How do I simulate C#'s {get; set;} in C++? - c++

Question

I am experimenting with lambda functions and managed to recreate a "get" functionality in C++. I can get the return value of a function without using parentheses. This is an example class, where I implement this:
using namespace std;
struct Vector2 {
float x;
float y;
float length = [&]()-> float {return sqrt(x * x + y * y); }();
float angle = [&]()-> float {return atan2(y, x); }();
Vector2() : x(0), y(0) {}
Vector2(float a, float b) : x(a), y(b) {}
~Vector2() {}
Vector2(Vector2& other) : x(other.x), y(other.y) {}
Vector2(Vector2&& other) = delete;
void operator =(Vector2&& other) noexcept{
x = other.x;
y = other.y;
}
};
int main()
{
Vector2 vec = Vector2(10, 17);
printf("%f\n%f\n%f\n%f\n", vec.x, vec.y, vec.length, vec.angle);
}
However, I am currently trying to also recreate the "set" functionality that C# has. But I'm failing. I tried to add this:
void angle = [&](float a)->void {
float l = length;
x = cos(a) * l;
y = sin(a) * l;
};
But am getting "Incomplete type is not allowed" error. I'm not sure if that's how it should look, even if I wasn't getting the error. Is it even possible to recreate the "set" functionality C# has in C++?
I know that I can just use a method SetAngle(float a){...}, but that's not really the point.

Answer 1

TL;DR: Don't
What you have isn't a getter, it's just a normal data member that's calculated once when the object is initialized.
In general, C++ doesn't support C#-style properties. The usual C++-style solution is to just use a pair of member functions (and maybe a data member, if you need to save the value separately), i.e.
struct Vector2 {
// ...
float length() const { return sqrt(x * x + y * y); }
void length(float l) {
float angle = angle();
float new_x = l * cos(angle);
float new_y = l * sin(angle);
x = new_x;
y = new_y;
}
// ...
};
You can get something close to a C#-style property, but you'll always run into edge-cases where they don't work perfectly. For example, here's something that will work in many cases:
template <typename T>
class Property
{
private:
std::function<T()> getter_;
std::function<void(const T&)> setter_;
public:
Property(std::function<T()> getter, std::function<void(const T&)> setter)
: getter_{getter},
setter_{setter}
{}
operator T()
{
return getter_();
}
const T& operator=(const T& val)
{
setter_(val);
return val;
}
};
struct Vector2
{
float x;
float y;
Property<float> length{
[this]() { return sqrt(x * x + y * y); },
[this](float l) {
float new_x = l * cos(angle);
float new_y = l * sin(angle);
x = new_x;
y = new_y;
}
}
Property<float> angle{
[this]() { return atan2(y, x); },
[this](float a) {
float l = length;
x = cos(a) * l;
y = sin(a) * l;
}
}
// ...
};
int main() {
Vector2 v;
v.x = 1;
v.y = 1;
v.angle = std::numbers::pi / 2;
std::cout << "(" << v.x << ", " << v.y << ")\n";
}
But this still falls apart in the edge cases, especially when you mix it with templates and/or auto type-deduction. For instance:
Vector2 v;
v.x = 1;
v.y = 1;
auto old_angle = v.angle;
v.angle = std::numbers::pi / 2;
// oops, this prints pi/2, not pi/4 like you probably expected
// because old_angle isn't a float, it's a Property<float> that
// references v
std::cout << old_angle << '\n';
Note also that there's a bug here. Consider this:
int main() {
Vector2 v1;
v1.x = 1;
v1.y = 1;
Vector2 v2 = v1;
v2.angle = std::numbers::pi / 2;
// Oops, assigning to v2.angle modified v1
std::cout << "(" << v1.x << ", " << v1.y << ")\n";
}
You could work around these issues by making Property non-copyable, but then you force any class that uses it to implement a custom copy-constructor. Also, while that would make the auto case "safe", it does so by turning it into a compile error. Still not ideal.

Answer 2

I agree with Miles. This is not the greatest idea, because it's unnatural for C++ developers, and you should write code that is first and foremost easy to read.
However, as an engineering challenge, here's a possible implementation:
#include <math.h>
#include <iostream>
template <typename T>
class Member
{
public:
operator T() const { return _value; }
void operator =(const T& value) const { _value = value; } void operator =(T&& value) { _value = std::move(value); }
private:
T _value;
};
class Angle
{
public:
Angle(const Member<float>& x, const Member<float>& y) :
_x(x), _y(y) {}
operator float() const { return atan2(_y, _x); }
private:
const Member<float>& _x, _y;
};
class Obj
{
public:
Member<float> x, y;
Angle angle;
Obj() : angle(this->x, this->y) {}
};
int main()
{
Obj o;
o.x = 3;
o.y = 5;
std::cout << o.x << ", " << o.y << " -> " << o.angle << std::endl;
}

Answer 3

While other solutions also seem to be possible, this one seems to be the most elegant :P
using namespace std;
struct Vector2 {
float x;
float y;
float init_length = [&]()-> float {return sqrt(x * x + y * y); }();
float init_angle = [&]()-> float {return atan2(y, x); }();
__declspec(property(get = GetAngle, put = SetAngle)) float angle;
__declspec(property(get = GetLength, put = SetLength)) float length;
Vector2() : x(0), y(0) {}
Vector2(float a, float b) : x(a), y(b) {}
~Vector2() {}
Vector2(Vector2& other) : x(other.x), y(other.y) {}
Vector2(Vector2&& other) = delete;
void operator =(Vector2&& other) = delete;
void Display() {
printf("%f\n%f\n%f\n%f\n\n", x, y, length, angle);
}
float GetLength() {
return sqrt(x * x + y * y);
}
float GetAngle() {
return atan2(y, x);
}
void SetLength(float l) {
float a = GetAngle();
x = cos(a) * l;
y = sin(a) * l;
}
void SetAngle(float a) {
float l = GetLength();
x = cos(a) * l;
y = sin(a) * l;
}
};
int main()
{
Vector2 vec = Vector2(10, 17);
vec.Display();
vec.length = 5;
vec.Display();
}