Let a constructor establish an invariant, and throw if it cannot

Reason

Leaving an object without its invariant established is asking for trouble. Not all member functions can be called.

Example

class Vector {  // very simplified vector of doubles
    // if elem != nullptr then elem points to sz doubles
public:
    Vector() : elem{nullptr}, sz{0}{}
    Vector(int s) : elem{new double[s]}, sz{s} { /* initialize elements */ }
    ~Vector() { delete [] elem; }
    double& operator[](int s) { return elem[s]; }
    // ...
private:
    owner<double*> elem;
    int sz;
};

The class invariant - here stated as a comment - is established by the constructors. new throws if it cannot allocate the required memory. The operators, notably the subscript operator, rely on the invariant.

See also: If a constructor cannot construct a valid object, throw an exception

Enforcement

Flag classes with private state without a constructor (public, protected, or private).