C++ Constructors and Encapsulation

An object should not escape in a half-configured state. A constructor receives required data, validates the contract, and either establishes a valid object or reports failure. This removes the period between default construction and a series of setter calls during which the object violates its own rules.

Require only what truly belongs to the invariant. Optional state can use a meaningful default or an optional representation rather than placeholder strings and magic numbers.

#include <iostream>
#include <stdexcept>

class Temperature
{
public:
  explicit Temperature(double celsius) : celsius_{celsius}
  {
      if (celsius_ < -273.15)
          throw std::invalid_argument{"Below absolute zero"};
  }

  double celsius() const { return celsius_; }

private:
  double celsius_;
};

int main()
{
  Temperature room{22.5};
  std::cout << room.celsius() << '\n';
}

Members are constructed before the constructor body runs. An initializer list supplies their construction arguments directly. Assigning inside the body first default-constructs a member and then replaces it, and it cannot initialize references or const members.

Members initialize in declaration order. Keep the initializer list in that same order to avoid misleading readers and warnings. Use std::move when a by-value parameter intentionally transfers its owned contents into a member.

Private data is useful when unrestricted assignment could violate an invariant. Expose operations named after domain behavior: deposit, try_withdraw, or rename_topic. Each operation validates its own transition and leaves the object valid on every return path.

The compiler can generate default construction, copying, moving, and destruction. Accept those operations only when they preserve the type's meaning. A value-like temperature can be copied; a unique resource owner should move but not copy; an entity may need a deliberate identity rule.

Prefer the Rule of Zero by composing standard types. Use = default to state an appropriate generated operation and = delete to reject an operation that would break the contract.