[FIXED] Why does the "size_of_array" template fucntion does not work with int a[n] (size defined at runtime) while it works fine with int a[4]?

Issue

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

this code works fine with

int a[4];
cout<< tell_me_size(a)<<endl;

while does not work with

int n;
cin>>n;
int a[n];
cout<< tell_me_size(a)<<endl;

The later gives the error "no matching function for call to ‘tell_me_size(int [n])"

Solution

According to the C++ 20 Standard (13.4.3 Template non-type arguments)

2 A template-argument for a non-type template-parameter shall be a converted constant expression (7.7) of the type of the template-parameter.

Pay attention to that variable length arrays like this

int n;
cin>>n;
int a[n];

are not a standard C++ feature.

Answered By - Vlad from Moscow

Answer Checked By - Cary Denson (PHPFixing Admin)