又被C++编译器给郁闷了～

继续我那个该死的 Log 工具，然后为了做刚写好的 C++ 反射模块的 Unit Test，于是就写了一个 C++ 代码来生成实验数据。

为了测试的普遍性，我写了几个随机函数，用来生成各种类型的数据。

然后由于还有数组，于是就像写一个随机生成数组内容的函数。

然后很开心的写了一个模板函数，传入数组，数组长度，以及数组元素的随机生成函数。

函数的 Definition 大概是这样的：

template<class T>

void RndArray(T target[], int length, T (*RndT)())

{

T* pEnd = target + length;

for(T* fo = target; fo< pEnd; fo++)

{

*fo = RndT();

}

}

把这段代码写在 util.cpp 里后，很开心的在头文件里写了一个 declaration:

大概是这样的：

template<class T>

void RndArray(T target[], int length, T (*RndT)());

然后编译～一切OK～

然后为了测试一下，于是在 Main 函数里写了这么一段：

#include "stdafx.h"

using namespace System;

using namespace util;

int main(array<System::String ^> ^args)

{

InitializeUtil();

INT temp[10];

RndArray<INT>(temp,10,RndINT);

for(int fo =0;fo<10;fo++)

Console::WriteLine("[{0}] = {1}",fo,temp[fo]);

Console::ReadKey();

return 0;

}

再编译～

Error~

估计是模板实例化的时候出问题，开始倒腾那段模板代码，倒腾了10分钟还是不行，于是仔细看看错误消息：

Error 1 error LNK2028: unresolved token (0A00000D) "void __cdecl util::RndArray<int>(int * const,int,int (__cdecl*)(void))" (??$RndArray@H@util@@$$FYAXQAHHP6AHXZ@Z) referenced in function "int __clrcall main(cli::array<class System::String ^ >^)" (?main@@$$HYMHP$01AP$AAVString@System@@@Z) EntityDataGenerator.obj EntityDataGenerator

Error 2 error LNK2019: unresolved external symbol "void __cdecl util::RndArray<int>(int * const,int,int (__cdecl*)(void))" (??$RndArray@H@util@@$$FYAXQAHHP6AHXZ@Z) referenced in function "int __clrcall main(cli::array<class System::String ^ >^)" (?main@@$$HYMHP$01AP$AAVString@System@@@Z) EntityDataGenerator.obj EntityDataGenerator

发现是 Link 时出错，说明代码不是 definition 的问题，估计会是 declaration 的问题，于是倒腾头文件：调整 declaration 的顺序，修改命名空间，检查 头文件引用……

倒腾了半个小时还是不行～MSDN 上也没有任何相关的内容～

思考了一下，发现其他的正常的函数是一切OK的，问题就处在模板上～

模板函数如果不做模板实例化就什么也没有，只有实例化以后才会存在。因此如果模板函数更接近于一种 declaration 而不是 definition， 于是把 .cpp 的内容全部挪到 .h 里～

编译～执行～一切正常～

于是去仔细读了读模板的规范，发现在最后面有这么几段话：

From the point of view of the compiler, templates are not normal functions or classes. They are compiled on demand, meaning that the code of a template function is not compiled until an instantiation with specific template arguments is required. At that moment, when an instantiation is required, the compiler generates a function specifically for those arguments from the template.

Because templates are compiled when required, this forces a restriction for multi-file projects: the implementation (definition) of a template class or function must be in the same file as its declaration. That means that we cannot separate the interface in a separate header file, and that we must include both interface and implementation in any file that uses the templates.

Since no code is generated until a template is instantiated when required, compilers are prepared to allow the inclusion more than once of the same template file with both declarations and definitions in a project without generating linkage errors.

就因为这个该死的原因，让我对这么10行不到的代码郁闷了1个多小时～学之不精害死人啊～

TimNew
------------
Release you passion
To Realize you potential
---------------------------------
Tip Of the Day:
Marcus Garvey  - "With confidence, you have won before you have started."

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