I need to construct an array that stores one type of data but appears as another type for memory saving. Only the subscript operator ([]) is required.
Say, I have an array arr that appears as fp32 but stores int8 internally:
When reading a value from arr by arr[i], overloading the subscript operator that returns by value just works. However when assigning a value with syntax like arr[i]=somevalue, as I cannot cast int8_t to float32_t with reference, I cannot modify the inside value.
Defining a separate method set(unsigned int index, float32_t value) would be a simple solution, but it will require modifying all other code interacting with the array, thus not favored.
I considered using a template datatype for internal storing, and overloading its operator=. However, in this case the returned type of the [] operator cannot be determined and I cannot proceed further.
// dummy cast functions
int8_t complex_downcast(double x) {
return x;
}
double complex_upcast(int8_t x) {
return x;
}
// current structure
template <typename T, unsigned int N> struct buffer {
int8_t v[N];
T get(int i) const {
return complex_upcast(v[i]);
}
T operator[](int i) const {
return get(i);
}
void set(int i, T v) {
this->v[i] = complex_downcast(v);
}
};
buffer<double, 1> buf;
buf.set(0, 2.1); // Sets 2.1, cast to int8 and stored
std::cout << buf[0] << std::endl; // (double) 2.0000000
// Want: replace buf.set(index, value) by buf[index] = value, where complex_downcast is required.
What you need to do is have the array class overload the operator[] to return a proxy object, and then have the proxy overload operator= for writing and operator T for reading. Then your proxy's operators can do whatever you want.
Try something like this:
template <typename T, unsigned int N> struct buffer {
int8_t v[N];
struct proxy {
int8_t *elem;
proxy& operator=(T value) {
// whatever you need...
*elem = static_cast<T>(value);
return *this;
}
operator T() const {
// whatever you need...
return static_cast<T>(*elem);
}
};
proxy operator[](int i) {
return proxy{ &v[i]; };
}
};