C++ library for Incremental Computing


Anchors is a C++ library for incremental computing based on lord/anchors for rust and janestreet/incremental for OCaml.

Quoting janestreet/incremental, it allows you build large calculations (of the kind you might build into a spreadsheet) that can react efficiently to changing data.

You can view the accompanying blog post for more information.

API Documentation.


An Anchor represents a node in the graph (think a spreadsheet cell) and you can either create an Anchor with a value or define an updater function to create an Anchor from one or more Anchors.

As a basic example, let's define an Anchor whose value is the sum of two other anchors.

#include <anchors/anchorutil.h>
#include <anchors/engine.h>
using namespace anchors;
Engine d_engine; // First set up the anchors engine
AnchorPtr<int> anchorA(Anchors::create(2));
AnchorPtr<int> anchorB(Anchors::create(3));
auto sum = [](int a, int b) { return a + b; }; // Updater function
auto anchorC(Anchors::map2<int>(anchorA, anchorB, sum)); // Note that the function will not be called until you `get` the value of `anchorC`.
Main library namespace.
Definition: anchor.h:32

Anchors follows a demand-driven model and will only (re)compute the value of an Anchor when you observe the Anchor and call get().

EXPECT_EQ(d_engine.get(anchorC), 5);

If you update the value of an Anchor that an observed Anchor depends on, subsequent calls for the observed Anchor will return the updated value.

d_engine.set(anchorA, 10);
EXPECT_EQ(d_engine.get(anchorC), 13);

More Examples

String Concatenation

auto username(Anchors::create(std::string("John")));
auto concatenate = [](const std::string& text) { return "Hello, " + text; };
auto greeting(Anchors::map<std::string>(username, concatenate));
EXPECT_EQ("Hello, John", d_engine.get(greeting));
d_engine.set(username, std::string("Samuel"));
EXPECT_EQ("Hello, Samuel", d_engine.get(greeting));

Using an Input of a Different Type

// Create the different anchors.
auto myOrders = Anchors::create(std::vector<int>{150, 200, 300});
// `maxOrder` and `minOrder` accept a vector and return an integer
AnchorPtr<int> maxOrder = Anchors::map<int, std::vector<int>>(
myOrders, [](const std::vector<int>& v) {
return *std::max_element(v.begin(), v.end());
AnchorPtr<int> minOrder = Anchors::map<int, std::vector<int>>(
myOrders, [](const std::vector<int>& v) {
return *std::min_element(v.begin(), v.end());
// `orderRange` depends on two derived anchors. None of the values will be computed until you call `get`.
AnchorPtr<int> orderRange = Anchors::map2<int>(
maxOrder, minOrder, [](int max, int min) { return max - min; });
// Observe multiple anchors at a time.
// Note that `orderRange` will still return the correct values below if you do not observe `maxOrder` and `minOrder`.
// However, their values will be undefined unless you call `get` for the unobserved values after `orderRange` has been computed.
std::vector<AnchorPtr<int>> anchorsToObserve{
maxOrder, minOrder, orderRange};
// Verify the expected values
EXPECT_EQ(d_engine.get(maxOrder), 300);
EXPECT_EQ(d_engine.get(minOrder), 150);
EXPECT_EQ(d_engine.get(orderRange), 150);
// Update the input list and re-verify
d_engine.set(myOrders, {300, 400, 800});
EXPECT_EQ(d_engine.get(maxOrder), 800);
EXPECT_EQ(d_engine.get(minOrder), 300);
EXPECT_EQ(d_engine.get(orderRange), 500);

Verifying That It Avoids Needless Computations

// Given an Anchor `result`, whose value is derived from (W + X) - Z,
// Anchors will not recompute the sum if only the value of Z changes after we have first computed `result`.
auto anchorW(Anchors::create(10));
auto anchorX(Anchors::create(4));
int additionCounter = 0;
auto anchorY(
Anchors::map2<int>(anchorW, anchorX, [&additionCounter](int a, int b) {
return a + b;
auto anchorZ(Anchors::create(5));
int subtractionCounter = 0;
auto result(
Anchors::map2<int>(anchorY, anchorZ, [&subtractionCounter](int a, int b) {
return a - b;
EXPECT_EQ(d_engine.get(result), 9);
EXPECT_EQ(additionCounter, 1);
EXPECT_EQ(subtractionCounter, 1);
d_engine.set(anchorZ, 7);
EXPECT_EQ(d_engine.get(result), 7);
EXPECT_EQ(additionCounter,1); // It shouldn't recompute anchorY because its value did not change
EXPECT_EQ(subtractionCounter, 2);

A Quadratic Formula Calculator

auto a(Anchors::create(2));
auto b(Anchors::create(-5));
auto c(Anchors::create(-3));
int bsquareCounter = 0;
int fourACCounter = 0;
auto negativeB = Anchors::map<double>(b, [](double b) { return -b; });
auto bSquare = Anchors::map<double>(b, [&bsquareCounter](double b) {
return b * b;
auto fourAC =
Anchors::map2<double>(a, c, [&fourACCounter](double x, double y) {
return 4 * x * y;
auto squareRoot = Anchors::map2<double>(
bSquare, fourAC, [](double x, double y) { return std::sqrt(x - y); });
int denominatorCounter = 0;
auto denominator = Anchors::map<double>(a, [&denominatorCounter](double a) {
return 2 * a;
using FunctionType = std::function<double(double&, double&, double&)>;
FunctionType x1Func = [](double x, double y, double z) {
return (x + y) / z;
FunctionType x2Func = [](double x, double y, double z) {
return (x - y) / z;
auto x1 = Anchors::map3<double>(negativeB, squareRoot, denominator, x1Func);
auto x2 = Anchors::map3<double>(negativeB, squareRoot, denominator, x2Func);
EXPECT_EQ(3, d_engine.get(x1));
EXPECT_EQ(-0.5, d_engine.get(x2));
EXPECT_EQ(1, bsquareCounter);
EXPECT_EQ(1, fourACCounter);
EXPECT_EQ(1, denominatorCounter);
d_engine.set(c, -7);
EXPECT_EQ(3.5, d_engine.get(x1));
EXPECT_EQ(-1, d_engine.get(x2));
// Only the value of C changed, so only anchors
// that depend on C should be recalculated
EXPECT_EQ(1, bsquareCounter);
EXPECT_EQ(2, fourACCounter);
EXPECT_EQ(1, denominatorCounter);


  • When you get an observed node, it will bring any other "stale" observed nodes up to date. An observed node is stale if any of its input has changed since it was last brought up to date.


You can use Anchors from a CMake project by extracting the file and adding the following:

# CMakeLists.txt
list(APPEND CMAKE_PREFIX_PATH <path_to_library_folder)
find_package(Boost REQUIRED) # Anchors requires a Boost installation
find_package(Anchors REQUIRED)
target_link_libraries(${YOUR_TARGET} PRIVATE Anchors::anchors)


This is still a work in progress, and some tasks I intend to work on in the near future are:

  • Implement lord/anchors optimization - scroll to "anchors, a hybrid solution".
  • Implement a setUpdater() function that allows you change the updater function for an Anchor.
  • Cycle Detection.
  • Add support for an Incremental.bind equivalent.
  • Support caching input parameters.
  • Support for map3, map4, etc.
  • More tests.