Add emplace_back to template class

Question

I got stuck implementing my own template class where I wanted to add my own implementation of emplace_back function. Since I am still learning template design I'll appreciate your input here.

template <typename T, typename ...Args>
class MydataStruct
{
public:
    //...
    void emplace_back(Args&&... args)
    {
        //...
        myqueue.emplace_back(args...);
    }
    //...
private:
    std::deque<T> myqueue;
};

Sample use:

MydataStruct<int> test;
test.emplace_back(1);

Whenever I am trying to compile this code I receive error that emplace_back is not defined. It only works with no arguments. How should I fix this?

score 8 · Answer 1 · edited May 23 '17 at 12:01

You should make the member function a member function template. Use variadic Forwarding References to capture the arguments, then std::forward the arguments to myqueue.emplace_back

template <typename T>
class MydataStruct
{
public:
    //...
    template<typename ...Args>
    void emplace_back(Args&&... args)
    {
        //...
        myqueue.emplace_back(std::forward<Args>(args)...);
    }
    //...
private:
    std::deque<T> myqueue;
};

François Andrieux · Answer 2 · 2017-01-06T22:02:56.430

3

The error is that you put your variadic template in your class' template parameters rather than add one to your method. Try this instead.

#include <deque>

template <typename T /*typename ...Args*/>
//        remove this ^^^^^^^^^^^^^^^^^
class MydataStruct
{
public:
    //...
    template<typename ...Args>
    // add this ^^^^^^^^^^^^^
    void emplace_back(Args&&... args)
    {
        myqueue.emplace_back(args...);
    }
private:
    std::deque<T> myqueue;
};

int bop()
{
    MydataStruct<int> test;
    test.emplace_back(1);
}

Edit: Note that this will not do what you want with rvalues. You will need to use std::forward.

edited Jan 06 '17 at 22:02

answered Jan 06 '17 at 21:19

François Andrieux

28,148
6
56
87

I see... Thank you very much for clarification. – sebap123 Jan 06 '17 at 21:21
Don't just add a note saying why your answer is wrong. Just write a correct answer? Also "rvalue", not "R value" – Barry Jan 06 '17 at 22:00

score 0 · Answer 3 · answered Jan 06 '17 at 21:24

0

to get your example to work you would have to do

MydataStruct<int,int> test;
test.emplace_back(1);

but moving the ...Args to the emplace_back function is the way to go...

answered Jan 06 '17 at 21:24

Biggy Smith

910
7
14

Add emplace_back to template class

3 Answers3