How to store parameters from variadic template member function into vector?

Question

#include <Windows.h>
#include <string>
#include <vector>
#include <iostream>
using namespace std;

class CTest
{
public:
    template<typename... Args>
    void AddStringsToVector(const std::string &First, Args&... args);
private:
    std::vector<std::string> m_stringsvec;
};

template<typename... Args>
void CTest::AddStringsToVector(const std::string &First, Args&... args)
{
    m_stringsvec.push_back(First);
    m_stringsvec.push_back(args...);

    for (auto &i : m_stringsvec)
        std::cout << i << std::endl;
}

void main()
{
    CTest test;
    test.AddStringsToVector("test1","test2","test3");
    system("pause");
}

only works when i pass two parameters:

test.AddStringsToVector("test1","test2");

If I pass any number of parameters other then two I get an error.

For example:

test.AddStringsToVector("test1","test2","test3");

Error: Severity Code Description Project File Line Error C2661 'std::vector>::push_back': no overloaded function takes 2 arguments

SACHIN GOYAL · Accepted Answer · 2015-12-29 06:16:52Z

6

Recursively call the function .

void CTest::AddStringsToVector()//function to break recursion 
{

}
template<typename... Args>
void CTest::AddStringsToVector(const std::string &First, Args&... args)
{
    m_stringsvec.push_back(First);
    AddStringsToVector(args...);


}

answered Dec 29, 2015 at 6:16

SACHIN GOYAL

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2 Comments

CinCout Over a year ago

Shouldn't the following be the breaking method?: void CTest::AddStringsToVector(const std::string &First) { m_stringsvec.push_back(First); }

vsoftco Over a year ago

@HappyCoder The above it's equally good, will break the recursion when Args = [].

Jarod42 · Accepted Answer · 2017-08-22 12:26:47Z

3

A non-recursive method:

class CTest
{
public:
    template<typename... Args>
    void AddStringsToVector(const std::string &first, const Args&... args)
    {
        m_stringsvec.push_back(First);
        int dummy[] = { 0, (m_stringsvec.push_back(args), 0)...}; // all magic is here
        (void) dummy; // Avoid unused variable warning.

        for (const auto &s : m_stringsvec) {
            std::cout << i << std::endl;
        }
    }
private:
    std::vector<std::string> m_stringsvec;
};

The real stuff is there:

int dummy[] = { 0, (m_stringsvec.push_back(args), 0)...};

(foo(), 0) uses comma operator. That do the first part (the job we want) and evaluate as 0.

With variadic expansion, it becomes (m_stringsvec.push_back(args), 0)....

put the whole in initializer list to guaranty order evaluation.

In C++17, it would be even simpler with folding expression:

    template<typename... Args>
    void AddStringsToVector(const std::string &first, const Args&... args)
    {
        m_stringsvec.push_back(First);
         (m_stringsvec.push_back(args), ...); // Folding expression

        for (const auto &s : m_stringsvec) {
            std::cout << i << std::endl;
        }
    }

edited Aug 22, 2017 at 12:26

answered Dec 29, 2015 at 9:55

Jarod42

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1 Comment

Jarod42 Over a year ago

@Jeka: explanation added.

user1095108 · Accepted Answer · 2015-12-29 10:22:14Z

1

Another non-recursive option (warning, brain compile):

struct swallow { template <typename ...T> swallow(T&& ...) noexcept { } };

class CTest
{
public:
    template<typename... Args>
    void AddStringsToVector(const std::string &first, const Args&... args)
    {
        m_stringsvec.push_back(First);
        swallow{(m_stringsvec.push_back(args), 0)...};

        for (const auto &s : m_stringsvec) {
            std::cout << i << std::endl;
        }
    }
private:
    std::vector<std::string> m_stringsvec;
};

answered Dec 29, 2015 at 10:22

user1095108

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