sizeof
returns the size in bytes of a type. One common usage pattern, especially in C, is to use sizeof
to determine the
size of an array. However, arrays decay to pointers when passed as arguments to a function, and if sizeof
is applied to such an argument,
it will return the size of the pointer, not of the array. A similar issue happens when the array is used in an arithmetic operation.
This rule raises issues when:

sizeof
is used to compute the array size of a pointer passed as a function argument.

sizeof
is called on the result of an arithmetic operation involving an array.
Note: C++17 provides a std::size
function that will correctly compute the number of elements of an array and fail to compile if
provided with a pointer. It is simpler and safer to use this variant when available. C++20 also provides the functions std::ssize
,
std::ranges::size
, and std::ranges::ssize
with similar effects.
Noncompliant code example
void fun(int *data, int array[10]) {
size_t const dataSize = sizeof data / sizeof(int); // Noncompliant, type of data is int *
size_t const arraySize = sizeof array / sizeof(int); // Noncompliant, type of array is int * too
int primes[] = { 1, 2, 3, 5, 7, 13, 17, 19};
size_t const primesSize2 = sizeof(primes + 1) / sizeof(int); // Noncompliant, type of primes + 1 is int *
}
Compliant solution
// Computing dataSize is now the responsibility of the caller
void fun(int *data, int dataSize, int (&array)[10]) {
size_t const arraySize = sizeof array / sizeof(int); // Compliant, no decay
int primes[] = { 1, 2, 3, 5, 7, 13, 17, 19};
size_t const primesSize = std::size(primes); // Better variant in C++17
size_t const primesSize2 = sizeof primes / sizeof(int) + 1; // Compliant, type of primes is int[8]
}