This sets all the entries in the array to the given entry. The default value of fill parameter should be zero.
void fill( int array[], std::size_t capacity, int value = 0 );
For templated implementations, you could use 0 as the default value, but it is better to use the default value of the templated type: T{}.
template <typename T> void fill( T array[], std::size_t capacity, T value = T{} );