====== C++ Basics ====== C++ is an object-oriented language. You can define new data types and their operations and then program with them much as you program with fundamental types such as %%int, char, float,%% etc. For example, you can define %%Matrix%% and %%Vector%% types and then write high-level linear algebra programs, like Matlab scripts. ====== Using classes ====== For example, you could program with typical %%Matrix%% and %%Vector%% types like this <code c++> int M = 13; int N = 11; Matrix A(M,N); // construct a matrix A Vector x(N); // construct a vector x for (int i=0; i<M; ++i) for (int j=0; j<N; ++ j) A(i,j) = drand48(); // assign random numbers to A for (int j=0; j<N; ++j) x(j) = drand48(); // ditto for x Vector y = A*x; // multiply A*x and then set y = A*x // print the results cout << "y's dimension is " << y.dim() << endl; cout << "y's value is " << y << endl; </code> Some C++ vocabulary * a user-defined type is called a **class**; e.g. %%Matrix%% is a class. Classes are roughly like fundamental types. * variables of user-defined types are often called **objects**, e.g. %%A%% is an object of type %%Matrix%%. * objects are initialized or **constructed** by **constructors**, e.g. the statement %%Matrix A(M,N);%% constructs an %%M x N%% Matrix object named %%x%%. The %%(M,N)%% is an argument list for the %%Matrix%% constructor, in this case the row and columns dimensions. Constructors typically allocate memory and assign initial values to the object's internal data structures. * Classes have **member functions** and **operators**. %%A*x%% calls the %%Matrix, Vector%% multiplication operator, and %%y.dim()%% calls the %%dim()%% member function of object %%y%%. * %%cout << y << endl%% prints object %%y%% to standard output followed by a new line. ====== Reading header files ====== Of course, the code above requires the %%Matrix, Vector%% classes and their member functions to be defined elsewhere. Usually classes are declared in **header files** (%%matrix.h%% and %%vector.h%%) and implemented in **source files** (%%matrix.cpp%% and %%vector.cpp%%). The header file declarations define the high-level user interface to the class, and the source file defines the low-level programming that actually makes the classes work. To use a well-written library you will usually not have look at the source files. But you will want to able to read and understand the header files. The declarations of the %%Matrix%% and %%Vector%% classes used above might look like this ((normally the class declarations would go in two files %%matrix.h%% and %%vector.h%% but I'll cram the text together here)) <code c++> class Matrix { public: Matrix(); // default ctor Matrix(int M, int N); // ctor for M x N matrix operator=(const Matrix& A); // assignment double& operator()(int i, int j); // get/set (i,j) element int rows() const; // return # rows int cols() const; // return # cols private: // ... ignore everything labelled private ... }; class Vector { public: Vector(); Vector(int dim); operator=(const Vector& x); // assignment double& operator()(int i); // get/set (i) element int dim() const; // return # rows private: // ...ignore... }; Vector operator*(const Matrix& A, const Vector& x); // Matrix * Vector operator operator ostream& operator<<(ostream& os, const Matrix& A); // Matrix print operator operator ostream& operator<<(ostream& os, const Vector& x); // Vector print operator </code> Each line in the class declaration declares an object or a function that can be used in programming with the class. how a declaration translate to usage is tricky. The following table will help you get started. Remember, "declaration" means how the function declaration appears in a header file, and "usage" means how it is used in high-level programming. ^Declaration^Usage^Meaning^ |%%Matrix()%% | %%Matrix A;%% | construct a 0x0 Matrix named A | |%%Matrix(int M, int N)%% | %%Matrix A(3,4);%% | construct a 3x4 Matrix named A | |%%operator=(const Matrix& A)%% | %%A = B;%% | assign Matrix B to Matrix A | |%%double& operator()(int i, int j)%% | %%A(i,j) = 0.34;%% | assign the i,j element of A... | | | %%double Aij = A(i,j)%% | ...or get the value of the i,j element | |%% rows()%% | %%int M = A.rows();%% | get the number of rows in A | |%%Vector operator*(const Matrix& A, const Vector& x);%% | %%Vector y = A*x;%% | multiply %%A%% times %%x%% | This barely scratches the surface of C++ programming but it's enough to get you started programming with channelflow. Please refer to C++ books and online documentation for more information.
