When should C++ define the constructor

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C.40: if the class contains invariants, define the constructor Reason (cause)

This is the purpose of the constructor.

Example (sample) 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 usually a good idea to represent invariants through Ensure in a constructor.

Note (Note)

For convenience, you can define a constructor for a class even if it does not contain invariants.

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 (note)

The initialization list of Clippers 11 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 constructor of Rec2 is redundant. At the same time, the default value of int provided by the member initializer will be better.

Enforcement (implementation recommendations)

If the class contains a user-defined copy operation but does not provide a constructor (a user-defined copy is an obvious sign that the class has an invariant)

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