Manipulating struct members with functions

[alanthreonus]

Let's say I have the following struct:

struct foo
{
   int a;
   int b;
};

Now let's say I want to manipulate a and b. I could write the following functions:

void barA(foo &baz)
{
   //do stuff to baz.a
}

void barB(foo &baz)
{
   //do stuff to baz.b
}

But what I would like to do is just write one general function that could manipulate either a or b. I'm not sure how to write this function, though. I imagine pointers are involved, but it's been a while since I've used C++, and my skills are rusty. Could someone help me out?

[Zhuge]

Couldn't you just take an int&, and pass foo_instance.a or foo_instance.b?

[Stewbond]

Try using a class instead:

class foo
{
private:
    int a;
    int b;
public:
    //contstructor
    foo(int a_in, int b_in)
    {
        a = a_in;
        b = b_in;
    }
    //getters
    int getA() { return a; }
    int getB() { return b; }

    //setters
    void setA(int a_in)
    {
        //Do stuff to A here
    }
    void setB(int b_in)
    {
        //Do stuff to B here
    }
    void setAB(int a_in, int b_in)
    {
        //Do stuff to a and b here
    }
    void setAorB(int v_in, bool SetA)
    {
        if (Set A) a = v_in;
        else b = v_in;
    }
};

Then in your function you could use:

int x=0, y=0;
foo baz (x,y);

//If you want to set A only:
baz.setA( x );
//If you want to set B only:
baz.setB( y );
//You can also try this to set B only (depending on your code in the set function):
baz.setAB( baz.getA(), y);
//Or you can try inputing one value as shown here and then flag which variable you want to set with a bool:
baz.setAorB ( x , true );

I hope this helps. If we want to be able to manipulate A or B we'll need to tell the struct or class which one we need to modify anyways.

[alanthreonus]

It appears that in my tired state last night, I did not correctly explain what the function should do. The function should be able to handle multiple instances of a struct type and be able to manipulate a specified combination of those structs' members. Ideally I'd like to be able to pass in the names of the members like this:

struct foo
{
   int a;
   int b;
   int c;
   int d;
};

void bar(foo baz1, foo baz2, ??? member1, ??? member2)
{
   //do stuff to baz1.member1

   //do stuff to baz2.member2

   foo baz3;
   //do stuff to baz3.member1
   //do stuff to baz3.member2
}

So the problem is that I don't know what to replace the ???s with or if this is even possible. A variation of Stewbond's setAorB function would probably work, but this way would be cleaner if it's doable.

[hanst99]

Can you please describe with pseudocode what it actually is you are trying to do?

[Stewbond]

How about:

struct foo
{
   int a;
   int b;
   int c;
   int d;
};
int main ()
{
    foo MyStruct1, MyStruct2;
    void bar( &MyStruct1 , &MyStruct2 , member1 , member2 );
}
void bar(foo *baz1, foo *baz2, int member1, int member2)
{
   *baz1.a = member1;
   *baz2.b = member2;

   foo baz3;  //This structure will be destroyed when the '}' is encountered. Only declare local things here.
}

[zb0S216C]

Maybe your could do something similar to this:

void Bar( foo *, int, foo *, int );

int main( )
{
    foo X = { 0, 0 };
    foo Y = { 0, 0 };

    Bar( &X, 0, nullptr, 0 );
}

void Bar( foo *A, int ANew, foo *B, int BNew )
{
    if( A != nullptr )
        A->a = ANew;

    if( B != nullptr )
        B->b = BNew;
}

_Wazzak