How Can I Efficiently Determine the Size of a C Array at Compile Time?

Understanding the "Size of Array" Template Function

This code defines a template function, GetArrLength, designed to determine the size of an array:

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

Breaking Down the Function's Parameters

The parameter T(&)[size] is declared as a reference to an array of size size of type T. This means it accepts a reference to any array whose type and size are specified as template parameters.

Template Parameter Matching

When calling this function, the compiler attempts to deduce the template parameters. For example, if we call it as follows:

int a[10];
GetArrLength(a);

The compiler will determine that T is int and size is 10, creating a parameter reference to an array of 10 integers.

Function Behavior

The function returns the value of the template parameter size, effectively providing the number of elements in the array.

Usage and Limitations

This code simplifies obtaining the array size but has some drawbacks. First, it utilizes a signed type for both the template parameter and the return value, which is problematic as sizes cannot be negative. For a more robust solution, an unsigned type such as std::size_t should be used.

Secondly, the result of this function is not a constant-expression, even though an array size should be. Constant-expression evaluation is essential for certain optimizations.

Constant-Expression Solution

A more advanced approach that provides a constant-expression result involves using type introspection and the sizeof operator:

template <std::size_t N>
struct type_of_size
{
    typedef char type[N];
};

template <typename T, std::size_t Size>
typename type_of_size<Size>::type&amp; sizeof_array_helper(T(&amp;)[Size]);

#define sizeof_array(pArray) sizeof(sizeof_array_helper(pArray))

This technique exploits the fact that the size of a character array equals its element count. The sizeof operator retrieves the size of the char array used to represent the template parameter size, providing a constant-expression evaluation of the array size.

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