問題描述
如果我在包含兩個不同翻譯單元的頭文件中定義一個函數(shù)(可能是一個類成員函數(shù)但未內(nèi)聯(lián))，我會收到鏈接錯誤，因為該函數(shù)是多重定義的.模板并非如此，因為在編譯器解析模板化類型的對象聲明之前，它們不是可編譯的類型.這讓我意識到我不知道編譯的模板代碼在哪里以及它是如何鏈接的，因為 C++ 不只是創(chuàng)建代碼的多個副本來定義 SomeTemplateClass.任何信息，將不勝感激.謝謝！
If I define a function (maybe a class member function but not inlined) in a header file that is included by two different translation units I get a link error since that function is multiply defined. Not so with templates since they are not compilable types until the compiler resolves a declaration of an object of a templatized type. This made me realize I don't know where compiled template code resides and how it is linked since C++ does not just create multiple copies of code to define SomeTemplateClass. Any info would be appreciated. Thanks!
推薦答案
C++編譯器使用了3種實現(xiàn)方案:
There are 3 implementation schemes used by C++ compilers:
貪婪實例化，編譯器在每個使用它的編譯單元中生成一個實例化，然后鏈接器丟棄除其中一個之外的所有實例化(這不僅僅是代碼大小優(yōu)化，它是必需的，以便函數(shù)地址、
static變量等都是唯一的).這是最常見的模型.
greedy instantiation, where the compiler generates an instantiation in each compilation unit that uses it, then the linker throws away all but one of them (this is not just a code-size optimization, it's required so that function addresses,
staticvariables, and the like are unique). This is the most common model.
查詢實例化，其中編譯器有一個已經(jīng)完成的實例化數(shù)據(jù)庫.當(dāng)需要實例化時，會檢查并更新數(shù)據(jù)庫.我知道的唯一使用它的編譯器是 Sun 的，默認(rèn)情況下不再使用它.
queried instantiation, where the compiler has a database of instantiations already done. When an instantiation is needed, the DB is checked and updated. The only compiler I know which uses this is Sun's, and it isn't used by default anymore.
迭代實例化，其中實例化由鏈接器進行(直接或通過將它們分配給編譯單元，然后重新編譯).這是 CFront 使用的模型 - 即歷史上它是第一個使用的模型 - 以及使用 EDG 前端的編譯器(與 CFront 相比進行了一些優(yōu)化).
iterated instantiation, where the instantiations are made by the linker (either directly or by assigning them to a compilation unit, which will then be recompiled). This is the model used by CFront -- i.e. historically it was the first one used -- and also by compilers using the EDG front-end (with some optimisations compared to CFront).
(參見 C++ 模板，David Vandevoorde 和 Nicolai Josuttis 的完整指南.另一個在線參考是 http://www.bourguet.org/v2/cpplang/export.pdf，更關(guān)注編譯模型，但仍然有實例化機制的描述).
(See C++ Templates, The Complete Guide by David Vandevoorde and Nicolai Josuttis. Another online reference is http://www.bourguet.org/v2/cpplang/export.pdf, which is more concerned about the compilation model but still has descriptions of the instantiation mechanisms).
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