Passing Array Into Function - Pointer vs Reference (C++ vs C)

Question

I have a broad level question regarding best practices for passing arrays into functions.

So in the past when I've been programming in C and I wanted a function to have it's input be an array, I would declare that functions input parameters to be a pointer. This worked relatively well.

However, I've began programming more in C++ and am trying to determine the best practice for passing arrays into functions. So I've noticed that it is popular in C++ to pass objects by reference such that expensive copying operations are avoided. However, when I google passing arrays into functions, I read statements saying that arrays are automatically passed by reference.... So what's the deal with this? Why are arrays automatically passed by reference? And let's say I don't want the function to modify the array, is it possible to pass const arrays?

I'm having a difficult time getting my test program to compile. So I'm curious if anyone could explain what it means to pass an array into a function in C++ and how that differs from C.

Thanks!

Answer 1

In both C and C++, declaring a function to take an array parameter, such as in the following example, in fact causes the function to take a pointer. For example:

void foo(int arr[]);

This function signature is identical to:

void foo(int *arr);

Thus when you try to pass an array in either C or C++ you're already avoiding any overhead of copying an array.

Why are arrays automatically passed by reference?

They're passed by reference only in a loose sense. They're not literally passed as a C++ reference, which would look like the following:

void foo(int (&arr)[10]); // arr is a reference to an array of 10 ints

The reason for the C behavior is because they thought passing arrays by value would never be used anyway because of the expensive copy. The reason C++ has the same behavior is simply for compatibility.

Experience has shown that the special behavior of array parameters was a bad idea, and so it is one of the many reason that one should avoid using raw arrays in C++. The problem is that passing an array either way is dangerous:

void foo(int arr[10]) { arr[9] = 0; }

void bar() {
    int data[] = {1, 2};
    foo(data);
}

The above code is wrong but the compiler thinks everything is fine and issues no warning about the buffer overrun.

Instead use std::array or std::vector, which have consistent value semantics and lack any 'special' behavior that produces errors like the above.

And let's say I don't want the function to modify the array, is it possible to pass const arrays?

You can:

void foo(int const arr[]);
void foo(int const *arr);

void foo(int const (&arr)[10]);

So I'm curious if anyone could explain what it means to pass an array into a function in C++ and how that differs from C.

If you use the syntax that works in C then it doesn't really differ at all.