具有多个类型名的C ++模板

Question

I'm learning about a technique used to find the number of elements in an array (so that I can hopefully starting writing sorting algorithms without requiring the length of the array to be passed with the array as a parameter) but in the tutorial, this line appears as the template declaration:

template <typename T, size_t N>

I honestly didn't know you could declare multiple typenames in one template declaration, but furthermore what does "size_t N" do? is this a variable declaration inside of a template declaration as well?

Answer 1

size_t is a type that resembles unsigned int . Having it in the template parameter just means that you pass a size_t , not a type. N probably represents the size of an array, which is an unsigned value. For example:

template<typename T, size_t N>
void zeroArray(T (&arr)[N]) {    //arr is a reference to an array 
    std::fill(arr, arr + N, 0);  //of N elements of type T
}

int main() {
    int arr[3];
    zeroArray<int, 3>(arr);
}

In the example, I could have said:

zeroArray(arr);

because both template arguments are deduced.

Answer 2

what does "size_t N" do? is this a variable declaration inside of a template declaration as well?

Yep, basically.

Template arguments may be types or values of integer type. There are a few other things they can be, too (see [C++11: 14.3.2/1] ) and I wouldn't call them "variables" per se , but...

Anyway, the values can be deduced just like types, too, so:

template <typename T, size_t N>
size_t array_size(const T (&)[N])
{
    return N;
}

int main()
{
   int  x[5];
   char y[10];
   std::string z[20];

   std::cout << array_size(x) << ',' << array_size(y) << ',' << array_size(z) << '\n';
}

Output: 5,10,20 .

Perfectly valid.