# ExprTk: a single header C++ engine for runtime math expressions > ExprTk is a header only C++ library that parses and evaluates mathematical expressions at runtime, with a built-in language covering vectors, strings, control flow and user defined functions. It suits C++ codebases that need a configurable formula layer, and it carries the cost of a very large header and a permissive but oddly classified licence. **ArashPartow/exprtk** — C++ Mathematical Expression Parsing And Evaluation Library https://www.partow.net/programming/exprtk/index.html - Repository: https://github.com/ArashPartow/exprtk - Website: https://www.partow.net/programming/exprtk/index.html - Stars: 1,035 · Forks: 325 - Language: C++ - License: NOASSERTION - Published: 2026-10-19 · Updated: 2026-10-19 · Language: en - Canonical page: https://hysenlabs.com/projects/arashpartow-exprtk ## One header carries the whole parser ExprTk is described as a run-time mathematical expression parsing and evaluation engine for C++, and the delivery mechanism is unusually simple. The library is a single header, exprtk.hpp, and the installation section says only that it should be placed in a project or system include path, for example /usr/include/. There is nothing to link against and nothing to build before you use it. What sits behind that header is a full expression language rather than a formula evaluator. The capabilities list names three types, scalar, vector and string, and then covers basic operators, assignment operators such as `:=` and `+=`, equality and inequality comparisons, a set of logic operators including `and`, `mand`, `mor`, `nand`, `nor`, `xnor`, `xor`, `shl` and `shr`, roughly thirty built-in functions, a full trigonometry set, control structures, loop statements and string processing with `in`, `like`, `ilike` and concatenation. Underneath the parser there is an optimizer. The capabilities list names constant folding, simple strength reduction and dead code elimination, plus a runtime check layer covering vector bounds, string bounds, loop iteration counts, execution time bounds, compilation checkpointing and assert statements. The library also does numerical integration and differentiation. So the honest description is a small embedded programming language with a numeric bent, not a spreadsheet formula parser. ## The expression language accepts more notation than most parsers The example expressions section is the most revealing part of the documentation, because it shows how much syntactic sugar the parser tolerates. Alongside the ordinary infix forms, ExprTk accepts implied multiplication, so `2x + 3y + 4z + 5w` is documented as equal to `2 * x + 3 * y + 4 * z + 5 * w`, and `3(x + y) / 2.9 + 1.234e+12` is equal to `3 * (x + y) / 2.9 + 1.234e+12`. Parentheses and square brackets are interchangeable in some positions, with `(x + y)3.3 + 1 / 4.5` documented as equal to `[x + y] * 3.3 + 1 / 4.5`. Curly braces and square brackets are also used in the examples as a grouping notation, for instance `({1/1}*[1/2]+(1/3))` and `sgn(+1.2^3.4z / -5.6y) <= {-7.8^9 / -10.11x }`. Conditionals appear in more than one form. The ternary operator is present, and so is an `if` form with logical keywords, such as `if(((x[2] + 2) == 3) and ((y + 5) <= 9), 1 + w, 2 / z)`. Loops are part of the language as well, with `while (x <= 100) { x -= 1; }` given as an example, alongside for, repeat-until, break and continue. The breadth is a double edged sword. It makes formulas readable to people who write them, and it means the grammar is large enough that you should read the language structure section before assuming a given expression is valid. ## Getting the header in and building the example suite Because ExprTk is header only, using it in a project takes one file copy. The examples in the repository are separate: exprtk_simple_example_01.cpp through exprtk_simple_example_24.cpp, plus exprtk_test.cpp, exprtk_benchmark.cpp and two functional test data files, exprtk_functional_test.txt and exprtk_functional_ext_test.txt. The Makefile in the repository builds the whole set. The documented commands are: ```bash make clean all make clean pgo make strip_bin make valgrind_check ``` A complete build is `make clean all`. There is a separate profile guided optimization target, `make clean pgo`, which is worth knowing about if expression evaluation sits in a hot path, and a `make strip_bin` target for stripping the produced executables. The Makefile compiles with `-pedantic-errors -Wall -Wextra -Werror -Wno-long-long` and `-O2 -DNDEBUG`, and it defines separate sanitizer option sets for address, memory, leak and undefined behaviour. The Makefile also builds sanitizer variants, with `-fsanitize=address`, `-fsanitize=memory`, `-fsanitize=leak` and `-fsanitize=undefined` available as separate option variables, and it defines a `valgrind` target. So the project ships its own checking machinery, which matters if you plan to run the engine on untrusted expression strings. The compiler compatibility section lists builds that are error and warning free under GNU Compiler Collection 3.5 and newer, Clang and LLVM 1.1 and newer, Microsoft Visual Studio C++ 7.1 and newer, Intel C++ 8.x, AMD Optimizing C++ 1.2, Nvidia C++ 19.x, PGI C++ 10.x, IBM XL C/C++ 9.x and Circle C++, plus C++ Builder XE4 and newer. ## Runtime checks and feature toggles decide what an expression can do The runtime check layer is what separates ExprTk from a parser that blindly evaluates text. The capabilities list names vector bounds, string bounds, loop iteration bounds, execution time bounds, compilation process checkpointing and assert statements. That set maps directly onto the failure modes of evaluating a string someone else wrote: an index past the end of a vector, a loop that never terminates, an expression that runs for too long. The documentation also has a dedicated section on enabling and disabling features, which is how you narrow the language for a given deployment. The section on compilation errors covers diagnostics, and the section on expression return values covers the different types a compiled expression can produce. A separate section on unknown unknowns is worth reading before you rely on any of this, since it is where the author discusses the limits of what the documentation and the engine can tell you. For organization, the documentation describes variable, vector and string definition, vector processing, user defined functions, expression dependents, and hierarchies of symbol tables. The symbol table hierarchy is the mechanism behind scoping, and the dependencies section covers how one expression can refer to another. ## The licence question is worth resolving before you ship The licence picture here needs a careful sentence, because the two sources in the repository do not line up on their face. The copyright notice section states that free use of the library is permitted under the guidelines and in accordance with the most current version of the MIT License. It gives the SPDX identifier `SPDX-License-Identifier: MIT` and a file copyright of `SPDX-FileCopyrightText : Copyright (C) 1999-2025 Arash Partow`, and the Makefile header repeats that the free use is permitted in accordance with the most current version of the MIT License, with the same SPDX identifier and the author line `Arash Partow (1999-2025)`. The repository's licence field, on the other hand, is marked as a custom licence that GitHub cannot classify, pointing at the LICENSE file. The repository contains license.txt rather than a standard LICENSE. Both statements can be true at once, since a notice file and a licence file are different documents and MIT requires that the copyright and permission notice travel with the source. But because the metadata does not resolve cleanly, read license.txt yourself and confirm what it grants before you ship a product containing the header. This is a licensing fact to verify, not a legal opinion. On maintenance, the repository is not archived, the last push was on 2026-08-01, and there are no GitHub releases, so version tracking runs through the readme.txt documentation and the release history in the download page, with a mirror repository and an exprtk-extras repository also listed as locations for the most recent version. ## Where ExprTk is the wrong choice Three situations argue against this library. The first is dependency size and build time. A single header that large has to be preprocessed by every translation unit that includes it, and the compile cost lands on every developer and every CI run. Projects with a strict build budget or a large number of targets will feel this. The second is type safety. The language has scalars, vectors and strings, but it is dynamically typed at expression level, so a mistake in a formula string is a runtime error rather than a compile error. The runtime check layer catches many of these, but it catches them at the point of evaluation, inside whatever request or job was running. The third is general purpose scripting. If what you actually need is to let users write small programs, an embedded language with a proper type system and a sandbox designed for it fits the job better than a numeric expression engine with loop and conditional support bolted on. The alternative approach within C++ is to write the parser yourself, which is what the size of the expression grammar in ExprTk tells you about the cost: the language supports implied multiplication, three grouping notations, a dozen logic operators in both symbolic and keyword form, loops, and calculus, and each of those features is a grammar rule to write, test and maintain. A third path is to bind a scripting language already present in your stack, which trades the header size for a runtime dependency and a different integration model. The difference in approach is not performance, it is who owns the grammar: with ExprTk the grammar is a well documented artifact you include, with either alternative it is code in your repository. ## Conclusion ExprTk fits C++ applications that need users or configurations to supply formulas, especially where vectors, conditionals and small custom functions have to work together in one expression. It is the wrong choice for a project that wants to ship a small dependency, since the engine lives in one very large header, and for anything that needs a language with a type system rather than a numeric one. Before adopting, check the licence file yourself, because the copyright notice cites MIT and SPDX-License-Identifier MIT while the repository's licence field is marked as a custom licence that GitHub cannot classify, and confirm that the C++ standard your project targets is covered by the compiler list, which starts at GNU 3.5, Clang 1.1 and Visual Studio 7.1. ## FAQ ### How do I add ExprTk to a C++ project? ExprTk is header only. Place exprtk.hpp in a project or system include path, for example /usr/include/, and include it. The repository's Makefile targets `make clean all` for a complete build of the examples and tests. ### Which C++ compilers does ExprTk support? The documentation lists error and warning free builds with GNU Compiler Collection 3.5+, Clang and LLVM 1.1+, Microsoft Visual Studio C++ 7.1+, Intel C++ 8.x+, AMD Optimizing C++ 1.2+, Nvidia C++ 19.x+, PGI C++ 10.x+, IBM XL C/C++ 9.x+, Circle C++ and C++ Builder XE4+. ### Does ExprTk support user defined functions and vectors? Yes. The documentation lists user defined functions, vector processing, variable, vector and string definition, and hierarchies of symbol tables, with types covering scalar, vector and string. ### What runtime checks does ExprTk perform? The capabilities list names vector bounds, string bounds, loop iteration limits, execution time bounds, compilation process checkpointing and assert statements, and a separate section covers enabling and disabling features. ### What licence does ExprTk use? The copyright notice states use is permitted under the most current version of the MIT License and gives the SPDX identifier MIT, while the repository metadata classifies the licence as a custom one that GitHub cannot classify and points at the LICENSE file, so read license.txt to confirm the terms. ## Sources - [ArashPartow/exprtk on GitHub](https://github.com/ArashPartow/exprtk) - [Issues](https://github.com/ArashPartow/exprtk/issues) - [Project website](https://www.partow.net/programming/exprtk/index.html) - [README](https://github.com/ArashPartow/exprtk/blob/master/README.md) --- Hysen Labs editorial analysis, written from the project's own repository and release notes. Cite the canonical page: https://hysenlabs.com/projects/arashpartow-exprtk