not is irrelevant to this question.

Now for your question. The amount of memory that you're using when working with files depends on what you're doing with them. The example you show is a list of file handles, not the files themselves. It would be inefficient to load the contents of a file into memory just by instantiating a reference to the file with a File object.

So your list is a list of file handles. This makes both of your proposed explanations incorrect since the data is not loaded into memory, though you're on the right track with the second explanation. The contents of the file is not in memory, so it will not use 40 MB of memory for 4 10 MB files. It's also not a pointer to the memory either. A pointer would still indicate that the file data is in memory and you File object just an address for that block. It's more accurate to says that it's a reference to the file given by the OS or a reference to the file's place in storage. The amount of memory that the file handle will take up is going to be relatively small, though I don't know any exact values.

Data will be loaded into memory if you explicitly request that data with a function like readAsBytes. Then the whole file will be loaded into memory and it will likely use the full 10 MB per file. There might be some optimization I don't know about, but it will be closer to the full size of the file. This will of course be in a separate variable and the File objects themselves will take up very little memory.

Answer 2

Dart uses pass-by-value for primitive types like int or String and uses pass-by-reference for more complex data types like List. Here the files are probably an object so it will take on the memory of the pointers.

This is a simple example for pass-by-value.

void main() {
  void addOne(int x) {
    x += 1;
    print("x here is $x");
  }
  
  int x = 1;
  addOne(x); //Output: "x here is 2"
  print(x); //1
}

Here the value "x" is passed by value so, even if the function incremented x, the value in the main function will still remain 1.

example for pass-by-reference:

void main() {
  void addList(List<int> x) {
    x.add(12);
    print("x here is $x");
  }

  List<int> x = [1, 2, 3, 4, 5];
  addList(x);  //x here is [1, 2, 3, 4, 5, 12]
  print(x);  //[1, 2, 3, 4, 5, 12]
}

Here the value of x is passed by reference so it would get updated in the main function as well

Passing the reference means to pass the memory location and not the actual value which is very efficient in case of big list or arrays or even complex objects. It can save much more memory than pass-by-value.

In your case the files variable is a List of File(List<File>) objects. That means that file1, file2, etc are File objects and shall thus be passed-by-reference in dart