# Cantera 4.0.0

*Under development*

## Breaking Changes

As a new major version, Cantera 4.0 includes a number of backwards-incompatible changes
which may require updates to user code. These changes are summarized here, along with
some guidance on how to migrate existing code.

### C++: Transition to `std::span` for array arguments

Methods operating on arrays have been modified to take `std::span` arguments instead of
pointers to unsized arrays or references to `std::vector`. This approach provides the
opportunity to ensure array sizes are correct while providing compatibility with
different data containers. For example, `Phase::getMoleFractions(double* x)` has been
replaced with `Phase::getMoleFractions(span<double> x)`.

The following code demonstrates how to use different container types with Cantera's
methods that take `span` arguments:

```{code-block} C++
using namespace Cantera;

auto soln = newSolution("mech.yaml");
auto phase = soln->thermo();
vector<double> x1(phase->nSpecies());
Eigen::ArrayXd x2 = Eigen::ArrayXd::Zero(phase->nSpecies());

// custom user container for multiple states
MyVector x3(4 * phase->nSpecies());
size_t offset = 2 * phase->nSpecies();

// Old call signatures (Cantera 3.2 and older)
phase->getMoleFractions(x1.data());
phase->getMoleFractions(x2.data());
phase->getMoleFractions(&x3[offset]);

// New call signatures (Cantera 4.0 and newer)
phase->getMoleFractions(x1);
phase->getMoleFractions(asSpan(x2)); // Eigen 3.4.0
phase->getMoleFractions(x2); // Eigen 5.0
phase->getMoleFractions(span<const double>(&x3[offset], phase->nSpecies()));
```

For methods that had an extra argument specifying array sizes, this argument has
been removed. For example,
`PlasmaPhase::setElectronEnergyLevels(const double* levels, size_t length)` is now just
`PlasmaPhase::setElectronEnergyLevels(span<const double> levels)`.

Additionally, a number of methods which previously returned references to `std::vector`
have been updated to return `std::span` instead. For example,
`const std::vector<double> Phase::molecularWeights()` has been replaced with
`span<const double> Phase::molecularWeights()`.