first attempt

include/task1.h

@@ -0,0 +1,6 @@
+#ifndef TEST01_H
+#define TEST01_H
+
+double CalculateSphereVolume(double radius);
+
+#endif

src/main1.cpp

@@ -0,0 +1,29 @@
+#include "test01.h"
+#define _CRT_SECURE_NO_WARNINGS
+#include <stdio.h>
+
+#define TASK_2
+
+#ifdef TASK_1
+int main()
+{
+	double radius = 0;
+	puts("Enter a radius:");
+	scanf_s("%lf", &radius);
+	printf("Volume of sphere is %.2lf\n",
+		CalculateSphereVolume(radius));
+	return 0;
+}
+#endif
+
+#ifdef TASK_2
+int main()
+{
+	double radius = 0;
+	puts("Enter a radius:");
+	scanf_s("%lf", &radius);
+	printf("Area of circle %.2lf\n",
+		3.14*radius*radius);
+	return 0;
+}
+#endif

src/task1.cpp

@@ -0,0 +1,8 @@
+#include "test01.h"
+#define _USE_MATH_DEFINES
+#include <math.h>
+
+double CalculateSphereVolume(double radius)
+{
+	return (double) 4 / 3 * M_PI * pow(radius, 3);
+}