CODEWITHSUNDEEP
c++cconstants
Michael
Michael

Reputation: 523

C++ Const Pointer Comparison

I have been writing a project in C, due to a third party vendor who writes C++ files with a .C extension I have had to add -c c++ to my build. This is causing minor issues with some of my C function pointers. I am currently getting the error

error: comparison between distinct pointer types ‘void (*)(uint32_t)’ {aka ‘void (*)(unsigned int)’} and ‘const void (*)(uint32_t)’ {aka ‘const void (*)(unsigned int)’} lacks a cast [-fpermissive]

Here is my function.

bool scheduler_remove_task(const void (*TickFct)(uint32_t period_ms)) {
    bool task_removed = false;
    if(TickFct != NULL) {
        uint8_t i;
        for(i=0; i<MAX_TASKS; i++) {
            if(tasks[i].TickFct == TickFct) {  // ERROR on this line
                tasks[i].TickFct = NULL;
                task_removed = true;
                logger_log(&sch_logger, LOG_INFO, "Task %s removed", tasks[i].task_name);
                break;
            }
        }
    }
    return task_removed;
}

I get the pointers are seen as different types but why does the const keyword matter when I am only comparing values? What is different about const in C++?

Is there a way to keep my comparison without removing const from the arguments list?

Upvotes: 1

Views: 213

Answers (1)

Artyer
Artyer

Reputation: 40791

const void (*TickFct)(uint32_t period_ms) declares TickFct as a pointer to a function which returns const void. This is a different function type than void(uint32_t).

You want a const-pointer to a function that returns void: void (*const TickFct)(uint32_t period_ms).

Or that might be clearer with a type alias:

// typedef void(* tick_fc_t)(uint32_t period_ms);  // C-style
using tick_fct_t = void(*)(uint32_t period_ms);

bool scheduler_remove_task(const tick_fc_t TickFct) {  // ...

Upvotes: 3

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