Value classes and constructor invariants

The idea

A class can hide representation and expose operations that preserve a rule. A constructor should establish that rule immediately; a member initializer list initializes fields before the constructor body runs. A const member function can observe ordinary members without modifying them through this. These constraints make a small value type easier to use correctly.

When an operation may fail, check the request before changing the object. Copying a class whose members are ordinary value types creates an independent value, but a class containing pointers requires an explicit ownership design. Prefer standard owning members and their established copy behavior instead of manually managing allocations. State the range of arithmetic inputs so preserving a business invariant does not conceal integer overflow.

#include <iostream>
#include <stdexcept>
class Seats {
  int left;
public:
  explicit Seats(int n) : left(n) {
    if (n < 0) throw std::invalid_argument("capacity");
  }
  bool reserve(int n) {
    if (n < 0 || n > left) return false;
    left -= n; return true;
  }
  int remaining() const { return left; }
};
int main() { Seats s(5); s.reserve(7); std::cout << s.remaining(); }

5

The excessive request returns before mutation.