StarMirror/CppCoreGuidelines
1
0
Fork 0
mirror of https://github.com/isocpp/CppCoreGuidelines.git synced 2024-03-22 13:30:58 +08:00
Code Issues Projects Releases Wiki Activity

C.43: rewording and examples suggested in #344

Browse Source
This commit is contained in:
Victor Villas 2015-10-29 20:32:20 -04:00
parent ff7bc575e8
commit ebd3a33d1d
1 changed files with 23 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

28
CppCoreGuidelines.md
View File

@ -3959,27 +3959,45 @@ 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).
* (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
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:
Date();
Date(int dd, int mm, int yyyy);
// ...
};
vector<Date> vd1(1000); // default Date needed here
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.
`{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.
##### 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
* Flag classes without a default constructor