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

问题描述

我正在尝试学习模板，但遇到了这个令人困惑的错误.我在头文件中声明了一些函数，我想制作一个单独的实现文件，在其中定义函数.这是调用标头(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，这是一个可用的头文件 (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 << "
";
    return;
}

使用实现(dumper2.cpp):

With implementation (dumper2.cpp):

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

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

奇怪的是，如果我将定义 dumpVector 的代码从 .h 移动到 .cpp 文件，我会收到以下错误.

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

那么为什么它以一种方式工作而不是另一种方式?明明编译器能找到test()，为什么找不到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模板的哪个版本.当您将函数的实现移动到 x.cpp 时，它位于与 main.cpp 不同的翻译单元中，并且 main.cpp 无法链接到特定的实例化，因为它不存在于该上下文中.这是 C++ 模板的一个众所周知的问题.有几种解决方案:

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) 像之前一样，将定义直接放在 .h 文件中.这有优点 &缺点，包括解决问题(专业版)，可能使代码的可读性降低在某些编译器上更难调试 (con) 并且可能会增加代码膨胀 (con).

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 中，#include "x.cpp" 来自 x.h.如果这看起来很奇怪而且是错误的，请记住 #include 只会读取指定的文件并编译它就好像该文件是 x.cpp 换句话说，这与上面的解决方案#1 所做的完全一样，但它将它们保存在单独的物理文件中.在做这种事情时，重要的是不要试图自己编译 #included 文件.出于这个原因，我通常给这些类型的文件一个 hpp 扩展名，以将它们与 h 文件和 cpp 文件区分开来.

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

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 << "
";
  return;

}

3) 由于问题在于尝试使用它的翻译单元不知道 dumpVector 的特定实例化，因此您可以在与定义模板的地方.只需添加: template void dumpVector(std::vector v, std::string sep); ... 到定义模板的文件中.这样做，您不再需要从 h 文件中#include hpp 文件:

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);

文件: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 << "
";
  return;
}

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

顺便说一句，总而言之，您的模板函数正在采用 vector by-value.您可能不想这样做，而是通过引用或指针传递它，或者更好的是，传递迭代器以避免创建临时 &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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