模板 (.tpp) 文件包含保护

问题描述

在编写模板化类时，我喜欢将实现移动到不同的文件 (myclass.tpp) 并将其包含在主标题的底部 (myclass.hpp>).

When writing templated classes, I like to move the implementation into a different file (myclass.tpp) and include it at the bottom of the main header (myclass.hpp).

我的问题是:我需要在 .tpp 文件中包含守卫还是将它们包含在 .hpp 文件中就足够了?

My Question is: do I need include guards in the .tpp file or is it sufficient to have them in the .hpp file?

示例代码:

myclass.hpp

#ifndef MYCLASS_HPP
#define MYCLASS_HPP

template<typename T>
class MyClass
{
public:
    T foo(T obj);
};

//include template implemetation
#include "myclass.tpp"

#endif

myclass.tpp

#ifndef MYCLASS_TPP //needed?
#define MYCLASS_TPP //needed?

template<typename T>
T MyClass<T>::foo(T obj)
{
    return obj;
}

#endif //needed?

推荐答案

从不需要包含守卫:它们非常有用、便宜、无破坏性和预期.所以是的，您应该使用标题保护来保护这两个文件:

Include guards are never needed: they're just terribly useful, cheap, non-disruptive and expected. So Yes, you should protect both files with header guards:

• 非常有用:它们允许您声明多个文件的依赖项，而无需跟踪已包含哪些文件.
• 便宜:这只是一些预编译标记.
• 非破坏性:它们非常适合 #include 的大多数用例(我有一个不知道如何编写宏的同事，所以他#included 实现文件 facepalm).
• 预期:开发人员知道它们是什么，但几乎没有注意到它们；相反，缺少包含保护的头文件会唤醒我们并添加到全局 wtf/line 计数器中.

• Terribly useful: they allow you to declare a dependency from multiple files without keeping track of which files have already been included.
• Cheap: this is just some precompilation tokens.
• Non-disruptive: they fit well with most use-cases of #include (I've had a colleague who didn't know how to write macros so he #included implementation files facepalm).
• Expected: developers know what they are and barely notice them; on the contrary a header file missing include guards wakes us up and adds to the global wtf/line counter.

我借此机会强调 StoryTeller 的评论:

I take the opportunity to highlight the comment from StoryTeller:

如果未定义 hpp 防护，我会更进一步并添加一个描述性的 #error 指令.只是为了提供一点保护，包括首先对人的保护.

翻译成:

#ifndef MYCLASS_TPP
#define MYCLASS_TPP

#ifndef MYCLASS_HPP
#error __FILE__ should only be included from myclass.hpp.
#endif // MYCLASS_HPP

template<typename T>
T MyClass<T>::foo(T obj)
{
    return obj;
}

#endif // MYCLASS_TPP

注意:如果翻译单元先#include 然后#include ，则不会触发任何错误，一切正常很好.

Notice: if a translation unit first #include <myclass.hpp> and then #include <myclass.tpp>, no error is fired and everything is fine.

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