A simple, little-overhead, allocation-free, and extensible C++ JSON writer, directly wrapping a std::ostream
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Motivation and design

When writing JSON messages, some C/C++ libraries work by building an in-memory object for the sole purpose of serialising it as JSON. Many libraries comprise multiple header and source files and/or expose a complex API.

minijson_writer is a simple decorator around a std::ostream. It directly writes on the stream without allocating additional memory, and does not throw exceptions unless the stream does.

Despite being a single header file of ~900 LOC, minijson_writer is complete and can be easily extended to support custom types.

minijson_reader is the independent counterpart for parsing JSON messages.


If the compiler does not support C++11, Boost 1.55 or better is required.

Basic usage

Writing a JSON object:

minijson::object_writer writer(stream); // wrap a std::ostream
writer.write("field1", 42); // integral types
writer.write("field2", true); // boolean
writer.write("field3", 42.42); // floating point types
writer.write("field4", "foo"); // char[] and char*
writer.write("field5", std::string("bar")); // std::string
writer.write("field6", minijson::null); // null type (on C++11 you can use nullptr)
writer.close(); // always call close() when you are done

Writing a JSON array:

minijson::array_writer writer(stream); // wrap a std::ostream
writer.write(42); // integral types
writer.write(true); // boolean
writer.write(42.42); // floating point types
writer.write("foo"); // char[] and char*
writer.write(std::string("bar")); // std::string
writer.write(minijson::null); // null type (on C++11 you can use nullptr)
writer.close(); // always call close() when you are done

Nested objects and arrays

Both object_writer andarray_writer have two methods called nested_object() and nested_array() returning another writer that can be used to write a nested object or a nested array, respectively.

minijson::object_writer writer(stream);
writer.write("name", "Los Angeles");
  minijson::object_writer position_writer = writer.nested_object("position");
  position_writer.write("n", 34.05);
  position_writer.write("w", 118.25);
  minijson::array_writer mayors_writer = writer.nested_array("mayors");

Arrays can be also written by using the write_array method, that accepts a range:

const char* mayors[] = { "Villaraigosa", "Garcetti" };
minijson::object_writer writer(stream);
writer.write("name", "Los Angeles");
writer.write_array("mayors", std::begin(mayors), std::end(mayors));

A utility write_array function is provided, that can be used to write top-level JSON arrays:

const char* mayors[] = { "Villaraigosa", "Garcetti" };
minijson::write_array(stream, std::begin(mayors), std::end(mayors));


As a (possibly) neater alternative to nested_object() and nested_array(), you can provide support for custom types by specialising minijson::default_value_writer:

struct position
  double n;
  double w;

namespace minijson

struct default_value_writer<position>
  void operator()(std::ostream& stream, const position& p,
                  writer_configuration configuration) const
    minijson::object_writer writer(stream, configuration);
    writer.write("n", p.n);
    writer.write("w", p.w);

} // namespace minijson

You can now write position instances right away:

const position p = { 34.05, 118.25 };

minijson::object_writer writer(stream);
writer.write("name", "Los Angeles");
writer.write("position", p);

You are encouraged to specialise default_value_writer for your custom types, but please note that specialising it for primitive types or types declared in the std namespace could break your future builds, in case contributors to this library decide to provide more specialisations.

As an alternative to specialising default_value_writer, you can provide a functor when you call write:

enum party

struct party_writer
  void operator()(std::ostream& stream, const party& p, minijson::writer_configuration) const
    const char* s;
    switch (p)
    case REPUBLICANS: s = "republicans"; break;
    case DEMOCRATS:   s = "democrats";   break;
    default:          s = "other";       break;
    minijson::default_value_writer<char*>()(stream, s); // always write quoted strings this way

// ...

minijson::object_writer writer(stream);
writer.write("name", "Los Angeles");
writer.write("governedBy", DEMOCRATS, party_writer());

Similarly, a functor can be provided to write_array (both the method and the standalone function) to determine how each item of the range has to be written.

Deprecation notice: in previous versions of minijson_writer, the third argument of type minijson::writer_configuration was not present. Old code will still compile, but users are encouraged to update their custom writers to the new signature. Failure to do so will break pretty-printing and possibly other upcoming features, and in the future may also break the build.


Pretty-printing can be enabled by passing a second argument of type minijson::writer_configuration to the constructor of object_writer and array_writer.
write_array also accepts an extra argument.

// enable pretty-printing with default settings (use 4 spaces)
minijson::object_writer writer(stream,

// use tabs
minijson::array_writer writer(stream,

// use 2 spaces
minijson::write_array(stream, elements.begin(), elements.end(),

When providing custom writers (either by specialising default_value_writer or by passing a functor), make sure you pass the writer_configuration argument down to the nested object/array writer that is responsible for writing the nested object/array (if any). Failure to do so will break pretty-printing and possibly other upcoming features.



ASCII and UTF-8 strings are supported. The output encoding depends on the encoding of the input strings (field names and values), and no transformations are performed besides escaping control characters.

The stream

  • You can use std::fixed, std::scientific, and std::setprecision on the stream to modify how floating-point numbers are represented
  • All the other format flags may be altered by this library, and never restored to their original value
  • The stream is never flushed, not even when close() is called
  • It is responsibility of the client to check the stream's error state flags


In general, no exceptions are thrown unless the stream does: in that case, basic exception safety is guaranteed. Constructors and destructors do not affect the stream, thus being no-throw.

Copy construction

Copying a writer is allowed (for C++03 compatibility, e.g. when storing writers into STL containers) and is safe, as long as only one copy is then used for writing on the stream.