C++模板编程:为什么T[]和R(A1，A2，A3)被视为类型？

template <typename T>
T calculateAverage(T arr[], int size) {
    T sum = 0;
    for (int i = 0; i < size; i++) {
        sum += arr[i];
    }
    return sum / size;
}

int intArray[] = {1, 2, 3, 4, 5};
double doubleArray[] = {1.1, 2.2, 3.3, 4.4, 5.5};

int intAvg = calculateAverage(intArray, 5);
double doubleAvg = calculateAverage(doubleArray, 5);

template <typename R, typename A1, typename A2, typename A3>
class FunctionWrapper {
private:
    R (*func)(A1, A2, A3);
public:
    FunctionWrapper(R (*f)(A1, A2, A3)) : func(f) {}

    R operator()(A1 arg1, A2 arg2, A3 arg3) {
        return func(arg1, arg2, arg3);
    }
};

int add(int a, int b, int c) {
    return a + b + c;
}

double multiply(double a, double b, double c) {
    return a * b * c;
}

FunctionWrapper<int, int, int, int> addWrapper(add);
FunctionWrapper<double, double, double, double> multiplyWrapper(multiply);

int result1 = addWrapper(1, 2, 3);
double result2 = multiplyWrapper(1.1, 2.2, 3.3);