C ++函数指针

Question

using namespace std;

int addition (int a, int b)
{
    return (a+b);
}

int subtraction (int a, int b)
{
    return (a-b);
}
int operation (int x, int y, int (*functocall)(int,int))
{
    int g;
    g = (*functocall)(x,y);
    return(g);
}
int main()
{
    int m,n;
    int (*minus)(int,int) = subtraction;

    m = operation (7,5,addition);
    n = operation (20,m,minus);
    cout << n;
    return 0;
}

Can anybody explain this line for me

int (*minus)(int,int) = subtraction;

Thanks a lot!

Answer 1

int (*minus)(int,int) = subtraction; is creating a variable called minus and assigning it a pointer to the function called subtraction . if the code is valid then the function subtraction would be declared int subtraction(int a, int b); .

the best way to deal with function pointers is to make them readable using typedef.

example:

typedef int (*math_op)(int,int); // new types is math_op

int subtraction (int a, int b)
{
    return (a-b);
}

math_op minus = subtraction;

later on these can be called like they are normal functions.

example:

int result = minus(10, 2); // result is now set to 8

your code rewritten:

using namespace std;

typedef int (*math_op)(int,int); // new types is math_op

int addition (int a, int b)
{
    return (a+b);
}

int subtraction (int a, int b)
{
    return (a-b);
}

int operation (int x, int y, math_op functocall)
{
    int g;
    g = functocall(x,y);
    return(g);
}

int main()
{
    int m,n;
    math_op minus = subtraction;

    m = operation (7,5,addition);
    n = operation (20,m,minus);
    cout << n;
    return 0;
}

Answer 2

int (*minus)(int,int)

says

A pointer to a function taking two ints as arguments returning an int.

The parentheses around (*minus) are there to make sure that the asterisk binds to the name of the typedef and not the return type (ie, the function does not return an int*).

Answer 3

"minus" is a name of a variable which is a pointer to a function taking two int arguments and returning another int. The function called "operation" takes 3 arguments: 2 ints and a pointer to a function which operates on 2 ints and return another one. When invoked, the operation function applies argument 3 to the arguments 1 and 2.