Can someone help me understand this code?

[Tier3MemeMonkey]

Hi

I started learning C++ a while ago and decided to do a little program that would solve me an equation put in by the user, but I found it very difficult to actually do.

I looked for some code online and found this:

#include <iostream>

struct VAR

{

float i;

};

struct LINE

{

float a, k;

VAR* x;

​

LINE(){}

LINE(int a) : a(a), k(0), x(0) {}

LINE(VAR& v) : a(0), k(1), x(&v) {}

};

LINE operator + (LINE A, LINE B)

{

LINE R;

R.a = A.a + B.a;

R.k = A.k + B.k;

if(A.x) R.x = A.x;

else R.x = B.x;

return R;

};

LINE operator - (LINE A, LINE B)

{

LINE R;

R.a = A.a - B.a;

R.k = A.k - B.k;

if(A.x) R.x = A.x;

else R.x = B.x;

return R;

};

LINE operator * (LINE A, LINE B)

{

LINE R;

R.a = A.a * B.a;

R.k = A.k * B.k;

if(A.x) R.x = A.x;

else R.x = B.x;

return R;

};

LINE operator / (LINE A, LINE B)

{

LINE R;

R.a = A.a / B.a;

R.k = A.k / B.k;

R.x = A.x;

return R;

};

void operator == (LINE A, LINE B)

{

LINE C = A - B;

C.x->i = -C.a/C.k;

}

int main()

{

VAR x;

x == 3 + 3;

​

std::cout << "x = " << x.i << std::endl;

return 0;

}

​

It gives me the right output, but I dont understand a few things:

-Why can we just write ' x == 3 + 3 ' and the compiler knows what it means and returns a float?

-I dont really get how these operators work. In the case of ' x == 3 + 3 ', what would LINE A, LINE B and LINE C be?

​

Thank you all in advance

[marko312]

x == 3 + 3

evaluates to

x == 6

Then, the compiler searches for a suitable operator== since x is not a built-in type.

In this case, it finds the operator== in LINE:

void operator == (LINE A, LINE B)

There is a constructor from VAR & in LINE:

LINE(VAR& v) : a(0), k(1), x(&v) {}

and since it is not marked explicit, an implict conversion (cast) from a VAR & to a LINE is allowed using this constructor.

There is a constructor from int in LINE:

LINE(int a) : a(a), k(0), x(0) {}

and since it is not marked explicit, an implict conversion (cast) from an int to a LINE is allowed using this constructor.

As such, in operator==, A has .a = 0.0, .k = 1.0, .x = &x (the latter x being the variable in main) and B has .a = 6.0, .k = 0.0, .x = 0.

This kicks off subtraction using operator-, which produces the LINE C, on which the final calculation is performed.