64. Constexpr ADC Temperature Table

1. Variables (True Constants)

Unlike const (which just means "read-only" and might typically sit in RAM or Flash), constexpr guarantees the value is known at compile time.

// C-Style
#define MAX_VOLTAGE 3.3

// C++ Style
constexpr float MAX_VOLTAGE = 3.3f;
constexpr float THRESHOLD = MAX_VOLTAGE / 2.0f; 
// Compiler calculates 1.65f and stores it directly. No runtime division.

2. Functions (The Powerhouse)

You can write functions that execute during compilation.

// Calculates factorial at compile-time
constexpr int factorial(int n) {
    return (n <= 1) ? 1 : (n * factorial(n - 1));
}

// Usage
// The compiler effectively writes: int buf_size = 120;
int buf_size = factorial(5); 

3. If/Loops (C++14 onwards)

Modern constexpr supports standard logic.

constexpr uint32_t calculate_prescaler(uint32_t sysclk, uint32_t target_freq) {
    uint32_t div = sysclk / target_freq;
    if (div > 0xFFFF) return 0xFFFF; // Clamp to 16-bit
    return div;
}

1. Zero-Overhead Lookup Tables

Instead of hardcoding a 256-byte CRC table or Sine Wave array (which is error-prone to type manually), you can write a constexpr function to generate it. The compiler fills the array in Flash for you.

struct Table { uint8_t data[256]; };

constexpr Table generate_crc() {
    Table t = {};
    // ... Fill t.data logic ...
    return t;
}

// Table is generated during compilation and stored in Flash (.rodata)
constexpr Table crc_table = generate_crc(); 

2. Safe Register Math

Calculating bitmasks or clock dividers often involves complex shifts and divisions. Using macros (#define) is messy and error-prone. constexpr functions allow you to write readable logic with validation checks (asserts) that run at compile time.

3. Static Assertions

You can use constexpr values in static_assert to prevent invalid configurations from even compiling.

constexpr int BUFFER_SIZE = 128;
static_assert(BUFFER_SIZE % 8 == 0, "Buffer must be aligned to 8 bytes");