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.
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1 changed files with 8 additions and 2 deletions
10
src/json.hpp
10
src/json.hpp
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@ -1853,7 +1853,8 @@ class basic_json
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recursively. Note that
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- strings and object keys are escaped using escape_string()
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- numbers are converted to a string before output using std::to_string()
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- integer numbers are converted to a string before output using std::to_string()
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- floating-point numbers are converted to a string using "%g" format
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@param prettyPrint whether the output shall be pretty-printed
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@param indentStep the indent level
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@ -1961,7 +1962,12 @@ class basic_json
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case (value_t::number_float):
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{
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return std::to_string(m_value.number_float);
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// 15 digits of precision allows round-trip IEEE 754 string->double->string
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unsigned int sz = (unsigned int)std::snprintf(nullptr, 0, "%.15g", m_value.number_float);
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std::vector<char> buf(sz + 1);
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std::snprintf(&buf[0], buf.size(), "%.15g", m_value.number_float);
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string_t formatted = buf.data();
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return formatted;
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}
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default:
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