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not_null<T>

CppCoreGuidelines not_null<T>

总结

一、当接口入参中，需要表达not null semantic的时候，可以使用not_null<T>。

这一点，其实就是:

1、CppCoreGuidelines I.12: Declare a pointer that must not be null as not_null

2、CppCoreGuidelines F.23: Use a not_null<T> to indicate that "null" is not a valid value

中表达的含义。

显然，not_null<T>需要实现"comparable to nullptr concept"，关于此的实现，参见:

1、https://github.com/microsoft/GSL/blob/main/include/gsl/pointers

二、一般，在实现move semantic的时候，会将source pointer置为nullptr，这就说明对于resource handle member，是不能够使用not_null<T>，因为它是可以为null的，显然不是not null。

三、当前，gsl对not_null的支持是不够的，它对于如下类型的支持是有限的:

1、not_null<zstring>

2、not_null<span>

参见:

github not_null cannot (and should not) be used with span #399

四、需要注意的是，not_null<T> 的设计是不限于pointer的，所有支持not null semantic的，都可以使用它，比如:

1、pointer

2、smart pointer

3、convertible to null_ptr_t

CppCoreGuidelines I.12: Declare a pointer that must not be null as not_null

CppCoreGuidelines F.23: Use a not_null<T> to indicate that "null" is not a valid value

Reason

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 might 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.

NOTE:

1、有些design-by-contract的思想

CppCoreGuidelines GSL: Guidelines support library # GSL.view: Views

not_null<T> // T is usually a pointer type (e.g., not_null<int*> and not_null<owner<Foo*>>) that must not be nullptr. T can be any type for which ==nullptr is meaningful.

Use not_null<zstring> for C-style strings that cannot be nullptr. ??? Do we need a name for not_null<zstring>? or is its ugliness a feature?

bfilipek How not_null can improve your code?

void Foo(int* pInt); // pInt cannot be null

I should actually use:

not_null<int *> pInt

wanghenshui 使用gsl::not_null封装raw pointer

拓展阅读

NOTE:

1、这一段的内容是有误的，gsl::not_null是不支持move的