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.

Also read [this](http://www.parashift.com/c++-faq/templates.html#faq-35.12) FAQ. — Prasoon Saurav, Apr 23 '11 at 07:30
Are you sure you want to pass `list` by value, that is, make a copy of the list just to print its elements and then get rid of the copy again? I suggest you change the signature to `template void printVector(const std::vector& list);` — fredoverflow, Apr 23 '11 at 07:31

kennytm · Accepted Answer · 2011-04-23T08:04:15.707

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:

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.)
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
//^^^^
```

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

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) {
    ...
}

Thank you. Would I be able to do something like: `using namespace std; #include "Sort_Search.h"` in the main .cpp instead of std:vector ? — Skinner927, Apr 23 '11 at 07:32
@Skinner927: You might want to read something like [this](http://stackoverflow.com/questions/1265039/using-std-namespace) first. — CB Bailey, Apr 23 '11 at 07:34
@Skinner927: No. Headers should avoid assuming the files they're included in will have declared, included, or defined anything in particular before including the header. And `using namespace std;` is not widely considered good C++ practice, making that particular assumption even worse. — Nicholas Knight, Apr 23 '11 at 07:36
@beduin: `std::vector` is not a type. You can't `typedef` it. — kennytm, Apr 23 '11 at 07:37
Thanks everyone. I've read that `using namespace std;` is frowned upon. I'm taking a class at school and that's what the instructor uses but I'll be sure to change my ways! Is there a problem with declaring `using std:vector;` in the header and .cpp ? — Skinner927, Apr 23 '11 at 07:40
@Skinner927: A very big problem: `using` declarations, like everything else, propagates from the header to the file it's included in, polluting the namespace and possibly surprising the person including it. The key thing to understand is that `#include` in C++ is quite literal: you're telling the compiler to take every byte of the header file, dump it in your cpp file, and then compile the result. The header is not a separately compiled entity, making it quite different from the sort of library modules you may be used to in languages like Java. — Nicholas Knight, Apr 23 '11 at 07:47
Java does not have pass by reference, only [pass by value](http://javadude.com/articles/passbyvalue.htm). To contrast, C# can pass by reference if you use the `ref` keyword. — fredoverflow, Apr 23 '11 at 08:01
@KennyTM: Thank you for that extremely informative post. I had a feeling that my problems were coming from the fact that the implementation and declaration of the template cannot be separate, but I could not find anything specifically saying you couldn't in my text book. My problems have arisen from my instructor defining the template prototypes in a header file, I then though that I needed to make a corresponding .cpp file. Thanks so much for the help! I can now finish this project and get some sleep. :) — Skinner927, Apr 23 '11 at 08:06

score 1 · Answer 2 · answered Apr 23 '11 at 07:56

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.

score 1 · Answer 3 · answered Apr 23 '11 at 10:17

1

What's actually wrong with your sample is the missing #include <vector> directive in Sort_Search.h. The remarks posted by others are also correct, of course.

answered Apr 23 '11 at 10:17

Paul Michalik

4,331
16
18

C++: Templates in Header files are destroying me

3 Answers3