Define a constructor if a class has an invariant

Reason

That's what constructors are for.

Example

class Date {  // a Date represents a valid date
              // in the January 1, 1900 to December 31, 2100 range
    Date(int dd, int mm, int yy)
        :d{dd}, m{mm}, y{yy}
    {
        if (!is_valid(d, m, y)) throw Bad_date{};  // enforce invariant
    }
    // ...
private:
    int d, m, y;
};

It is often a good idea to express the invariant as an Ensures on the constructor.

Note

A constructor can be used for convenience even if a class does not have an invariant. For example:

struct Rec {
    string s;
    int i {0};
    Rec(const string& ss) : s{ss} {}
    Rec(int ii) :i{ii} {}
};

Rec r1 {7};
Rec r2 {"Foo bar"};

Note

The C++11 initializer list rule eliminates the need for many constructors. For example:

struct Rec2{
    string s;
    int i;
    Rec2(const string& ss, int ii = 0) :s{ss}, i{ii} {}   // redundant
};

Rec2 r1 {"Foo", 7};
Rec2 r2 {"Bar"};

The Rec2 constructor is redundant. Also, the default for int would be better done as a default member initializer.

See also: construct valid object and constructor throws.

Enforcement

• Flag classes with user-defined copy operations but no constructor (a user-defined copy is a good indicator that the class has an invariant)