如何将函数指针分配给函数

Question

I have this program and I'm getting:

lvalue required as left operand of assignment

because of the line function_a = function .

int function_a(int j){
    return j+10;
}

int function_b(int j){
    return j;
}

void set_a(int (*function)(int)){
    function_a = function;
}

int main(){
    int a = function_a(2);
    printf("%d, ", a);

    set_a(function_b);

    int b = function_a(2);
    printf("%d", b);
}

I want to set function_a to function_b in function set_a . So I'm expecting output 12, 2 . What I should do to assign this properly?

Answer 1

A function definition cannot be replaced by assignment, ie a function definition is not an lvalue to which you can assign something, and that's why you get the error; But you can define a function pointer and assign different function definitions, and call the function pointer just as if it were a function in the sense of an ordinary function definition:

int function_a(int j){
    return j+10;
}

int function_b(int j){
    return j;
}

int (*f)(int) = function_a;

void set_a(int (*function)(int)){
    f = function;
}


int main(int argc, const char *argv[])
{

    int a = f(2);
    printf("%d, ", a);

    set_a(function_b);

    int b = f(2);
    printf("%d", b);

    return 0;
}