JSON Expression Parser in C++

Overview

This C++20 console application allows evaluating expressions on a JSON file. It first parses the string in the JSON file, creating a JSONObject, and then it parses the command and shows the result. This application was created as a solution to the test task proposed by the 2025 JetBrains Internship Project LLDB Debugger for Windows that was required for application.

Instructions

cmake 3.22 or above is required, as well as gtest for the test executable:

sudo apt-get install cmake
sudo apt-get install libgtest-dev

To compile the console application, in the root directory:

cmake .
make

To run the application (see examples below):

./json_eval <file> "command"

To run tests:

./json_test

Task

Write a C++ console application that allows evaluating expressions on a JSON file.

Consider the following scenario:

###################################################

# test.json is a JSON file with the content:

# {"a": { "b": [ 1, 2, { "c": "test" }, [11, 12] ]}}

# ./json_eval is the console application

###################################################

The application should support:

• Trivial expressions with JSON paths:

$ ./json_eval test.json "a.b[1]"
2
$ ./json_eval test.json "a.b[2].c"
test
$ ./json_eval test.json "a.b"
[ 1, 2, { "c": "test" }, [ 11, 12 ] ]

• Expressions in the subscript operator []:

$ ./json_eval test.json "a.b[a.b[1]].c"
test

• Intrinsic functions: min, max, size:

min, max return the min, max element from the passed array or arguments.

$ ./json_eval test.json "max(a.b[0], a.b[1])"
2
$ ./json_eval test.json "min(a.b[3])"
11

size - returns size of the passed object, array or string

$ ./json_eval test.json "size(a)"
1
$ ./json_eval test.json "size(a.b)"
4
$ ./json_eval test.json "size(a.b[a.b[1]].c)"
4

Number literals:

$ ./json_eval test.json "max(a.b[0], 10, a.b[1], 15)"
15

Feel free to implement any suitable interface for the application.

Requirements:

• The implementation should be as fast as possible.
• Implement reasonable error reporting. For example, if the JSON doesn't have a required field, the application should report an error. If the JSON is invalid, the library should report that as well. Note that the expression can also be invalid.
• Add automated tests.
• Use any modern C++ standard: C++11, C++14, C++17, or C++20.
• You may use any functions from the C++ standard library.
• It is allowed to use a third-party library only for writing unit tests, such as GTest or Catch2, etc. It is forbidden to use third-party libraries for any other purposes, such as parsing JSON. Include a clear README with instructions on how to build the application.

Bonus

Besides the required tasks, this implementation also provides the following:

• More detailed error reporting, letting the user know where the failure took place
• Nested intrinsic function calls
• Intrinsic function calls in array indexing