Howto extend std::list and create custom function?

Question

Have a custom std::list class helper:

#ifndef __WLIST_H

    #define __WLIST_H

    #include <list>
    using namespace std;

    template <class T >
    class WList : public std::list<T>
    {
        public:
            void wAdd();
            void wRemove();
            bool wContains();
            void wGet();
            void wClear();
            int  wSize();

    };

#endif /*__WLIST_H*/

Howto declare the custom add new item?, in the class declare:

#include "WList.h"

template<class T>
void WList<T>::wAdd(/*???*/)
{
    this->push_back(/*???*/);
}

Howto set the object type in the argument?

Can't you just pass a class of type `T` to `wAdd` (`void WList::wAdd(T object_to_add)`) and then add it like `this->push_back(object_to_add)`? — apalomer, Jan 21 '18 at 21:54
If you don't know the answer to *Howto declare the custom add new item?*, you need to understand the fundamentals of the language from [a good book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). Also, don't derive from `std::list`. The standard prohibits it. — R Sahu, Jan 21 '18 at 21:56
@apalomer does not work, the compiler says an error: `WList::wAdd(Rectangle) is not defined` — e-info128, Jan 21 '18 at 22:29
Spanish? That's a link to [The Definitive C++ Book Guide and List](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — R Sahu, Jan 21 '18 at 22:33
@e-info128 you have to implement it in the header otherwise it won't be instantiated. — apalomer, Jan 21 '18 at 22:38

apalomer · Answer 1 · 2018-01-24T08:19:45.717

Although you should not inherit from stl containers I will give you the answer with your same example.

When using a function or a class template you have to implement the code in the header file, otherwise, the compiler will not instantiate the right functions. When defining function templates the compiler creates a specific class of the function template for the given class. For example, when you do WList<Rectangle> the compiler creates an instance of the WList for the template Rectangle. To do so the compiler needs all the function/class templates in the code that is compiling, otherwise, some functions (the ones that are not in the header file) will be missing and a func is not defined or undefined reference to func error happens.

Your code should be something like:

WList.h

#include <list>

template <typename T >
class WList : public std::list<T>
{
public:
    void wAdd(T obj)
    {
        this->push_back(obj);
    }    
};

or

#include <list>

 template <typename T >
 class WList: public std::list<T>
 {
     public:
         void wAdd(T obj);

 };

 template<typename T>
 void WList<T>::wAdd(T obj)
 {
     this->push_back(obj);
 }

Main

int main() {

    WList<double> list;
    list.wAdd(2);
}

A more proper way of doing this would be without inheriting from the stl container:

#include <list>

template <typename T >
class WList
{
public:
    void wAdd(T obj)
    {
        m_data.push_back(obj);
    }    
protected:
    std::list<T> m_data;
};

Moreover, note that using template<typename T> is better than template<class T> to avoid static ambiguity between class as template and class as class definition

An even more proper way: `m_data.push_back(std::move(obj));`. — Daniel Langr, Jan 22 '18 at 09:02
only if @e-info128 uses [c++11 though](http://en.cppreference.com/w/cpp/utility/move), isn't it? — apalomer, Jan 22 '18 at 09:07
and if declare function name in the header but declare functionality in the class? is `WList::wAdd(T obj)`? — e-info128, Jan 24 '18 at 01:59
You can declare the function outside of the class definition, but if the function is not declared in the header file then it won't be instantiated when the class for a given type is instantiated. Look at my edit. — apalomer, Jan 24 '18 at 08:18

score 0 · Answer 2 · answered Jan 22 '18 at 09:08

In C++11, if it's available:

Templates need to be defined in a header file, you can write inside a class:

void wAdd(T&& obj) { this->push_back(std::move(obj)); }   
void wAdd(const T& obj) { this->push_back(obj); }

You might also want to consider emplace semantics:

template <typename... Ts>
void wEmplace(Ts&&... args) { this->emplace_back(std::forward<Ts>(args)...); }

And do not use using namespace std; in a header file!

Howto extend std::list and create custom function?

2 Answers2