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This article mainly explains "what is the role of C++ typedef typename", interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Next, let the editor take you to learn "what is the role of C++ typedef typename?"

The typedef typename role of C++

Some of the grammars of C++ are puzzling, including:

Typedef typename std::vector::size_type size_type

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For details, see C++ Primer (Fifth Edition) P584

Some do not understand the grammar sometimes although know what it probably means, ignore it, in fact, but the heart is always diaphragm, let's get to the bottom of it.

Vector::size_type

To understand the above grammar, you must first see the meaning of vector::size_type. Referring to "STL Source Code Analysis", it is not difficult to find, in fact:

Template class vector {public: / /... typedef size_t size_type; / /...}

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This makes it clear that vector::size_type is the nested type definition of vector, which is actually equivalent to the size_t type.

In other words:

Vector::size_type ssize;// is equivalent to size_t ssize.

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Why use the typename keyword

So the question is, why add the typename keyword?

Typedef std::vector::size_type size_type;//why not?

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In fact, the compiler doesn't know what a vector::size_type is until the template type is instantiated. In fact, there are three possibilities:

Static data member

Static member function

Nesting type

Then the role of typename is reflected at this time-the definition is no longer ambiguous.

Summary

So according to the above two analyses

Typedef typename std::vector::size_type size_type

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The true face of the statement is:

Typedef creates aliases for existing types, while typename tells the compiler that std::vector::size_type is a type, not a member.

At this point, I believe you have a deeper understanding of "what is the role of C++ typedef typename". You might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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