无法使用我认为应该是编译时常数的内容初始化静态数组

Question

I'm trying to initialize a static array with the size based on what I think should be a compile-time constant.

This is easily fixable but simply not using static, but I didn't expect this issue to show up at all. This might cause me other issues in the future.

The relevant sections:

typedef struct {
    GPIO_TypeDef* Port;
    uint16_t Pin;
} PortPin;

typedef struct {
    I2C_HandleTypeDef *Handle;
    uint16_t Address;
    PortPin Interrupt;
    PortPin Shutdown;
} VL53L1_Dev_t;

//This one is bigger but I've kept it short to keep it readable. It's 3 elements in my code
static VL53L1_Dev_t Sensors[] = {
    {//1
            &hi2c2,
            0x52,
            {
                    GPIOA,
                    GPIO_PIN_11
            },
            {
                    GPIOA,
                    GPIO_PIN_10
            }
    }
}
static const int sensorCount = sizeof(Sensors)/sizeof(Sensors[0]);
static uint8_t encodedData[sensorCount * 2];//Compiler doesn't like this part

Since Sensors is fully defined on compile-time, I expected sensorCount to also be a compile-time constant. Which apparently it isn't because I can't use it to declare and initialize static arrays. I don't understand why.

Answer 1

The size of an array declared at file scope must be an integer constant expression . It is defined in section 6.6p6 of the C standard :

An integer constant expression shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, _Alignof expressions, and floating constants that are the immediate operands of casts. Cast operators in an integer constant expression shall only convert arithmetic types to integer types, except as part of an operand to the sizeof or _Alignof operator .

So the contents of a variable, even if declared const , are not part of such an expression.

If however you define sensorCount as a macro, the expression sizeof(Sensors)/sizeof(Sensors[0]) is a compile time constant.

#define SENSOR_COUNT  (sizeof(Sensors)/sizeof(Sensors[0]))
static uint8_t encodedData[SENSOR_COUNT * 2];