Tutorial: Refactor MathFunctions code

Propagate the refactor in Step 10 MathFunctions through all of the
steps. Use MathFunctions/MathFunctions.cxx instead of Tutorial.cxx
to determine which sqrt library is called. Adds .h files which
correspond to their .cxx files by name.

Help/guide/tutorial/Step4/MathFunctions/CMakeLists.txt

@@ -1,9 +1,15 @@
-add_library(MathFunctions mysqrt.cxx)
+add_library(MathFunctions MathFunctions.cxx mysqrt.cxx)
 
 # state that anybody linking to us needs to include the current source dir
 # to find MathFunctions.h, while we don't.
 target_include_directories(MathFunctions
-          INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
-          )
+                           INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
+                           )
+
+# should we use our own math functions
+option(USE_MYMATH "Use tutorial provided math implementation" ON)
+if (USE_MYMATH)
+  target_compile_definitions(MathFunctions PRIVATE "USE_MYMATH")
+endif()
 
 # TODO 3: Link to tutorial_compiler_flags

Help/guide/tutorial/Step4/MathFunctions/MathFunctions.cxx

@@ -0,0 +1,19 @@
+#include "MathFunctions.h"
+
+#include <cmath>
+
+#ifdef USE_MYMATH
+#  include "mysqrt.h"
+#endif
+
+namespace mathfunctions {
+double sqrt(double x)
+{
+// which square root function should we use?
+#ifdef USE_MYMATH
+  return detail::mysqrt(x);
+#else
+  return std::sqrt(x);
+#endif
+}
+}

Help/guide/tutorial/Step4/MathFunctions/MathFunctions.h

@@ -1 +1,5 @@
-double mysqrt(double x);
+#pragma once
+
+namespace mathfunctions {
+double sqrt(double x);
+}

Help/guide/tutorial/Step4/MathFunctions/mysqrt.cxx

@@ -1,7 +1,9 @@
+#include "mysqrt.h"
+
 #include <iostream>
 
-#include "MathFunctions.h"
-
+namespace mathfunctions {
+namespace detail {
 // a hack square root calculation using simple operations
 double mysqrt(double x)
 {
@@ -22,3 +24,5 @@ double mysqrt(double x)
   }
   return result;
 }
+}
+}

Help/guide/tutorial/Step4/MathFunctions/mysqrt.h

@@ -0,0 +1,7 @@
+#pragma once
+
+namespace mathfunctions {
+namespace detail {
+double mysqrt(double x);
+}
+}