Make sure output of NaN, Infinity and -Infinity is identical on all platforms (#717)

Specifically, printing `.nan`, `.inf`, and `-.inf`, respectively, as per the spec section 10.2.1.4.

include/yaml-cpp/emitter.h

@@ -7,10 +7,13 @@
 #pragma once
 #endif
 
+#include <cmath>
 #include <cstddef>
+#include <limits>
 #include <memory>
 #include <sstream>
 #include <string>
+#include <type_traits>
 
 #include "yaml-cpp/binary.h"
 #include "yaml-cpp/dll.h"
@@ -153,7 +156,28 @@ inline Emitter& Emitter::WriteStreamable(T value) {
 
   std::stringstream stream;
   SetStreamablePrecision<T>(stream);
-  stream << value;
+
+  bool special = false;
+  if (std::is_floating_point<T>::value) {
+    if ((std::numeric_limits<T>::has_quiet_NaN ||
+         std::numeric_limits<T>::has_signaling_NaN) &&
+        std::isnan(value)) {
+      special = true;
+      stream << ".nan";
+    } else if (std::numeric_limits<T>::has_infinity) {
+      if (value == std::numeric_limits<T>::infinity()) {
+        special = true;
+        stream << ".inf";
+      } else if (value == -std::numeric_limits<T>::infinity()) {
+        special = true;
+        stream << "-.inf";
+      }
+    }
+  }
+
+  if (!special) {
+    stream << value;
+  }
   m_stream << stream.str();
 
   StartedScalar();

test/integration/emitter_test.cpp

@@ -253,8 +253,9 @@ TEST_F(EmitterTest, ScalarFormat) {
   out << DoubleQuoted << "explicit double-quoted scalar";
   out << "auto-detected\ndouble-quoted scalar";
   out << "a non-\"auto-detected\" double-quoted scalar";
-  out << Literal << "literal scalar\nthat may span\nmany, many\nlines "
-                    "and have \"whatever\" crazy\tsymbols that we like";
+  out << Literal
+      << "literal scalar\nthat may span\nmany, many\nlines "
+         "and have \"whatever\" crazy\tsymbols that we like";
   out << EndSeq;
 
   ExpectEmit(
@@ -526,9 +527,10 @@ TEST_F(EmitterTest, SimpleComment) {
 
 TEST_F(EmitterTest, MultiLineComment) {
   out << BeginSeq;
-  out << "item 1" << Comment(
-                         "really really long\ncomment that couldn't "
-                         "possibly\nfit on one line");
+  out << "item 1"
+      << Comment(
+             "really really long\ncomment that couldn't "
+             "possibly\nfit on one line");
   out << "item 2";
   out << EndSeq;
 
@@ -984,6 +986,45 @@ TEST_F(EmitterTest, ValueOfBackslash) {
   ExpectEmit("foo: \"\\\\\"");
 }
 
+TEST_F(EmitterTest, Infinity) {
+  out << YAML::BeginMap;
+  out << YAML::Key << "foo" << YAML::Value
+      << std::numeric_limits<float>::infinity();
+  out << YAML::Key << "bar" << YAML::Value
+      << std::numeric_limits<double>::infinity();
+  out << YAML::EndMap;
+
+  ExpectEmit(
+	  "foo: .inf\n"
+	  "bar: .inf");
+}
+
+TEST_F(EmitterTest, NegInfinity) {
+  out << YAML::BeginMap;
+  out << YAML::Key << "foo" << YAML::Value
+      << -std::numeric_limits<float>::infinity();
+  out << YAML::Key << "bar" << YAML::Value
+      << -std::numeric_limits<double>::infinity();
+  out << YAML::EndMap;
+
+  ExpectEmit(
+	  "foo: -.inf\n"
+	  "bar: -.inf");
+}
+
+TEST_F(EmitterTest, NaN) {
+  out << YAML::BeginMap;
+  out << YAML::Key << "foo" << YAML::Value
+      << std::numeric_limits<float>::quiet_NaN();
+  out << YAML::Key << "bar" << YAML::Value
+      << std::numeric_limits<double>::quiet_NaN();
+  out << YAML::EndMap;
+
+  ExpectEmit(
+	  "foo: .nan\n"
+	  "bar: .nan");
+}
+
 class EmitterErrorTest : public ::testing::Test {
  protected:
   void ExpectEmitError(const std::string& expectedError) {
@@ -1034,5 +1075,5 @@ TEST_F(EmitterErrorTest, InvalidAlias) {
 
   ExpectEmitError(ErrorMsg::INVALID_ALIAS);
 }
-}
-}
+}  // namespace
+}  // namespace YAML