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This article introduces the relevant knowledge of "C++11 inline namespace example analysis". In the actual case operation process, many people will encounter such difficulties. Next, let Xiaobian lead you to learn how to deal with these situations! I hope you can read carefully and learn something!

Introduction to namespaces

With the expansion of software development scale, the possibility of duplication of class names and function names is increasing. The simplest solution is renaming, which is not a problem when adding code to an existing class library, but not when combining two code libraries that have never met.

C++ solves this problem by introducing namespaces. Suppose there are two namespaces:

Two namespaces, std_namespace1 and lstd_namespace2, are defined in the code. Normal classes with the same name are defined in both namespaces. It doesn't matter whether the member functions of these two classes are the same, so let's just use the same name here.

After having a namespace, you can distinguish between these two Normal classes with the same name by using the method of [namespace name:: class name].

Using the using statement allows the contents of a namespace to be freed as if they existed in the outer namespace.

One drawback of this approach is that duplicate class names can occur, which requires careful control by the programmer.

inline namespace

C++11 introduces inline namespaces, which feature that you can use the contents of the namespace directly in the outer namespace without using the using statement, and without using namespace prefixes. First look at the code:

The declaration method for inline namespaces is to add the inline keyword before the original declaration syntax. In addition, the above code has the following characteristics:

The namespaces of the two declarations have the same name, and they belong to the same namespace. This is a feature that C++ namespaces have always had.

The first time the namespace is declared with the inline keyword, this is called explicit inlining; the second time the inline keyword is not used, but since inline has been declared the first time, the namespace declared here is also inline. This situation becomes implicit inline.

With inline namespaces declared, you can use them directly in outer namespaces without applying prefixes.

In the above code, test_inline_namespace is outside linline_namespace1, so you can use Inline1 and Inline2 directly. test_inline_namespace2 is in the outer layer, so you just need to use the outer namespace inline_test prefix.

It looks like inline_namespace doesn't exist.

serious question

As mentioned earlier, inline namespaces act as if they don't exist, which raises a serious question: What's the point of it? To answer this question, let's take a closer example of actual development. Suppose you have the following class library code:

Simple classes, and of course the code to use them is equally simple:

So far, it's a foreshadowing, and then the point is: Suppose our team has upgraded its library, and at the same time wants:

The user code is not affected unless the user wants to change it.

Free access to new library features

You can still use the original class library if you want

The solution, of course, is to use inline namespaces. The first is to deal with the class library:

Namespaces are defined in the code for each version of the class library, with the latest version defined as an inline namespace. With this preparation, the user code can look like this:

When using the latest class libraries, it is as if the namespace has not been defined; if you really need the original class libraries, you can add the generic version prefix to the class name.

It is also important to note that due to the existence of implicit inline syntax, when ver3 appears in the future, just move the only inline keyword before the first occurrence of ver3 definition!

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