c++ - Is there anything wrong with sizeof(array)/sizeof(array[0])?

Question

One of my colleagues has recently said that the above statement is not type safe and I should use something else as you need as much type safe structures as possible to reduce the amount of possible bugs.

Whilst I agree on being type safe, I'm a little confused as this is the type of code in question (only the contents and length of data[] is modified)

unsigned char data[] = {1,2,3,4,5};
int data_len = sizeof(data) / sizeof(data[0]); 

Where is the part that is not type safe?

Needless to say, other than the comment, the colleague will not explain further.

PS: This is used to copy initialisation data into a class from the constructor, no C++11 compiler exists here, so we can't use std::array or other fancy array initialisation. techniques.

Answer 1

Maybe your colleague meant that using this expression with pointers will give an unexpected result. This mistake is made very often by beginners. For example

void f( unsigned char data[] )
{
   int data_len = sizeof(data) / sizeof(data[0]); 
   //...
}

//...

unsigned char data[] = {1,2,3,4,5};
f( data );

So in general case it would be more safely to use a template function instead of the expression. For example

template <class T, size_t N>

inline size_t size( const T ( & )[N] )
{
   return N;
}

Take into account that there is template structure std::extent in C++ 11 that can be used to get the size of a dimension.

For example

int a[2][4][6];

std::cout << std::extent<decltype( a )>::value << std::endl;
std::cout << std::extent<decltype( a ), 1>::value << std::endl;
std::cout << std::extent<decltype( a ), 2>::value << std::endl;