// temp.h
template <typename T>
struct A {
T value;
}
// foo.h #include "temp.h" void foo();
// foo.cpp
#include "foo.h"
void foo() { A<int> _foo; }
// bar.h #include "temp.h" void bar();
// bar.cpp
#include "bar.h"
void bar() { A<int> _bar; }
// main.cpp
#include "foo.h"
#include "bar.h"
int main() { foo(); bar(); return 0; }
我认为它是在调用foo()时发生的,因为它是第一次使用A< int>,所以A< int>在foo.o实施.
并且,当调用bar()时,它链接到A< int>在foo.o.
我对吗？或实例化发生两次？
解决方法
我不确定其他编译器,这是g如何处理它,从7.5 Where’s the Template?：
Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed,and not at all otherwise. There are two basic approaches to this problem,which are referred to as the Borland model and the Cfront model.
- Borland model:
Borland C++ solved the template instantiation problem by adding the code equivalent of common blocks to their linker; the compiler emits template instances in each translation unit that uses them,and the linker collapses them together. The advantage of this model is that the linker only has to consider the object files themselves; there is no external complexity to worry about. The disadvantage is that compilation time is increased because the template code is being compiled repeatedly. Code written for this model tends to include definitions of all templates in the header file,since they must be seen to be instantiated.
> Cfront模型：The AT&T C++ translator,Cfront,solved the template instantiation problem by creating the notion of a template repository,an automatically maintained place where template instances are stored. A more modern version of the repository works as follows: As individual object files are built,the compiler places any template definitions and instantiations encountered in the repository. At link time,the link wrapper adds in the objects in the repository and compiles any needed instances that were not prevIoUsly emitted. The advantages of this model are more optimal compilation speed and the ability to use the system linker; to implement the Borland model a compiler vendor also needs to replace the linker. The disadvantages are vastly increased complexity,and thus potential for error; for some code this can be just as transparent,but in practice it can been very difficult to build multiple programs in one directory and one program in multiple directories. Code written for this model tends to separate definitions of non-inline member templates into a separate file,which should be compiled separately.
这是g如何实现它,重点是我的：
G++ implements the Borland model on targets where the linker supports it,including ELF targets (such as GNU/Linux),Mac OS X and Microsoft Windows. Otherwise G++ implements neither automatic model.
那就是说：用g,每个翻译单元都有自己的实例.该页面再次提到：
…,but each translation unit contains instances of each of the templates it uses. The duplicate instances will be discarded by the linker,but in a large program,this can lead to an unacceptable amount of code duplication in object files or shared libraries.
您可以选择避免它(当然是第一个也是最后一个选项是明确的)：
Duplicate instances of a template can be avoided by defining an explicit instantiation in one object file,and preventing the compiler from doing implicit instantiations in any other object files by using an explicit instantiation declaration,using the extern template Syntax
Compile your template-using code with
-frepo. The compiler generates files with the extension.rpolisting all of the template instantiations used in the corresponding object files that could be instantiated there; the link wrapper,collect2,then updates the.rpofiles to tell the compiler where to place those instantiations and rebuild any affected object files. The link-time overhead is negligible after the first pass,as the compiler continues to place the instantiations in the same files.Compile your code with
-fno-implicit-templatesto disable the implicit generation of template instances,and explicitly instantiate all the ones you use.
