CODEWITHSUNDEEP
c++carrayssizeof
Dardar
Dardar

Reputation: 751

How to know the size of a const array?

given the following code:

const myStr codesArr[] =  {"AA","BB", "CC"};

myStr is a class the wraps char*. I need to loop over all the items in the array but i don't know the number of items. I don't want to define a const value that will represent the size (in this case 3)

Is it safe to use something like:

const int size = sizeof(codesArr) / sizeof(myStr);

what to do?

Upvotes: 18

Views: 23632

Answers (3)

ecatmur
ecatmur

Reputation: 157314

The safe and clear way to do this is to use std::extent (since C++11):

const size_t size = std::extent<decltype(codesArr)>::value;

Upvotes: 38

Karthik T
Karthik T

Reputation: 31952

The best way(pre c++11) to get size of an array is given at this link. It is a convoluted template trick, but has many advantages over other approaches as is discussed in the above link. The method is to use the following macro+function:

template <typename T, size_t N>
char ( &_ArraySizeHelper( T (&array)[N] ))[N];

#define countof( array ) (sizeof( _ArraySizeHelper( array ) ))

Its advantages, among others, are that it is purely a compile time operation, which is better than doing it in run time, and , before c++11's constexpr, is the only method that allows you to use the result of the operation as the size of another array ( since its a compile time constant

Upvotes: 1

Luchian Grigore
Luchian Grigore

Reputation: 258548

You can use int size = sizeof(codesArr) / sizeof(myStr); as long as you don't pass it as parameter to a function, because then the array will decay into a pointer and lose size information.

You can also use a template trick:

template<typename T, int sz>
int size(T(&)[sz])
{
    return sz;
}

Upvotes: 19

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