Merge pull request #1286 from cppalliance/integral_conversion

Author: mborland

doc/modules/ROOT/nav.adoc

@@ -4,7 +4,8 @@
 ** xref:examples.adoc#examples_construction[Basic Construction]
 ** xref:examples.adoc#examples_basic_math[Basic Arithmetic]
 ** xref:examples.adoc#examples_conversions[Conversions]
-*** xref:examples.adoc#examples_binary_floating_conversions[Binary Floating Point]
+*** xref:examples.adoc#examples_integer_conversions[Integer Conversions]
+*** xref:examples.adoc#examples_binary_floating_conversions[Binary Floating Point Conversions]
 ** xref:examples.adoc#examples_promotion[Promotion and Mixed Decimal Arithmetic]
 ** xref:examples.adoc#examples_charconv[`<charconv>`]
 ** xref:examples.adoc#examples_generic_programming[Generic Programming]

doc/modules/ROOT/pages/examples.adoc

@@ -72,10 +72,42 @@ Decimal Result: 100
 [#examples_conversions]
 == Conversions
 
+[#examples_integer_conversions]
+=== Integral Conversions
+
+.This https://github.com/cppalliance/decimal/blob/develop/examples/integral_conversions.cpp[example] shows how to construct an integral value from decimal type and vice versa, as well as the behavior
+
+====
+[source, c++]
+----
+include::example$integral_conversions.cpp[]
+----
+
+.Expected Output
+....
+Decimal QNAN converts to Integer Max
+Decimal INF converts to Integer Max
+  decimal64_t pi: 3.141592653589793
+std::uint32_t pi: 3
+
+Conversions will be lossless
+  decimal64_t digits10: 16
+std::uint32_t digits10: 9
+   std::uint32_t max: 4294967295
+decimal64_t from max: 4294967295
+
+Conversions will be lossy
+  decimal32_t digits10: 7
+std::uint64_t digits10: 19
+   std::uint64_t max: 18446744073709551615
+decimal32_t from max: 1.844674e+19
+....
+====
+
 [#examples_binary_floating_conversions]
 === Binary Floating Point Conversions
 
-.This https://github.com/cppalliance/decimal/blob/develop/examples/binary_float_conversions.cpp[example] shows show to construct a binary floating point value from decimal type and vice versa
+.This https://github.com/cppalliance/decimal/blob/develop/examples/binary_float_conversions.cpp[example] shows how to construct a binary floating point value from decimal type and vice versa, as well as the behavior
 ====
 [source,c++]
 ----

examples/integral_conversions.cpp

@@ -0,0 +1,78 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+//
+// This file demonstrates how to convert various types to decimal types and back,
+// along with edge case handling
+
+#include <boost/decimal/decimal32_t.hpp>        // For type decimal32_t
+#include <boost/decimal/decimal64_t.hpp>        // For type decimal64_t
+#include <boost/decimal/cmath.hpp>              // For decimal support of cmath functions
+#include <boost/decimal/iostream.hpp>           // For decimal support of <iostream> and <iomanip>
+#include <boost/decimal/numbers.hpp>            // For decimal support of <numbers>
+#include <iostream>
+#include <limits>
+#include <cstdint>
+
+int main()
+{
+    using boost::decimal::decimal32_t;      // Type decimal32_t
+    using boost::decimal::decimal64_t;      // Type decimal64_t
+
+    // Non-finite values construct std::numeric_limits<TargetIntegerType>::max()
+    constexpr decimal64_t decimal_qnan {std::numeric_limits<decimal64_t>::quiet_NaN()};
+    const std::uint32_t int_from_nan {static_cast<std::uint32_t>(decimal_qnan)};
+
+    // Note here that we must use boost::decimal::isnan for decimal types,
+    // as it is illegal to overload std::isnan
+    if (boost::decimal::isnan(decimal_qnan) && int_from_nan == std::numeric_limits<std::uint32_t>::max())
+    {
+        std::cout << "Decimal QNAN converts to Integer Max\n";
+    }
+
+    // Same thing happens with decimal infinities since integers don't have an infinity
+    constexpr decimal32_t decimal_inf {std::numeric_limits<decimal32_t>::infinity()};
+    const std::uint64_t int_from_inf {static_cast<std::uint64_t>(decimal_inf)};
+
+    // Same as the above but with INF instead of NAN
+    if (boost::decimal::isinf(decimal_inf) && int_from_inf == std::numeric_limits<std::uint64_t>::max())
+    {
+        std::cout << "Decimal INF converts to Integer Max\n";
+    }
+
+    // For finite values the construction of the resulting integer matches the behavior
+    // you are familiar with from binary floating point to integer conversions.
+    // Namely, the result will only have the integer component of the decimal
+
+    // Construct the decimal64_t version of pi using our pre-computed constants from <boost/decimal/numbers.hpp>
+    constexpr decimal64_t decimal_pi {boost::decimal::numbers::pi_v<decimal64_t>};
+    const std::uint32_t int_from_pi {static_cast<std::uint32_t>(decimal_pi)};
+
+    std::cout << std::setprecision(std::numeric_limits<decimal64_t>::digits10)
+              << "  decimal64_t pi: " << decimal_pi << '\n'
+              << "std::uint32_t pi: " << int_from_pi << "\n\n";
+
+    // Constructing a decimal value from an integer is lossless until
+    // the number of digits in the integer exceeds the precision of the decimal type
+
+    std::cout << "Conversions will be lossless\n"
+              << "  decimal64_t digits10: " << std::numeric_limits<decimal64_t>::digits10 << "\n"
+              << "std::uint32_t digits10: " << std::numeric_limits<std::uint32_t>::digits10 << "\n";
+
+    constexpr decimal64_t decimal_from_u32_max {std::numeric_limits<std::uint32_t>::max()};
+    std::cout << "   std::uint32_t max: " << std::numeric_limits<std::uint32_t>::max() << "\n"
+              << "decimal64_t from max: " << decimal_from_u32_max << "\n\n";
+
+    // In the construction of lossy values the rounding will be handled according to
+    // the current global rounding mode.
+
+    std::cout << "Conversions will be lossy\n"
+          << "  decimal32_t digits10: " << std::numeric_limits<decimal32_t>::digits10 << "\n"
+          << "std::uint64_t digits10: " << std::numeric_limits<std::uint64_t>::digits10 << "\n";
+
+    constexpr decimal32_t decimal_from_u64_max {std::numeric_limits<std::uint64_t>::max()};
+    std::cout << "   std::uint64_t max: " << std::numeric_limits<std::uint64_t>::max() << "\n"
+              << "decimal32_t from max: " << decimal_from_u64_max << '\n';
+
+    return 0;
+}

test/Jamfile

@@ -216,6 +216,7 @@ run ../examples/to_from_file.cpp ;
 run ../examples/addition.cpp ;
 run ../examples/debugger.cpp ;
 run ../examples/binary_float_conversions.cpp ;
+run ../examples/integral_conversions.cpp ;
 
 # Test compilation of separate headers
 compile compile_tests/bid_conversion.cpp ;