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为了向后兼容“C模板”，C ++中是否存在typename关键字？

[英]Does the typename keyword exist in C++, for backwards compatibility with “C templates?”

原文 2010-02-25 19:53:37 0 7 c++/ c/ templates/ standards

I'm taking a C++ class, and my teacher mentioned in passing that the typename keyword existed in C++ (as opposed to using the class keyword in a template declaration), for backwards compatibility with “C templates.” 我正在学习C ++课程，而我的老师在传递中提到C ++中存在typename关键字（而不是在模板声明中使用class关键字），以便向后兼容“C模板”。

This blew my mind. 这引起了我的注意。 I've never seen or heard tell of anything like C++'s templates (except, perhaps, the preprocessor… and that's not really the same thing at all) in ANSI C. So, did I miss something huge somewhere, or is this a really esoteric extension by gcc or something, or is my teacher way off-base? 我从来没有见过或听说过C ++的模板之类的东西（除了，也许是预处理器......在ANSI C中根本不是同一个东西）。所以，我是否错过了某个地方的巨大东西，或者这是一个非常深奥的东西通过gcc或其他东西扩展，还是我的老师离开基地？

7 个解决方案

I think your teacher is off base. 我认为你的老师不在基地。

See Stan Lippman's post: Why C++ Supports both Class and Typename for Type Parameters for the real reason why C++ supports both. 请参阅Stan Lippman的帖子：为什么C ++支持类型参数的类和类型名称，这是C ++支持这两者的真正原因。

Perhaps the phrase your teacher was aiming for was along the lines of "...for backwards compatibility with C types ", ie, recognizing the problem that template<class T> is misleading when T is a C-style built-in type such as char or int , as others have said. 也许你的老师所针对的短语是“......为了与C 类型向后兼容”，即，当T是C风格的内置类型时，识别template<class T>会产生误导的问题。正如其他人所说的那样，作为char或int 。 But that's not a class! 但那不是一堂课！ :-) :-)

A while back a few GCC folks were suggesting that making the template machinery available to the C compiler would be a good way to implement C99's <tgmath.h> , but that doesn't appear to have come to anything. 不久之前，一些GCC人员建议将模板机制提供给C编译器将是实现C99的<tgmath.h>的好方法，但这似乎没有任何结果。

No, ANSI C does not support templates. 不，ANSI C不支持模板。

http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/topic/com.ibm.xlcpp8a.doc/language/ref/keyword_typename.htm http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/topic/com.ibm.xlcpp8a.doc/language/ref/keyword_typename.htm

Your teacher is making things up. 你的老师正在努力。 There's no such thing as templates in C. The typename keyword exists for two reasons: C中没有模板这样的东西typename关键字的存在有两个原因：

It makes more sense to say template<typename T> than template<class T> since T can be non-class types like int or double . 说template<typename T>比template<class T>更有意义，因为T可以是非类型类型，如int或double 。
It can be used to resolve parsing ambiguities in declarations like A::B * foo; 它可用于解析A::B * foo;等声明中的解析歧义A::B * foo; . 。 Does this declare a variable named foo , or is it a multiplication expression? 这是声明一个名为foo的变量，还是一个乘法表达式？ (Answer: It's parsed as the latter. To make it a declaration write typename A::B *foo; which tells the compiler to interpret A::B as a type name, not a variable name.) （答案：它被解析为后者。要使它成为声明，写入类型名称typename A::B *foo;它告诉编译器将A::B解释为类型名称，而不是变量名称。）

See http://pages.cs.wisc.edu/~driscoll/typename.html for a detailed explanation. 有关详细说明，请参见http://pages.cs.wisc.edu/~driscoll/typename.html 。

No, there's no such thing as a C template. 不，没有C模板这样的东西。 typename isn't even a keyword in C. typename甚至不是C中的关键字。

I wish to say that C really don't have a native template stuff, however, you can make it works fine with some kind of a MetaProgramming , take a look around the web you will find how to... 我想说C真的没有原生模板的东西，但是，你可以通过某种MetaProgramming使其工作正常，看看你会发现如何...

Another important thing to say is that C is a programming language to general purpose, so a lot of things like Object Orientation, template and some other things can be done with a little more effort. 另一个重要的事情是C是一种通用的编程语言，因此可以通过更多努力完成诸如面向对象，模板和其他一些事情。

Projects like Gnome are proof that it can be done and very well. 像Gnome这样的项目证明它可以做得非常好。

PS: Sorry about my terrible english!!! PS：对不起我糟糕的英语！

This doesn't seem right. 这似乎不对。 typename is not a reserved word at all in C. 在C中，typename根本不是保留字。

Perhaps they misspoke/remembered and were thinking of "C with Classes". 也许他们错过了/记住了，并且正在考虑“C with Classes”。