C++ - Tutorial 006. Structures

C++, struct

Одним из первых шагов в построении новых типов данных является организация данных в структуре, объединяющая несколько различных переменных с разными типами данных. Объявления структуры с помощью ключевого слова struct .

Например, объявим структуру Vector , в котором будет храниться указатель на начало массива элементов типа double и переменная с количеством этих элементов.

struct Vector {
    int sz;       // Number of elements
    double∗ elem; // Pointer to elements
};

A variable of type Vector can be declared in the code as follows:

Vector v;

However, this declaration is not useful in itself, since it is necessary to initialize this structure with some array of elements with a given number of elements. We can do this with the following function.

void vector_init(Vector& v, int s)
{
    v.elem = new double[s]; // Allocating memory for an array of elements
    v.sz = s;
}

In this function, a reference to the Vector object and the number of elements that need to initialize this vector are passed as arguments. Since an object of type Vector is passed as a non-constant object, then we can modify it.

The new operator allocates memory in a so-called free storage (dynamic memory or a simple heap).

The simple use of Vector looks like this:

double read_and_sum(int s)
    // Reading integers from standard input to return their sum, consider s positive
{
    Vector v;
    vector_init(v,s); // Allocate memory for s elements for v
    for (int i=0; i!=s; ++i)
        cin>>v.elem[i]; // Read data into an array of elements
 
    double sum = 0;
    for (int i=0; i!=s; ++i)
        sum+=v.elem[i];  // Summarize all the elements
    return sum;
}

There is still a long way to go before the Vector becomes flexible and elegant, like a vector from a standard library.

To access the structure elements, you can use the dot ( dot . ) If access using a name or link is used, or -> if access is through a pointer. For example:

void f(Vector v, Vector& rv, Vector∗ pv)
{
    int i1 = v.sz; // access through name
    int i2 = rv.sz; // access through reference
    int i4 = pv−>sz; // access through pointer
}