C++核心准则E4,5:设计并构建不变量

E.4: Design your error-handling strategy around invariants

E.4:围绕不变量设计错误处理策略

Reason（原因）

To use an object it must be in a valid state (defined formally or informally by an invariant) and to recover from an error every object not destroyed must be in a valid state.

为了使用对象，它一定要处于有效状态（通过不变量形式化或非形式化定义）并且为了从错误中恢复，所有没有销毁的对象必须处于有效状态。

Note（注意）

An invariant is a logical condition for the members of an object that a constructor must establish for the public member functions to assume.

不变量是一个适用于对象成员的逻辑条件，这个条件必须有构造函数建立，可以作为公有成员函数的前提条件。

Enforcement（实施建议）

???

E.5: Let a constructor establish an invariant, and throw if it cannot

E.5:让构造函数建立不变量，如果不能就抛异常

Reason（原因）

Leaving an object without its invariant established is asking for trouble. Not all member functions can be called.

建立一个对象却没有建立不变量是在找麻烦。不是所有成员函数都是可以被调用的。

Example（示例）

class Vector {  // very simplified vector of doubles
    // if elem != nullptr then elem points to sz doubles
public:
    Vector() : elem{nullptr}, sz{0}{}
    Vector(int s) : elem{new double[s]}, sz{s} { /* initialize elements */ }
    ~Vector() { delete [] elem; }
    double& operator[](int s) { return elem[s]; }
    // ...
private:
    owner<double*> elem;
    int sz;
};

The class invariant - here stated as a comment - is established by the constructors. new throws if it cannot allocate the required memory. The operators, notably the subscript operator, relies on the invariant.

See also: If a constructor cannot construct a valid object, throw an exception

类不变量-这里通过注释声明-通过构造函数建立了。如果不能分配要求的内存，new操作会抛出异常。运算符，特别是下标运算符依靠不变量。参见：如果不能构建有效的对象，就抛出异常。

Enforcement（实施建议）

Flag classes with private state without a constructor (public, protected, or private).

标记那些没有构造函数（公有的，私有的或保护的）却有私有成员的类。

原文链接https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#e4-design-your-error-handling-strategy-around-invariants