Question

Note: I am using the g++ compiler (which is I hear is pretty good and supposed to be pretty close to the standard).

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So, I think I’ve learned that passing a pointer-to-an-array or passing the actual array as an argument to another function always results in the passing of a pointer-to-that-array.

Similarly, I think I’ve learned that passing a pointer-to-a-function or passing the actual function as an argument to another function always results in the passing of a pointer-to-that-function.

Is it possible to pass a copy of the array (without explicitly making a copy of the array inside the called function)?

Is it possible to pass a copy of the function (without explicitly making a copy of the function inside the called function)? Is that ever useful? What does that even mean?

I understand the difference between passing an array or its copy; what is the difference between passing a function or its copy?

Answer 1

C doesn’t make it possible to pass arrays by value or more generally to copy them (unless by hand). C++ inherits that, so no it is not possible. The better approach would be to use std::array (for C++0x; boost::array is identical and available for C++03), which is a copyable type and can be passed by value.

int i[] = { 1, 2, 3 }; // type is int[3]
int j[3] = i; // error: can't copy an array
// C style would be to let j unitialized and to copy the values of
// i individually:
// int j[3]; std::copy(i, i + 3, j);

std::array<int, 3> ii = {{ 1, 2, 3 }};
std::array<int, 3> jj = ii; // Okay

It is not possible in either languages to copy functions and unlike arrays it is not possible to do it ‘by hand’. ‘Copying a function’ is not meaningful in those languages.

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Is it possible to pass a copy of the function (without explicitly
making a copy of the function inside the called function)? Is that
ever useful? What does that even mean?

In what language would ‘copying a function’ be meaningful?

Lua allows reading the byte-code of a function and it can be reused at a later point (or in another interpreter altogether) to reconstitute a function. I also assume that Lisp, under appropriate circumstances, can do the same with its code-as-data approach. Note that in both those languages it would not be common to do that and it’s not idiomatic to call it ‘copying a function’.

This sort of thing is less copying in the int i = j; sense that i is a copy, but is usually closer to the concept of marshalling/serialization. That is to say, the transformation of runtime data* into a more persistent form that can be stored and/or transmitted (e.g. across a network).

*: in those languages (among others) and unlike in C and C++, functions are data.