What does this C++ code mean?

Question

The following code returns the size of a stack-allocated array:

template
int siz(T (&) [size])
{
    return size;
}
         


        

Answer 1

T (&) [size] is a reference to an array. It needs to be a reference because the following program is not legal:

#include 

int sz(int *) { std::cout << "wtf?" << std::endl; return 0; }


int sz(int [4]) { std::cout << "4" << std::endl; return 0; }

int main() {
  int test[4];
  sz(test);
}

This program fails to compile with:

test.cc: In function ‘int sz(int*)’:
test.cc:6:5: error: redefinition of ‘int sz(int*)’
test.cc:3:5: error: ‘int sz(int*)’ previously defined here

because int sz(int [4]) is identical to int sz(int *).

The parenthesis are required to disambiguate here because T& [size] looks like an array of references which is otherwise illegal.

Normally if the parameter wasn't anonymous you would write:

template
int sz(T (&arr) [size])

To give the array the name arr. In this instance though all your example code cared about was the deduced size and hence the anonymous argument avoids warnings about unused arguments.