tools:cplusplus

# C++ for Beginners

This is a very short introduction into a couple of concepts of C++. It should enable a beginner without knowledge in C++ to properly use EEROS in an own simple application.

## Namespaces

With namespaces you can group entities such as classes or enums under a choosen name. In this way the global scope can be split into subscopes with distinguished names. EEROS heavely uses namespaces.
In order to use declarations from these namespaces we can either import such a namespace or use the scope operator '::'. Let's look at an example.

using namespace eeros::logger;
int main(int argc, char **argv) {
StreamLogWriter w(std::cout);
Logger<LogWriter>::setDefaultWriter(&w);
Logger<LogWriter> log;

eeros::control::Constant<std::array<double,4>> c1;

using eeros::control::Gain;
Gain<> g1;
}

With 'using namespace eeros::logger' all the declarations in this namespace are made visible and can be directly used.
The block 'Const' within the namespace 'eeros::control' is used with the aid of the scope operator.
The block 'Gain' within the namespace 'eeros::control' is made available by 'using eeros::control::Gain;' and can be referred to further on without qualifying with the namespace.

## Using Constructors

Constructors - as any other function - can be overloaded. Many blocks you may want to use in your control system can be instantiated in various ways. Let us consider an example with a Gain block.

using namespace eeros::control;
int main(int argc, char **argv) {
Gain<> g1(10);
Gain<> g2;
g2.setGain(10);
}

The first constructor initializes a gain block and sets its gain value to 10. The second instance of the same block is created with a gain value defaulting to 1. This value could then be subsequently set to 10.

## Templates

Function and class templates are widely used within the eeros framework. Templates allow the parametrization of many blocks and signals. This should be demonstrated with the example of a Constant block. Such a block should be able to contain a single constant value of any type. It should also be able to hold a multidimensional value.

using namespace eeros::control;
using namespace eeros::math;
int main(int argc, char **argv) {
Constant<> c1(2.5);  // type double
Constant<int16_t> c2(10);  // type int16_t
Constant<Matrix<2,1>> c3({2,1.5});  // type Matrix
c1.run();
log.info() << c1.getOut().getSignal().getValue();
}

The square brackets - even if left empty - are necessary, because a Constant block requires a signal type for its output. The block is instantiated per default with a value of type double. The second declaration chooses a different type. The third declaration uses a type different from an arithmetic type. The type Matrix is a built-in type which itself can be parametrized with its dimensions.
Do not forget to call the method run. Without this value of the block is not written to its output signal.