# 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++");
 
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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";
 
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# 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;
 
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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;
}
 
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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;
}
 
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Output

Could not find "cool"

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