What is array to pointer decay?

It's said that arrays "decay" into pointers. A C++ array declared as int numbers [5] cannot be re-pointed, i.e. you can't say numbers = 0x5a5aff23. More importantly the term decay signifies loss of type and dimension; numbers decay into int* by losing the dimension information (count 5) and the type is not int [5] any more. Look here for cases where the decay doesn't happen.

If you're passing an array by value, what you're really doing is copying a pointer - a pointer to the array's first element is copied to the parameter (whose type should also be a pointer the array element's type). This works due to array's decaying nature; once decayed, sizeof no longer gives the complete array's size, because it essentially becomes a pointer. This is why it's preferred (among other reasons) to pass by reference or pointer.

Three ways to pass in an array¹:

void by_value(const T* array)   // const T array[] means the samevoid by_pointer(const T (*array)[U])void by_reference(const T (&array)[U])

The last two will give proper sizeof info, while the first one won't since the array argument has decayed to be assigned to the parameter.

^{1 The constant U should be known at compile-time.}

Arrays are basically the same as pointers in C/C++, but not quite. Once you convert an array:

const int a[] = { 2, 3, 5, 7, 11 };

into a pointer (which works without casting, and therefore can happen unexpectedly in some cases):

const int* p = a;

you lose the ability of the sizeof operator to count elements in the array:

assert( sizeof(p) != sizeof(a) );  // sizes are not equal

This lost ability is referred to as "decay".

For more details, check out this article about array decay.

Here's what the standard says (C99 6.3.2.1/3 - Other operands - Lvalues, arrays, and function designators):

Except when it is the operand of the sizeof operator or the unary & operator, or is a string literal used to initialize an array, an expression that has type ‘‘array of type’’ is converted to an expression with type ‘‘pointer to type’’ that points to the initial element of the array object and is not an lvalue.

This means that pretty much anytime the array name is used in an expression, it is automatically converted to a pointer to the 1st item in the array.

Note that function names act in a similar way, but function pointers are used far less and in a much more specialized way that it doesn't cause nearly as much confusion as the automatic conversion of array names to pointers.

The C++ standard (4.2 Array-to-pointer conversion) loosens the conversion requirement to (emphasis mine):

An lvalue or rvalue of type “array of N T” or “array of unknown bound of T” can be converted to an rvalue of type “pointer to T.”

So the conversion doesn't have to happen like it pretty much always does in C (this lets functions overload or templates match on the array type).

This is also why in C you should avoid using array parameters in function prototypes/definitions (in my opinion - I'm not sure if there's any general agreement). They cause confusion and are a fiction anyway - use pointer parameters and the confusion might not go away entirely, but at least the parameter declaration isn't lying.