assigning function pointer to array of function pointers

Question

In the below code, I am trying to assign function pointer to array of function pointers. I get the error message

error: initializer element is not constant and then a note says near initialization for 'stateTable2[1]'.

In main, I tried to assign function pointer to another function pointer and no issue.

void function1 (void) // Function definition
{

}

void function2 (void) // Function definition
{

}

void (*fptr)(void) = function2;

void (*stateTable2[]) (void) = {function1,fptr};

int main()
{
    void(*fp)(void) = fptr;
    return 0;
}

In main, you are declaring, not assigning. Just try `fptr = function1;` — Mawg says reinstate Monica, Mar 15 '18 at 10:43
In `main` you have an initialization at runtime. As with assignments you don't need constant expression here. For initialization of data objects with static duration you need a constant expression. — Gerhardh, Mar 15 '18 at 10:46
... therefore `void (*stateTable2[]) (void) = {function1,fptr};` -> `void (*stateTable2[]) (void) = {function1, function2};` — Jabberwocky, Mar 15 '18 at 10:47

Vlad from Moscow · Accepted Answer · 2018-03-15T10:55:33.577

You may not initialize an array with the static storage duration with a non-constant object.

Either declare the array like

void (*stateTable2[]) (void) = { function1, function2 };

or move the declaration

void (*stateTable2[]) (void) = {function1,fptr};

inside main making the array as an array with automatic storage duration.

Here is a demonstrative program.

#include <stdio.h>

void function1 (void) // Function definition
{
    printf( "Hello " );
}

void function2 (void) // Function definition
{
    puts( "World!" );
}

void (*fptr)(void) = function2;

int main(void) 
{
    void ( *stateTable2[] ) (void) = { function1, fptr };
    void(*fp)(void) = fptr;

    fp();

    for ( size_t i = 0; i < sizeof( stateTable2 ) / sizeof( *stateTable2 ); i++ )
    {
        stateTable2[i]();
    }

    return 0;
}

Its output is

World!
Hello World!

How come preprocessor can understand the function name but not function pointer? I thought initializing fptr happens during preprocessing (hence it is known during compile time). But it looks this is not the case. — Rajesh, Mar 15 '18 at 16:54

assigning function pointer to array of function pointers

1 Answers1