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]?

Question

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])"

`int a[n]` where `n` is a runtime variable is not a valid c++ — Jane Doe, Jun 03 '22 at 18:23
`cin>>n; int a[n];` is not legal C++. add the `-pedantic-errors` compiler flag and the code will no longer compile. — NathanOliver, Jun 03 '22 at 18:29
The big 3 compilers saying VLAs are non-standard - live - https://godbolt.org/z/E9saMrs9q — Richard Critten, Jun 03 '22 at 18:31
Legal, illegal, either way, the compiler can't instantiate a template around a value the user will provide in the future. — user4581301, Jun 03 '22 at 18:32

score 1 · Accepted Answer · answered Jun 03 '22 at 18:28

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.

1 Answers1