# A Very Simple C++ Program¶

A short C++ program that uses Cantera is shown below. This program reads in a specification of a gas mixture from an input file, and then builds a new object representing the mixture. It then sets the thermodynamic state and composition of the gas mixture, and prints out a summary of its properties.

#include "cantera/thermo.h"
#include <iostream>

using namespace Cantera;

// The actual code is put into a function that
// can be called from the main program.
void simple_demo()
{
// Create a new phase
std::unique_ptr<ThermoPhase> gas(newPhase("h2o2.cti","ohmech"));

// Set its state by specifying T (500 K) P (2 atm) and the mole
// fractions. Note that the mole fractions do not need to sum to
// 1.0 - they will be normalized internally. Also, the values for
// any unspecified species will be set to zero.
gas->setState_TPX(500.0, 2.0*OneAtm, "H2O:1.0, H2:8.0, AR:1.0");

// Print a summary report of the state of the gas
std::cout << gas->report() << std::endl;
}

// the main program just calls function simple_demo within
// a 'try' block, and catches CanteraError exceptions that
// might be thrown
int main()
{
try {
simple_demo();
} catch (CanteraError& err) {
std::cout << err.what() << std::endl;
}
}


Before you can run this program, it first needs to be compiled. On a Linux system using the GCC compiler, a typical command line for compiling this program might look like this:

g++ -o combustor -pthread -O3 -std=c++0x -I/opt/cantera-2.3.0/include -L/opt/cantera-2.3.0/lib -lcantera -lsundials_cvodes -lsundials_ida -lsundials_nvecserial combustor.cpp


The locations of the Cantera header files (specified by the -I option) and the libraries (specified by the -L option) will vary depending on where you installed Cantera, and the list of libraries (such as sundials_cvodes) will vary depending on what options you used when compiling Cantera. For more advanced and flexible methods of compiling programs which use the Cantera C++ library, see Compiling Cantera C++ Programs.

This program produces the output below:

      temperature             500  K
pressure          202650  Pa
density        0.361163  kg/m^3
mean mol. weight         7.40903  amu

1 kg            1 kmol
-----------      ------------
enthalpy    -2.47725e+06       -1.835e+07     J
internal energy    -3.03836e+06       -2.251e+07     J
entropy         20700.1        1.534e+05     J/K
Gibbs function    -1.28273e+07       -9.504e+07     J
heat capacity c_p         3919.29        2.904e+04     J/K
heat capacity c_v         2797.09        2.072e+04     J/K

X                 Y          Chem. Pot. / RT
-------------     ------------     ------------
H2            0.8         0.217667         -15.6441
H              0                0
O              0                0
O2              0                0
OH              0                0
H2O            0.1         0.243153         -82.9531
HO2              0                0
H2O2              0                0
AR            0.1          0.53918         -20.5027


As C++ programs go, this one is very short. It is the Cantera equivalent of the “Hello, World” program most programming textbooks begin with. But it illustrates some important points in writing Cantera C++ programs.

## Catching CanteraError exceptions¶

The entire body of the program is put inside a function that is invoked within a try block in the main program. In this way, exceptions thrown in the function or in any procedure it calls may be caught. In this program, a catch block is defined for exceptions of type CanteraError. Cantera throws exceptions of this type, so it is always a good idea to catch them.

## The report function¶

The report() function generates a nicely-formatted report of the properties of a phase, including its composition in both mole (X) and mass (Y) units. For each species present, the non-dimensional chemical potential is also printed. This is handy particularly when doing equilibrium calculations. This function is very useful to see at a glance the state of some phase.