C++核心準(zhǔn)則R.37: 不要使用從破損的智能指針?獲取的指針或引用

R.37: Do not pass a pointer or reference obtained from an aliased smart pointer

R.37:?不要使用從破損的智能指針獲取的指針或引用

Reason（原因）

Violating this rule is the number one cause of losing reference counts and finding yourself with a dangling pointer. Functions should prefer to pass raw pointers and references down call chains. At the top of the call tree where you obtain the raw pointer or reference from a smart pointer that keeps the object alive. You need to be sure that the smart pointer cannot inadvertently be reset or reassigned from within the call tree below.

違反本規(guī)則是引用計(jì)數(shù)丟失和發(fā)生懸空指針的第一號理由。函數(shù)更應(yīng)該沿著調(diào)用鏈向下傳遞原始指針和引用。你應(yīng)該在調(diào)用樹的最頂端，從可以保證對象存在的智能指針獲得原始指針或引用。你需要保證智能指針不會在調(diào)用樹的下面被不小心重置或者重新賦值。

Note（注意）

To do this, sometimes you need to take a local copy of a smart pointer, which firmly keeps the object alive for the duration of the function and the call tree.

要做到這點(diǎn)，你需要獲取一個(gè)智能指針的局部拷貝，通過它可以在函數(shù)和調(diào)用樹執(zhí)行期間將對象牢牢地鎖定。

Example（示例）

Consider this code:

考慮以下代碼：

// global (static or heap), or aliased local ...
shared_ptr g_p = ...;

void f(widget& w)
{
    g();
    use(w);  // A
}

void g()
{
    g_p = ...; // oops, if this was the last shared_ptr to that widget, destroys the widget
}

The following should not pass code review:

下面的代碼應(yīng)該無法通過代碼評審：

void my_code()
{
    // BAD: passing pointer or reference obtained from a non-local smart pointer
    //      that could be inadvertently reset somewhere inside f or its callees
    f(*g_p);

    // BAD: same reason, just passing it as a "this" pointer
    g_p->func();
}

The fix is simple -- take a local copy of the pointer to "keep a ref count" for your call tree:

為了改正這個(gè)問題--獲取指針的局部拷貝以便為調(diào)用樹“保持引用計(jì)數(shù)”。

void my_code()
{
    // cheap: 1 increment covers this entire function and all the call trees below us
    auto pin = g_p;

    // GOOD: passing pointer or reference obtained from a local unaliased smart pointer
    f(*pin);

    // GOOD: same reason
    pin->func();
}

Enforcement（實(shí)施建議）

• (Simple) Warn if a pointer or reference obtained from a smart pointer variable (Unique_pointer?or?Shared_pointer) that is non-local, or that is local but potentially aliased, is used in a function call. If the smart pointer is a?Shared_pointer?then suggest taking a local copy of the smart pointer and obtain a pointer or reference from that instead.

• （簡單）如果函數(shù)調(diào)用時(shí)使用了一個(gè)從非局部智能指針變量（Unique_pointer or Shared_pointer）獲取的指針或者引用，報(bào)警。智能指針是局部變量但是可能是別名時(shí)也報(bào)警。如果智能指針是一個(gè)Shared_pointer，建議獲取一個(gè)智能指針的局部拷貝然后從該拷貝獲取指針或引用。
  

原文鏈接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#r37-do-not-pass-a-pointer-or-reference-obtained-from-an-aliased-smart-pointer

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