Get angle between 2 objects ignoring Y axis

AngleBetween.cpp

#include <cmath>

struct vec3
{
	float x, y, z;
};

float length(vec3 v)
{
	return sqrt(v.x * v.x + v.y * v.y + v.z * v.z);
}

float dot(vec3 a, vec3 b)
{
	return a.x * b.x + a.y * b.y + a.z * b.z;
}

float cross(vec3 a, vec3 b)
{
	vec3 out;
	out.x = a.y * b.z - b.y * a.z;
	out.y = a.z * b.x - b.z * a.x;
	out.z = a.x * b.y - b.x * a.y;
	return out;
}

float getRelAngleBetweenXZ(vec3 a, vec3 b)
{
	a.y = 0.0f;
	b.y = 0.0f;
	float cosTheta = dot(a, b) / length(a) * length(b);
	float rad = acosf(cosTheta);
	vec3 cp = cross(a, b);
	if (cp.y > 0)
	{
		rad *= -1;
	}

	return rad;
}