C++核心准则编译边学-F.23 使用not_null<T>表明“空”是无效值

F.23: Use a not_null<T> to indicate that "null" is not a valid value（使用not_null<T>表明“空”是无效值）

Reason（原因）

Clarity. A function with a not_null<T> parameter makes it clear that the caller of the function is responsible for any nullptr checks that may be necessary. Similarly, a function with a return value of not_null<T> makes it clear that the caller of the function does not need to check for nullptr.

清晰性。一个使用not_null<T>参数的函数可以明确地表明：如果有必要，调用者有责任进行空指针检查。类似的，返回not_null<T>的函数向调用者清晰的表明了不需要进行nullptr的检查。

Example（示例）

not_null<T*> makes it obvious to a reader (human or machine) that a test for nullptr is not necessary before dereference. Additionally, when debugging, owner<T*> and not_null<T> can be instrumented to check for correctness.

not_null<T*>向读者（人或机器）表明不需要在解引用之前进行nullptr检查。另外，在调试的时候，onwer<T*>和not_null<T>可用于正确性检查。

Consider:(考虑以下代码：)

int length(Record* p);

When I call length(p) should I check if p is nullptr first? Should the implementation of length() check if p is nullptr?

在调用length(p)的时候需要首先检查p是否是nullptr吗？实现length()的时候应该检查p是否为nullptr吗？

// it is the caller's job to make sure p != nullptr
int length(not_null<Record*> p);

// the implementor of length() must assume that p == nullptr is possible
int length(Record* p);

Note（注意）

A not_null<T*> is assumed not to be the nullptr; a T* may be the nullptr; both can be represented in memory as a T* (so no run-time overhead is implied).

not_null<T*>被假定不会是nullptr；T*可能是nullptr；两者在内存上都表现为T*（因此不会付出额外的代价)。

Note（注意）

not_null is not just for built-in pointers. It works for unique_ptr, shared_ptr, and other pointer-like types.

not_null不止适用于内置指针。它也适用于unique_ptr和shared_ptr以及其他类似指针的类型。

Enforcement（实施建议）

• (Simple) Warn if a raw pointer is dereferenced without being tested against nullptr (or equivalent) within a function, suggest it is declared not_null instead. (简单)处于某个函数中的裸指针如果没有进行nullptr（或等价的）检查就解引用，则报警。建议定义为not_null。
• (Simple) Error if a raw pointer is sometimes dereferenced after first being tested against nullptr (or equivalent) within the function and sometimes is not. (简单)如果一个裸指针在解引用之前，有时会进行防空检查有时又不检查，报错。
• (Simple) Warn if a not_null pointer is tested against nullptr within a function. (简单)如果not_null指针在函数内部进行了防空判断，报警。