C++ templated class default constructor fails

Question

So I am creating a Stack class for an assignment in C++. The core of the assignment is to familiarize us with templates. I have read my book over and over and looked question after question on here.

I need to have my Stack class be able to be constructed by

Stack s2;

but I get an error when I compile my test.cpp and can only compile when i construct as

Stack<T> s1;

where T is a std::string, int, etc. How do build my Stack so I can use both constructors?

Stack.cpp

#include <iostream>
#include <vector>
#include <stdlib.h>
#include <string>

using namespace std;

template<typename T> 
class Stack {
public:
    Stack();
    void Push(T val);
    T Pop();
    void Print();

private:
    vector<T> vecT;
};

template <typename T>
Stack<T>::Stack() { }

template <typename T>
void Stack<T>::Push(T val) { vecT.push_back(val); }

template <typename T>
T Stack<T>::Pop() { vecT.pop_back(); }

template <typename T>
void Stack<T>::Print() {
    cout << "[ ";
    for(int i=0; i<vecT.size(); i++) {
        cout << vecT[i] << " ";
    }
    cout << "]";
}

test.cpp

#include <iostream>
#include <vector>
#include <stdlib.h>
#include <string>

#include "Stack.cpp"

using namespace std;

int main() {
    Stack<string> s1;
        s1.Push("values1");
        s1.Push("values2");
        s1.Print();

    Stack s2;
        s2.Push("values1");
        s2.Push("values2");
        s2.Print();
}

[*Never, under no circumstances, and without exceptions, should you ever `using namespace std;` at global scope.*](https://stackoverflow.com/questions/1452721) — Henri Menke, Jun 20 '17 at 04:15

score 1 · Answer 1 · answered Jun 19 '17 at 20:40

1

What about a default template parameter?

template<typename T = std::string> 
class Stack {

Anyway, with

Stack<T> s1;

and

Stack s2;

you're not using different constructors; you're using, in both cases, the same default (no arguments) constructor.

answered Jun 19 '17 at 20:40

max66

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This will become `Stack<> s2;`. `Stack s2` won't compile unless you have C++1z – Passer By Jun 20 '17 at 05:55

Henri Menke · Answer 2 · 2017-06-20T21:23:39.993

Instead of templating everything you could just use a stack of a variant type (since C++17, before use Boost.Variant). With the code below you can now push int, double, and std::string onto the stack. Furthermore, Pop() is missing a return statement. Also, pop_back() of vector returns nothing.

#include <iostream>
#include <vector>
#include <string>
#include <variant>

class Stack {
    using Variant = std::variant<int,double,std::string>;
public:
    Stack();
    void Push(Variant val);
    void Pop();
    void Print();

private:
    std::vector<Variant> vecT;
};

Stack::Stack() : vecT() {}

void Stack::Push(Variant val) { vecT.push_back(val); }

void Stack::Pop() { vecT.pop_back(); }

void Stack::Print() {
    std::cout << "[ ";
    for ( auto const& v : vecT )
        std::visit([] (auto&& arg) { std::cout << arg << " "; }, v);
    std::cout << "]\n";
}

int main() {
    Stack s1;
        s1.Push("values1");
        s1.Push("values2");
        s1.Print();

    Stack s2;
        s2.Push("values1");
        s2.Push("values2");
        s2.Print();
}

With Boost.Variant you get a C++98 compatible solution.

#include <iostream>
#include <vector>
#include <string>
#include <boost/variant.hpp>

class Stack {
    typedef boost::variant<int,double,std::string> Variant;
    typedef std::vector<Variant>::iterator Iterator;

    std::vector<Variant> vecT;

    struct visitor : public boost::static_visitor<void>
    {
        template < typename T >
        void operator()(T const& arg) const { std::cout << arg << " "; }
    };
public:
    Stack();
    void Push(Variant val);
    void Pop();
    void Print();
};

Stack::Stack() : vecT() {}

void Stack::Push(Variant val) { vecT.push_back(val); }

void Stack::Pop() { vecT.pop_back(); }

void Stack::Print() {
    std::cout << "[ ";
    for ( Iterator it = vecT.begin(); it != vecT.end(); ++it )
        boost::apply_visitor( visitor(), *it );
    std::cout << "]\n";
}

int main() {
    Stack s1;
        s1.Push("values1");
        s1.Push("values2");
        s1.Print();

    Stack s2;
        s2.Push("values1");
        s2.Push("values2");
        s2.Print();
}

I don't think I have. I installed the normal g++ compiler for Linux mint about a month ago and left the normal settings? — Zhawn, Jun 20 '17 at 19:27
@Zhawn The second part of the updated answer now contains a C++98 compatible solution. — Henri Menke, Jun 20 '17 at 21:24

score 0 · Answer 3 · answered Jun 21 '17 at 07:14

You could use a default template parameter:

template<typename T = int> 
class Stack {

Then you could construct using:

Stack<> s2;

Also, the constructors are identical, you aren't calling a different one each time, but rather the template argument differs.

C++ templated class default constructor fails

3 Answers3