Making a structure in class - c++

Question

I am making my first steps in learning OOP . And here is the first problem which I can't solve.
The max function in this class should return the maximum of two numbers . I want to keep the numbers in the private scope and the functions in the public scope . But when I want to use variables from struct data{} in the public scope the compiler says that the variables are not declared . Please tell me why I get these errors .
class myclass{
private:
struct data{
int q ;
int w;
};
public:
void get(int a, int b){
struct data = {a , b}; // here I want to pass the variables to data struct
}
int max (){ // this function returns the biggest number
if(q>w)
return q;
else
return w;
}
};

Answer 1

struct data{
int q ;
int w;
};
only declares a type, not an object, so there are no q and w members anywhere inside your class instances. You need the declare an instance of the struct:
struct {
int q;
int w;
} data;
Then, you can write max as:
int max()
{
if (data.q > data.w)
return data.q;
else
return data.w;
}
(I've no idea what your get method is supposed to do, so I have no replacement for that.)

Answer 2

In C++ "class" and "struct" are close to being synonymous (the same thing). The ONLY difference is that a "struct" defaults to being "public" accessibility while a "class" defaults to private.
Once you understand this, it should become obvious that what you are doing is defining a sub-type within your class.
class myclass {
private: // <- not required, you already said that by saying "class".
struct data {
// <-- this is a class definition with "public:" just here.
...
};
};
C++ allows you to nest class/structure definitions so that you can, for example, create structures that marshal parameters or return values.
class Database {
class Result { ... };
};
...
class Exam {
class Result { ... };
};
These two result classes avoid namespace collision, by being Database::Result and Exam::Result instead of just "Result".
However - these are only definitions. They do not - as shown - have any effect on the outlying class, that is: they aren't being used to add a member to the class.
Your code:
class myclass{
private:
struct data{ // <-- this is a TYPE declaration, struct myclass::data
int q ; //
int w; //
}; // <-- no member name here so does not affect myclass itself.
public:
void get(int a, int b){
struct data = {a , b}; // here I want to pass the variables to data struct
}
int max (){ // this function returns the biggest number
if(q>w)
return q;
else
return w;
}
};
Declares a type "myclass::data" but does not add a member of type "myclass::data" to the class. The line "struct data = " is illegal, you're trying to assign values to a TYPE.
It should probably be written as
class MyClass {
int m_q;
int m_w;
public:
void set(int q, int w) {
m_q = q;
m_w = w;
}
int max() const {
return (m_q > m_w) ? m_q : m_w;
// or #include <algorithm> and return std::max(m_q, m_w);
}
};
You only need to hoist q & w into a struct if you are going to reuse that structural definition outside the confines of the class, e.g. in derived or parallel classes where you may want to add more of the same type of thing, in which case, you could perhaps do the following, but if you do it this exact way you'll eventually kick yourself for breaking encapsulation:
class MyClass {
public:
struct Data {
int m_q;
int m_w;
};
private:
Data m_data;
void set(int q, int w) {
m_data.m_q = q;
m_data.m_w = w;
}
int max() const {
return (m_data.m_q > m_data.m_w) ? m_data.m_q : m_data.m_w;
}
};
A better way, if this coupling of members needs to be externally visible to some degree would be:
class MyClass {
public:
class Data {
int m_q;
int m_w;
public:
Data() : m_q(0), m_w(0) {}
Data(int q, int w) : m_q(0), m_w(0) {}
void set(int q, int w) {
m_q = w;
m_w = w;
}
int q() const { return m_q; }
int w() const { return m_w; }
int max() const { return (m_q > m_w) ? m_q : m_w;
};
private:
Data m_data;
public:
MyClass() : m_data() {} // or = default
MyClass(int q, int w) : m_data(q, w) {}
MyClass(const Data& data) : m_data(data) {}
// Read-only access
const Data& data() const { return m_data; }
// To allow write access, e.g. for set:
Data& data() { return m_data; }
};
It's kinda overkill for such a simple case, but welcome to C++: the boilerplate language.

Answer 3

You have defined the structure but there is no object of that type. You should declare an object and you will not get any error.
class myclass{
private:
struct data{
int q ;
int w;
}var;
public:
void get(int a, int b){
var .q= a;
var.w=b; // here I want to pass the variables to data struct
}
int max (){ // this function returns the biggest number
if(var.q>var.w)
return var.q;
else
return var.w;
}
};