C.40: Define a constructor if a class has an invariant
C.40:如果类包含不变式，则定义构造函数
Reason(原因)
That's what constructors are for.
这就是构造函数存在的目的。
Example（示例）
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.
在构造函数中通过Ensure表现不变式通常都是一个好主意。
Note（注意）
A constructor can be used for convenience even if a class does not have an invariant. For example:
为了方便起见，即使类不包含不变式也可以为类定义构造函数。
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（注意）
The C++11 initializer list rule eliminates the need for many constructors. For example:
C++11的初始化列表消除了很多构造函数存在的必要性。例如：
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 member initializer.
Rec2的构造函数是多余的。同时成员初始化器提供的int的默认值会做得更好。
See also: construct valid object and constructor throws.
参见：【构建合法对象】和【构造函数抛出异常】
相关链接
成员初始化器：
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rc-in-class-initializer
构建合法对象：
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rc-complete
构造函数抛出异常：
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rc-throw
Enforcement（实施建议）
-
Flag classes with user-defined copy operations but no constructor (a user-defined copy is a good indicator that the class has an invariant)
-
如果类包含用户定义的拷贝操作但是没有提供构造函数（用户定义的拷贝是类具有不变式的明显标志）
原文链接
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c40-define-a-constructor-if-a-class-has-an-invariant
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