Add first complete and working example

Author: mborland

doc/modules/ROOT/pages/basics.adoc

@@ -119,27 +119,19 @@ For more information see: xref:cfenv.adoc[]
 The entire library can be accessed using the convenience header `<boost/decimal.hpp>`.
 A short example of the basic usage:
 
+.This https://github.com/cppalliance/decimal/blob/develop/examples/first_example.cpp[example] shows the very basics on how to make a complete and working program using the decimal library
+====
 [source, c++]
 ----
-#include <boost/decimal.hpp>
-#include <iostream>
-#include <iomanip>
-
-int main()
-{
-    using namespace boost::decimal;
-
-    // Outputs 0.30000000000000004
-    std::cout << std::setprecision(17) << 0.1 + 0.2 << std::endl;
-
-    // Construct the two decimal values
-    constexpr decimal64_t a {1, -1}; // 1e-1 or 0.1
-    constexpr decimal64_t b {2, -1}; // 2e-1 or 0.2
+include::example$first_example.cpp[]
+----
 
-    // Outputs 0.30000000000000000
-    std::cout << a + b << std::endl;
+.Expected Output
+....
+Using doubles:
+0.1 + 0.2 = 0.30000000000000004
 
-    return 0;
-}
-
-----
+Using decimal64_t:
+0.1_DD + 0.2_DD = 0.3000000000000000
+....
+====

examples/first_example.cpp

@@ -10,15 +10,17 @@ int main()
 {
     using namespace boost::decimal::literals; // The literals are in their own namespace like std::literals
 
-    // First we
-    std::cout << std::setprecision(17)
+    // First we show the result of 0.1 + 0.2 using regular doubles
+    std::cout << std::fixed << std::setprecision(17)
+              << "Using doubles:\n"
               << "0.1 + 0.2 = " << 0.1 + 0.2 << "\n\n";
 
     // Construct the two decimal values
     // We construct using the literals defined by the library
     constexpr boost::decimal::decimal64_t a {0.1_DD};
     constexpr boost::decimal::decimal64_t b {0.2_DD};
 
-    std::cout << "Using decimals: "
+    // Now we display the result of the same calculation using decimal64_t
+    std::cout << "Using decimal64_t:\n"
               << "0.1_DD + 0.2_DD = " << a + b << std::endl;
 }