Skip to content


using number_integer_t = NumberIntegerType;

The type used to store JSON numbers (integers).

RFC 8259 describes numbers as follows:

The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t and number_float_t are used.

To store integer numbers in C++, a type is defined by the template parameter NumberIntegerType which chooses the type to use.


Default type

With the default values for NumberIntegerType (std::int64_t), the default value for number_integer_t is std::int64_t.

Default behavior

  • The restrictions about leading zeros is not enforced in C++. Instead, leading zeros in integer literals lead to an interpretation as octal number. Internally, the value will be stored as decimal number. For instance, the C++ integer literal 010 will be serialized to 8. During deserialization, leading zeros yield an error.
  • Not-a-number (NaN) values will be serialized to null.


RFC 8259 specifies:

An implementation may set limits on the range and precision of numbers.

When the default type is used, the maximal integer number that can be stored is 9223372036854775807 (INT64_MAX) and the minimal integer number that can be stored is -9223372036854775808 (INT64_MIN). Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization, too large or small integer numbers will be automatically be stored as number_unsigned_t or number_float_t.

RFC 8259 further states:

Note that when such software is used, numbers that are integers and are in the range [-2^{53}+1, 2^{53}-1] are interoperable in the sense that implementations will agree exactly on their numeric values.

As this range is a subrange of the exactly supported range [INT64_MIN, INT64_MAX], this class's integer type is interoperable.


Integer number values are stored directly inside a basic_json type.



The following code shows that number_integer_t is by default, a typedef to std::int64_t.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
    std::cout << std::boolalpha << std::is_same<std::int64_t, json::number_integer_t>::value << std::endl;



Version history

  • Added in version 1.0.0.

Last update: May 1, 2022