C++核心准则C.61:拷贝操作应该具有拷贝的效果

C.61:拷贝操作应该具有拷贝的效果

That is the generally assumed semantics. After x = y, we should have x == y. After a copy x and y can be independent objects (value semantics, the way non-pointer built-in types and the standard-library types work) or refer to a shared object (pointer semantics, the way pointers work).

这是一个约定俗成的语义。当x=y被执行之后，我们应该也可以认为x==y。拷贝动作之后，x和y可以是独立的两个对象（值语义，象非指针内置类型和标准库类型那样）或者同一个共享对象的不同参照（指针语义，象指针的行为那样）。

class X {   // OK: value semantics
public:
    X();
    X(const X&);     // copy X
    void modify();   // change the value of X
    // ...
    ~X() { delete[] p; }
private:
    T* p;
    int sz;
};

bool operator==(const X& a, const X& b)
{
    return a.sz == b.sz && equal(a.p, a.p + a.sz, b.p, b.p + b.sz);
}

X::X(const X& a)
    :p{new T[a.sz]}, sz{a.sz}
{
    copy(a.p, a.p + sz, p);
}

X x;
X y = x;
if (x != y) throw Bad{};
x.modify();
if (x == y) throw Bad{};   // assume value semantics

class X2 {  // OK: pointer semantics
public:
    X2();
    X2(const X2&) = default; // shallow copy
    ~X2() = default;
    void modify();          // change the pointed-to value
    // ...
private:
    T* p;
    int sz;
};

bool operator==(const X2& a, const X2& b)
{
    return a.sz == b.sz && a.p == b.p;
}

X2 x;
X2 y = x;
if (x != y) throw Bad{};
x.modify();
if (x != y) throw Bad{};  // assume pointer semantics

Prefer value semantics unless you are building a "smart pointer". Value semantics is the simplest to reason about and what the standard-library facilities expect.

除非你在构建某种“智能指针”，否则值语义更好。值语义最容易理解而且也是标准库功能期待的。

(Not enforceable)无

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c61-a-copy-operation-should-copy

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