C ++模板函数在头中编译但不是实现 [英] C++ template function compiles in header but not implementation

问题描述

我试图学习模板，我遇到了这个混淆的错误。我在一个头文件中声明一些函数，我想要一个单独的实现文件，其中的函数将被定义。这里是调用头（dum.cpp）的代码：

I'm trying to learn templates and I've run into this confounding error. I'm declaring some functions in a header file and I want to make a separate implementation file where the functions will be defined. Here's the code that calls the header (dum.cpp):

#include <iostream>
#include <vector>
#include <string>
#include "dumper2.h"

int main() {
    std::vector<int> v;
    for (int i=0; i<10; i++) {
        v.push_back(i);
    }
    test();
    std::string s = ", ";
    dumpVector(v,s);
}

现在，这是一个工作头文件（dumper2.h）：

Now, here's a working header file (dumper2.h):

#include <iostream>
#include <string>
#include <vector>

void test();

template <class T> void dumpVector( std::vector<T> v,std::string sep);

template <class T> void dumpVector(std::vector<T> v, std::string sep) {
    typename std::vector<T>::iterator vi;

    vi = v.begin();
    std::cout << *vi;
    vi++;
    for (;vi<v.end();vi++) {
        std::cout << sep << *vi ;
    }
    std::cout << "\n";
    return;
}

执行（dumper2.cpp）：

With implementation (dumper2.cpp):

#include <iostream>
#include "dumper2.h"

void test() {
    std::cout << "!olleh dlrow\n";
}

奇怪的是，如果我移动定义dumpVector的代码。 h到.cpp文件，我得到以下错误。

The weird thing is that if I move the code that defines dumpVector from the .h to the .cpp file, I get the following error.

g++ -c dumper2.cpp -Wall -Wno-deprecated
g++ dum.cpp -o dum dumper2.o -Wall -Wno-deprecated
/tmp/ccKD2e3G.o: In function `main':
dum.cpp:(.text+0xce): undefined reference to `void dumpVector<int>(std::vector<int, std::allocator<int> >, std::basic_string<char, std::char_traits<char>, std::allocator<char> >)'
collect2: ld returned 1 exit status
make: *** [dum] Error 1

那么为什么它工作的方式而不是其他？显然，编译器可以找到 test（），为什么不能找到 dumpVector ？

So why does it work one way and not the other? Clearly the compiler can find test(), so why can't it find dumpVector?

推荐答案

您遇到的问题是，编译器不知道要实例化的模板的哪些版本。当你将函数的实现移动到x.cpp时，它与main.cpp处于不同的转换单元，而main.cpp不能链接到特定的实例化，因为它在该上下文中不存在。这是C ++模板的一个众所周知的问题。有几个解决方案：

The problem you're having is that the compiler doesn't know which versions of your template to instantiate. When you move the implementation of your function to x.cpp it is in a different translation unit from main.cpp, and main.cpp can't link to a particular instantiation because it doesn't exist in that context. This is a well-known issue with C++ templates. There are a few solutions:

1）只需将定义直接放在.h文件中，就像以前一样。这有优点&包括解决问题（pro），可能使代码更不可读和不可读，

1) Just put the definitions directly in the .h file, as you were doing before. This has pros & cons, including solving the problem (pro), possibly making the code less readable & on some compilers harder to debug (con) and maybe increasing code bloat (con).

2）将实现放在x.cpp和 #includex.cpp从 xh 内。如果这看起来很时髦和错误，只要记住 #include 只是读取指定的文件并编译它，就好像该文件是 x.cpp 换句话说，这确实是上面的解决方案＃1，但它保持在单独的物理文件。当做这样的事情，关键是你不试图编译它自己的 #include d文件。因此，我通常给这些类型的文件一个 hpp 扩展名，以区别于 h 文件和从 cpp 档案。

2) Put the implementation in x.cpp, and #include "x.cpp" from within x.h. If this seems funky and wrong, just keep in mind that #include does nothing more than read the specified file and compile it as if that file were part of x.cpp In other words, this does exactly what solution #1 does above, but it keeps them in seperate physical files. When doing this kind of thing, it is critical that you not try to compile the #included file on it's own. For this reason, I usually give these kinds of files an hpp extension to distinguish them from h files and from cpp files.

#include <iostream>
#include <string>
#include <vector>

void test();
template <class T> void dumpVector( std::vector<T> v,std::string sep);
#include "dumper2.hpp"

文件：dumper2.hpp

File: dumper2.hpp

template <class T> void dumpVector(std::vector<T> v, std::string sep) {
  typename std::vector<T>::iterator vi;

  vi = v.begin();
  std::cout << *vi;
  vi++;
  for (;vi<v.end();vi++) {
    std::cout << sep << *vi ;
  }
  std::cout << "\n";
  return;

}

3）由于问题是特定的实例化 dumpVector 对于尝试使用它的翻译单元是未知的，您可以在与定义模板的同一翻译单元中强制执行它。只需添加： template void dumpVector< int>（std :: vector< int> v，std :: string sep）; 被定义为。这样，您不再需要中的 hpp 文件 #include / code>档案：

3) Since the problem is that a particular instantiation of dumpVector is not known to the translation unit that is trying to use it, you can force a specific instantiation of it in the same translation unit as where the template is defined. Simply by adding this: template void dumpVector<int>(std::vector<int> v, std::string sep); ... to the file where the template is defined. Doing this, you no longer have to #include the hpp file from within the h file:

#include <iostream>
#include <string>
#include <vector>

void test();
template <class T> void dumpVector( std::vector<T> v,std::string sep);

档案：dumper2.cpp

File: dumper2.cpp

template <class T> void dumpVector(std::vector<T> v, std::string sep) {
  typename std::vector<T>::iterator vi;

  vi = v.begin();
  std::cout << *vi;
  vi++;
  for (;vi<v.end();vi++) {
    std::cout << sep << *vi ;
  }
  std::cout << "\n";
  return;
}

template void dumpVector<int>(std::vector<int> v, std::string sep);

顺便说一下，作为一个合计，你的模板函数取一个 vector by-value 。你可能不想这样做，并通过引用或指针传递它，或者，更好的是，传递迭代器，以避免做一个临时&复制整个向量。

By the way, and as a total aside, your template function is taking a vector by-value. You may not want to do this, and pass it by reference or pointer or, better yet, pass iterators instead to avoid making a temporary & copying the whole vector.

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