In recent months, I've been improving my C++ skills, after all plain C is really starting to show it's age. So, variadic templates, auto typed variables, lambda expressions and even fold expressions, I looked at all that stuff, pretty interesting overall.
I've found that from the point of view of 8 bit MCU programming, the constexpr modifier is quite useful. Everyone knows these 8 bit chips have limited processing power, so being able to do complex calculations at compile time is great, in my opinion.
This small example creates a constant array in flash memory, containing 8 bit samples forming one period of a sine wave. Normally, the samples would be calculated by some external program and converted to some ugly array, but in this case the compiler does the job. (note: this code requires C++14, so you need to enable that, for example, in AS7, add the option -std=gnu++14 to the compiler)
#include <stdint.h>
#include <avr/io.h>
#include <avr/pgmspace.h>
#define _USE_MATH_DEFINES
#include <math.h>
// We need to return an array, which is not possible in C/C++
// So we return a structure that contains the array.
struct array {
uint8_t _[256];
};
// This function returns an array structure filled with a calculated period of a sine wave (8 bit resolution).
// Since it is defined as constexpr, the compiler knows that the result of the function can be determined at compile time.
// No actual AVR code is generated, just data.
constexpr array table_generator() {
array tmp {};
double angle {};
for (int i = 0; i < 256; i++) {
angle = i * M_PI / 128;
tmp._[i] = 128 + 127 * sin(angle);
}
return tmp;
}
// Fill flash memory with the sine wave table
const PROGMEM array sine_wave = table_generator();
// A define to pretend we are dealing with a simple array
#define SINE_WAVE (&(sine_wave._[0]))
// Use the table so it's not optimized away
int main () {
for (int i = 0; i < 256; i ++) {
PORTD = SINE_WAVE[i];
}
}
