Is it possible to get private variable value using class instance name? - c++

Question

For example, I've written a class called Length:
class Length {
public:
void setValue(float);
private:
float value_;
};
void
Length::setValue(float newValue) {
value_ = newValue;
}
void print(float value) {
std::cout << value;
}
void computeStuff(float value) {
//do the computing
}
int main() {
Length width;
width.setValue(5);
std::cout << width; // <-- this is actually just an example
//what I actually want is:
print(width); // print 5
//or perhaps even
computeStuff(width);
return 0;
}
Now how to make width return value_ or 5?

Answer 1

Technically, width is not an instance name, it's a name of a variable of type Length. You can change your code to retrieve a variable in two ways:
Add a friend operator << for Length that does the printing, or
Add an implicit conversion operator from Length to float.
The first approach works only for output. You cannot pull the value directly:
friend ostream& operator <<(ostream& out, const Length& len) {
out << len.value_;
return out;
}
The second approach looks like this:
class Length {
...
public:
operator float() const { return value_; }
};

Answer 2

You must overload operator<< for your custom type, something like:
class Length
{
..
friend std::ostream& operator<<(std::ostream& os, const Length& o);
..
}
std::ostream& operator<<(std::ostream& os, const Length& o)
{
os << o.value_;
return os;
}
Mind that this
must be non member
is nothing special , just an operator overload applied to a standard way of inserting things into stream of <iostream>

Answer 3

You need to define an operator() method to print the value 5.

Answer 4

You need to overload the << operator for your class. You could also use a function to do the operator's work.
Operator <<
#include <iostream>
class Length {
friend std::ostream& operator<<(std::ostream& os, const Length& l);
public:
void setValue(float);
private:
float value_;
};
void
Length::setValue(float newValue) {
value_ = newValue;
}
std::ostream& operator<<(std::ostream& os, const Length& l)
{
os << l.value_;
return os;
}
int main() {
Length width;
width.setValue(5);
std::cout << width << std::endl; // print 5
return 0;
}
function:
#include <iostream>
class Length {
friend std::ostream& print(std::ostream &,const Length &l);
public:
void setValue(float);
private:
float value_;
};
void
Length::setValue(float newValue) {
value_ = newValue;
}
std::ostream& print(std::ostream &os, const Length &l)
{
os << l.value_;
return os;
}
int main() {
Length width;
width.setValue(5);
print(std::cout, width) << std::endl;
return 0;
}