CODEWITHSUNDEEP
cstructstaticconstants
bgarrood
bgarrood

Reputation: 487

Defining static const value form a struct

I am using C99 in Keil to program a microprocessor, and having difficulty using a struct to define a static const. The following compliles without issue:

static uint8_t const addr1 = 0x76;
static uint8_t const addr = addr1;

However the following does not:

typedef struct
{
    uint8_t const           address;    
} bmp280_t;

static bmp280_t bmp280_0 = {
        .address    =   0x76,
    };

static uint8_t const addr2 = bmp280_0.address;

The last line causes a compilation error:

......\main.c(97): error: #28: expression must have a constant value static uint8_t const addr2 = bmp280_0.address;

I have tried to replicate in visual studio, but both cases do not compile. I don't know if that is because it is compiling as cpp or using a different standard..

Upvotes: 1

Views: 367

Answers (1)

jfMR
jfMR

Reputation: 24738

The initializer you are using (i.e.: bmp280_0.address) is not a compile-time constant. You can however do the following:

#define ADDRESS 0x76   

typedef struct
{
    uint8_t const           address;    
} bmp280_t;

static bmp280_t bmp280_0 = {
        .address    =   ADDRESS,
    };

static uint8_t const addr2 = ADDRESS;

That is, define a preprocessor macro ADDRESS which will result in the compile-time constant 0x76 when replaced by the preprocessor and use this preprocessor macro as initializer.

Upvotes: 4

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