Fixes serialization of small floats

Now uses std::snprintf() to generate a "%.15g" formatted string for JSON values of type number_float. 15 decimals digits are enough to round-trip an IEEE 754 double from string->double->string and get an identical result. std::snprintf is called twice. Once to determine the required buffer size and then again after allocating a buffer of that size. Note that the buffer size *could* be hardcoded for better performance. "%.15g" should result in strings of maximum length 23, plus one character for the terminating null for a buffer size of 24.

Fixes serialization of small floats
7bfcbe28 · Joshua C. Randall · ef3ad895 · 7bfcbe28
Commit 7bfcbe28 authored Feb 28, 2015 by Joshua C. Randall
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json.hpp src/json.hpp +8 -2

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--- a/src/json.hpp
+++ b/src/json.hpp
@@ -1853,7 +1853,8 @@ class basic_json
    recursively. Note that
    - strings and object keys are escaped using escape_string()
-    - numbers are converted to a string before output using std::to_string()
+    - integer numbers are converted to a string before output using std::to_string()
+    - floating-point numbers are converted to a string using "%g" format
    @param prettyPrint    whether the output shall be pretty-printed
    @param indentStep     the indent level
@@ -1961,7 +1962,12 @@ class basic_json
            case (value_t::number_float):
            {
-                return std::to_string(m_value.number_float);
+                // 15 digits of precision allows round-trip IEEE 754 string->double->string
+                unsigned int sz = (unsigned int)std::snprintf(nullptr, 0, "%.15g", m_value.number_float);
+                std::vector<char> buf(sz + 1);
+                std::snprintf(&buf[0], buf.size(), "%.15g", m_value.number_float);
+                string_t formatted = buf.data();
+                return formatted;
            }
            default: