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Merge pull request #369 from villasv/master

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C.43: rewording and examples suggested in #344
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Gabriel Dos Reis 2015-11-03 02:05:30 -08:00
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@ -3960,27 +3960,45 @@ Another reason is been to delay initialization until an object is needed; the so
* (Simple) Every constructor should initialize every member variable (either explicitly, via a delegating ctor call or via default construction). * (Simple) Every constructor should initialize every member variable (either explicitly, via a delegating ctor call or via default construction).
* (Unknown) If a constructor has an `Ensures` contract, try to see if it holds as a postcondition. * (Unknown) If a constructor has an `Ensures` contract, try to see if it holds as a postcondition.
### <a name="Rc-default0"></a> C.43: Give a class a default constructor ### <a name="Rc-default0"></a> C.43: Ensure that a class has a default constructor
##### Reason ##### Reason
Many language and library facilities rely on default constructors, e.g. `T a[10]` and `std::vector<T> v(10)` default initializes their elements. Many language and library facilities rely on default constructors to initialize their elements, e.g. `T a[10]` and `std::vector<T> v(10)`.
##### Example ##### Example , bad
class Date { class Date { // BAD: no default constructor
public: public:
Date(); Date(int dd, int mm, int yyyy);
// ... // ...
}; };
vector<Date> vd1(1000); // default Date needed here vector<Date> vd1(1000); // default Date needed here
vector<Date> vd2(1000, Date{Month::october, 7, 1885}); // alternative vector<Date> vd2(1000, Date{Month::october, 7, 1885}); // alternative
The default constructor is only auto-generated if there is no user-declared constructor, hence it's impossible to initialize the vector `vd1` in the example above.
There is no "natural" default date (the big bang is too far back in time to be useful for most people), so this example is non-trivial. There is no "natural" default date (the big bang is too far back in time to be useful for most people), so this example is non-trivial.
`{0, 0, 0}` is not a valid date in most calendar systems, so choosing that would be introducing something like floating-point's NaN. `{0, 0, 0}` is not a valid date in most calendar systems, so choosing that would be introducing something like floating-point's NaN.
However, most realistic `Date` classes have a "first date" (e.g. January 1, 1970 is popular), so making that the default is usually trivial. However, most realistic `Date` classes have a "first date" (e.g. January 1, 1970 is popular), so making that the default is usually trivial.
##### Example
class Date {
public:
Date(int dd, int mm, int yyyy);
Date() = default; // See also C.45
// ...
private:
int dd = 1;
int mm = 1;
int yyyy = 1970;
// ...
};
vector<Date> vd1(1000);
##### Enforcement ##### Enforcement
* Flag classes without a default constructor * Flag classes without a default constructor