假设一个头文件定义了一个函数模板。现在假设有两个实现文件#include 这个头文件，并且它们每个都有一个对函数模板的调用。在两个实现文件中，函数模板都是用相同的类型实例化的。

// header.hh
template <typename T>
void f(const T& o)
{
    // ...
}

// impl1.cc
#include "header.hh"

void fimpl1()
{
    f(42);
}

// impl2.cc
#include "header.hh"

void fimpl2()
{
    f(24);
}

链接器可能会提示 f() 的多个定义。具体来说，如果 f() 不是模板，那么情况确实如此。

• 为什么链接器不提示 f() 的多个定义？
• 标准中是否规定链接器必须优雅地处理这种情况？换句话说，我总能指望类似上面的程序编译链接吧？
• 如果链接器足够聪明，可以消除一组函数模板实例化的歧义，为什么它不能对常规函数做同样的事情，因为它们与实例化函数模板的情况相同？

最佳答案

Gnu C++ 编译器手册有 a good discussion of this .摘录:

C++ templates are the first language feature to require more intelligence from the environment than one usually finds on a UNIX system. 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. This 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 model

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 be 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.

When used with GNU ld version 2.8 or later on an ELF system such as GNU/Linux or Solaris 2, or on Microsoft Windows, G++ supports the Borland model. On other systems, G++ implements neither automatic model.

关于c++ - 一个函数模板的多个定义，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/235616/