C++: Templates in Header files are destroying me

Question

I'm fluent in Java, but very new to C++. I'm definitely not understanding what is wrong -- at all.

Here's the code:

// Sort_Search.h
#ifndef SORT_SEARCH_H
#define SORT_SEARCH_H  

using std::vector;

template<typename T> void printVector(vector<T> &list);

#endif



// Sort_Search.cpp

#include <iostream>
#include <vector>

using std::vector;

template<typename T>
void printVector(vector<T> &list) {
    // print every member of the list
    for(int i = 0; i < (int)list.size(); i++) {
        // insert a comma where needed
        if(i != 0)
            cout << ", ";
        cout << list[i];
    }
}

I keep getting the same errors:

sort_search.h(6): error C2182: 'printVector' : illegal use of type 'void'

sort_search.h(6): error C2998: 'int printVector' : cannot be a template definition

There are more templates causing similar errors in the same files. I figured if I can fix one, I'll figure out how to fix the rest. I've tried every single thing I can think of.

Thanks so much for any help. I'm going crazy over here. haha.

Answer 1

In the header, you need to provide the namespaces.

template<typename T> void printVector(std::vector<T> list);
//                                    ^^^^^

---

There are several things you need to consider:

1. In C++, parameters (except arrays) are always passed as value type if you don't specify it, unlike Java where every objects are passed as reference type. That means, if the function signature is printVector(std::vector<T> list), the list will be copied when feed into printVector. This is often undesirable. Therefore, you need to change it to pass by reference by adding an & to the type:

   template<typename T> void printVector(std::vector<T>& list);
   //                                                  ^
   

   but making it a reference means modification of list inside printVector will be propagated out. You often don't want to accidentally modify the list. This can be enforced by making the parameter a constant:

   template<typename T> void printVector(const std::vector<T>& list);
   //                                    ^^^^^
   

   (Making it a const-reference also have the advantage that it can accept rvalues.)

2. Also unlike Java, in C and C++ #include does not know if you have included the header once before. #include is simply a "copy-and-paste" mechanism. That means, if somehow the compiler sees

   #include "Sort_Search.h"
   ...
   #include "Sort_Search.h"
   

   then 2 copies of printVector will be defined, and that leads to compiler error. This is possible if two different headers a.h and b.h includes Sort_Search.h and some source file include both a.h and b.h. To avoid this, we always need to provide an #include guard which prevents the file to be included more than once:

   #ifndef SORT_SEARCH_H_m6f2kyhdncxflxr
   #define SORT_SEARCH_H_m6f2kyhdncxflxr
   //^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   
   template<typename T> void printVector(const std::vector<T>& list);
   
   #endif
   //^^^^
   

3. A vector<T> is not a built-in type, so you need to #include <vector> to let the compiler knows the existence of such type.

   #ifndef SORT_SEARCH_H_m6f2kyhdncxflxr
   #define SORT_SEARCH_H_m6f2kyhdncxflxr
   
   #include <vector>
   //^^^^^^^^^^^^^^^
   
   template<typename T> void printVector(const std::vector<T>& list);
   
   #endif
   

4. Finally, a template is implemented differently than generics in Java or C#. It is like an AST-level copy-and-paste mechanism. Every time you call printVector, the compiler will determine what T is (say, int), and then create a new function by replacing every T by int.

   Because of this, the implementation of a template cannot be separated from the declaration. Or, the implementation is part of the declaration. Therefore, for correctness, the printVector have to be moved into the header:

   #ifndef SORT_SEARCH_H_m6f2kyhdncxflxr
   #define SORT_SEARCH_H_m6f2kyhdncxflxr
   
   #include <vector>
   #include <iostream>
   
   template<typename T> void printVector(const std::vector<T>& list) {
      for (int i = 0; i < list.size(); ++ i) { ... }
   }
   
   #endif
   

   Or, if you still want to separate the .cpp from .h, you could include the .cpp from the .h:

   #ifndef SORT_SEARCH_H_m6f2kyhdncxflxr
   #define SORT_SEARCH_H_m6f2kyhdncxflxr
   
   #include <vector>
   
   template<typename T> void printVector(const std::vector<T>& list);
   
   #include "Sort_Search.cpp"
   //^^^^^^^^^^^^^^^^^^^^^^^^
   
   #endif
   

   // Sort_Search.cpp:
   #include <iostream>
   template<typename T> void printVector(const std::vector<T>& list) {
       ...
   }
   

Answer 2

Make your life a million times easier and define all of your template functions / classes inline. There is no benefit to having a separate .h and .cpp file as both need to be included at compile time anyway so you get nothing but pain for splitting them up.