C++ Structs, Classes, and Objects

Separate variables can represent one concept, but the relationship remains informal. A type makes that relationship explicit: a rectangle owns dimensions and provides operations that use them. Callers no longer need to remember which free function accepts which loose values.

Model concepts that have rules, behaviors, or recurring combinations. Do not create a class for every primitive variable; an abstraction should reduce the number of facts callers must coordinate.

#include <iostream>
#include <string>

struct StudySession
{
  std::string topic;
  int minutes{};

  bool is_long() const { return minutes >= 60; }
};

int main()
{
  StudySession session{"C++", 75};
  std::cout << session.topic << ": " << std::boolalpha
            << session.is_long() << '\n';
}

An object is an instance of a type. Its data members hold state, member functions expose behavior, and its lifetime begins after initialization and ends at destruction. Member order and ownership determine how composed resources are created and released.

A const member function can be called on const objects and documents observation. Mark accessors and calculations const when they do not modify the object's logical state.

Members of a struct are public by default; members of a class are private by default. Both can have constructors, methods, inheritance, operators, templates, and destructors. The common convention is a struct for simple aggregates or value-like data and a class for an invariant-protecting interface.

Objects are copied by value unless an interface borrows them or the type disables copying. Standard-library members already know how to copy or move themselves, so a type composed from strings and vectors often needs no custom destructor or copy code.

This is the Rule of Zero: let well-designed members manage resources. Write custom copy, move, or destruction behavior only when the type directly owns a lower-level resource that requires it.