Access struct property by variable value - c++

Question

I've got a struct "margin" in my class with 4 properties. Instead of writing four different getter/setter methods, I figured I could do it in a better way:
class myClass {
private:
struct margin {
int bottom;
int left;
int right;
int top;
}
public:
struct getMargin();
void setMargin(string which, int value);
};
But how can I set the property of the struct corresponding with the string "which" from within the function setMargin()? For example, if I call myClass::setMargin("left", 3), how can I then set "margin.left" to "3"? Preferably while keeping the struct POD? I really can't figure this out...
And on a side note, is this really better than writing many getter/setter methods?
Thanks!

Answer 1

First, your idea is terrible... :)
Note you don't even have a margin member (added below)
I'd use an enum for this, if you don't want setters/getters for each property:
class myClass {
private:
struct margin {
int bottom;
int left;
int right;
int top;
} m; // <--- note member variable
public:
enum Side
{
bottom, left, rigth, top
};
struct getMargin();
void setMargin(Side which, int value);
};
and have a switch statement inside setMargin.
void myClass::setMargin(Side which, int value)
{
switch (which)
{
case bottom:
m.bottom = value;
break;
//....
}
}

Answer 2

class myClass {
private:
int margin[4];
public:
enum Side
{
bottom, left, rigth, top
};
void setMargin(Side which, int value);
};
void myClass::setMargin(Side which, int value)
{
margin[which]=value;
}

Answer 3

Either Luchian's or Gir's suggestion would be my preference. If you really want need a look up by a string, though, it is probably best to do that with an associative container.
class MyClass {
std::map<std::string, int> m_;
public:
bool setMargin(std::string which, int value) {
std::map<std::string, int>::iterator i = m_.find(which);
if (i == m_.end()) return false;
i->second = value;
return true;
}
};
This is only useful if you have a dynamic interface that allows your user to define their own margins by name.

Answer 4

If you can make margin public, you can get rid of get,set methods:
class myClass {
public:
struct Margin {
int bottom;
int left;
int right;
int top;
};
Margin margin;
};
myClass mc;
mc.margin.bottom = mc.margin.left;

Answer 5

You can use "relative pointer" which points the distance from the address of struct to point a special element. For example:
SetMargin(FIRST,5);
Which FIRST is from an enum value and is 0.
SetMargin(SECOND,100);
SECOND is 4 since int is 4 bytes in my system
implementation:
void SetMargin(enum margin_elements,int a)
{
int *relative=struct_pointer+(int*)margin_elements;
*relative_pointer=a;
return;
}