Using Objects

Warning! This chapter is in a very early draft state!

Before we can use objects we need to create them based on a class. The standard library has some classes that we can instantiate in our applications.

Creating Objects

One of the most used classes from the std library is the string class.

Let us create an instance of the string class.

// Creating an object with a default constructor
std::string empty;

// Creating an object with an initialization constructor
std::string hello("Hello from C++");

Note the difference between object instantiation with a default constructor and instantiation with an initialization constructor.

In C++ we do not add parentheses () when using the default constructor. If you do, C++ thinks you are calling a function.

string vs String

Note that normally we will define class names using capital letters. This is a common practice in object oriented programming. Too bad C++ did not follow this convention. This results in std classes that start with lower case letters

The string class is a bit special as it also allows you to create a string object using the following syntax:

std::string myName = "Nico De Witte";

Calling Methods

Once an object is created, one can start calling methods on the object using the . operator.

Let's take a look at an example where we call the length() method on an std::string object.

std::string hello("Hello from C++");

cout << "Length of \"" << hello
  << "\" is " << hello.length() << endl;

Output

Length of "Hello from C++" is 14

The std::string class defines a lot of useful methods we can call on std::string objects. Make sure to take a look at them at https::/cplusplus.com/reference/string/string (opens new window).

Passing Arguments

Some methods will require you to pass external information to the method so it can do its work.

Passing data to the method is achieved by passing a comma separated list of arguments.

Let's take the find() method of std::string as an example. It searches the string for a given substring:

std::string text = "Hello from the cool world of C++";
std::string cool = "cool";

std::size_t positionCool = text.find(cool);
if (positionCool != std::string::npos) {
  cout << "Found \"" << cool << "\" at position " << positionCool << endl;
} else {
  cout << "Could not find \"" << cool << "\"" << endl;
}

As a second argument we an also supply the index of where to start searching from in the string.

std::string text = "Hello from the cool world of C++";
std::string cool = "cool";

std::size_t positionCool = text.find(cool, 20);
if (positionCool != std::string::npos) {
  cout << "Found \"" << cool << "\" at position " << positionCool << endl;
} else {
  cout << "Could not find \"" << cool << "\"" << endl;
}

Output

Could not find "cool"