calculator

calculator.hpp is a minimal, header-only C++ library for parsing and evaluating integer arithmetic expressions, e.g. "10 * (7 - 1)". It is designed for simplicity, it supports all C++ integer types (e.g. int8_t, unsigned int, long, __int128_t), but does not support functions, comparison operators, or user variables. It compiles with any C++ compiler that supports C++11 or later.

calculator is a simple but fast operator-precedence parser.

In version 2.0 (2025), error handling has been significantly improved. The library now detects virtually all integer overflows and underflows, helping prevent incorrect results. An extensive test suite further ensures the correctness and robustness of the implementation. This calculator is being used in production by the primesieve and primecount C/C++ libraries.

Supported operators

calculator.hpp uses the same operator precedence and associativity as the C++ programming language and also supports the power operator and basic scientific notation for integers.

<table> <tr align="center"> <td><b>Operator</b></td> <td><b>Description</b></td> </tr> <tr align="left"> <td>|</td> <td>Bitwise Inclusive OR</td> </tr> <tr align="left"> <td>&</td> <td>Bitwise AND</td> </tr> <tr align="left"> <td>~</td> <td>Unary Complement </td> </tr> <tr align="left"> <td>&lt;&lt;</td> <td>Shift Left</td> </tr> <tr align="left"> <td>&gt;&gt;</td> <td>Shift Right</td> </tr> <tr align="left"> <td>+</td> <td>Addition</td> </tr> <tr align="left"> <td>-</td> <td>Subtraction</td> </tr> <tr align="left"> <td>*</td> <td>Multiplication</td> </tr> <tr align="left"> <td>/</td> <td>Division</td> </tr> <tr align="left"> <td>%</td> <td>Modulo</td> </tr> <tr align="left"> <td>^, **</td> <td>Raise to power</td> </tr> <tr align="left"> <td>e, E</td> <td>Integer Scientific Notation</td> </tr> </table>

C++ API

Main functions defined in calculator.hpp:

// Default eval() returns signed 64-bit
std::int64_t calculator::eval(const std::string& expression);

// Any integer type, e.g.: int8_t, unsigned short, __int128_t, ...
template <typename T>
T calculator::eval<T>(const std::string& expression);

How to use it

calculator::eval("1+2") takes a string with an integer arithmetic expression as an argument, evaluates the arithmetic expression and returns the result. If the expression string is not a valid integer arithmetic expression a calculator::error exception is thrown.

#include "calculator.hpp"
#include <cassert>
#include <iostream>

int main()
{
    try
    {
        // Default eval() uses int64_t
        std::int64_t result = calculator::eval("(0 + (255 & 1000)*3) / -2");
        std::cout << result << std::endl;

        // GCC/Clang 128-bit integers
        __int128_t res = calculator::eval<__int128_t>("2^100");
        assert(res == __int128_t(1) << 100);
    }
    catch (calculator::error& e)
    {
        std::cerr << e.what() << std::endl;
    }

    return 0;
}

Scientific notation

calculator.hpp also supports basic scientific notation for integers (without the dot syntax).

std::int64_t res = calculator::eval("1e5");
assert(res == 100000);

std::int64_t res = calculator::eval("2e5");
assert(res == 200000);