Printing std::strings produces no output

Question

I was building a small template stack class for a side project and it appeared to be working correctly. However, when I tried it with strings it doesn't appear to work. I have no compilation errors or warnings. I simply get no output. I'm a little rusty in C++ but I wasn't expecting to get blocked by a problem that seems this simple.

My main function (for testing):

#include <iostream>
#include <fstream>
#include <string>
#include "myStack.h"

int main()
{

    bool repeat = true;
    int option = -1;

    std::cout << "Option (1 - String | 2 - Integer) : ";
    std::cin >> option;
    std::cout << "\n";

    switch (option)
    {

    case 1:
    {
        myStack<std::string> stringStack;
        stringStack.push("!");
        stringStack.push("there");
        stringStack.push("Hello");
        stringStack.show();
        break;
    }

    case 2:
    {
        myStack<int> intStack;
        intStack.push(3);
        intStack.push(2);
        intStack.push(1);
        intStack.show();
        break;
    }

    default:
        break;
    }

    std::cout << "\n";

    return 0;
}

Relevant parts of my stack class:

#pragma once

template <typename T>
class myStack
{

  private:
    T *elements;
    size_t capacity;

  public:
    myStack();
    T top();
    size_t size();
    void push(T pushed);
    void pop();
    bool isEmpty();
    void show(std::ostream &out = std::cout);
};

template <typename T>
myStack<T>::myStack()
{
    this->elements = NULL;
    this->capacity = 0;
}


template <typename T>
void myStack<T>::push(T pushed)
{
    this->elements = (T *)realloc(this->elements, (this->capacity + 1) * sizeof(T));
    this->elements[this->capacity] = pushed;
    this->capacity++;
}

template<typename T>
void myStack<T>::show(std::ostream &out)
{
    for (int i = this->capacity - 1; i >= 0; i--)
    {
        out << this->elements[i] << std::endl;
    }
}

By the way, you seem to be missing warnings when actually compiling the source files. The warnings are present only for the linking. — Qaz
– Qaz, Commented Apr 7, 2019 at 5:50
You can't store std::string that way. It is not trivially copyable. — Retired Ninja
– Retired Ninja, Commented Apr 7, 2019 at 5:54
Allocating memory for a std::string using realloc() (or malloc(), or calloc()) gives undefined behaviour. If you must dynamically allocated a std::string, or a set of them, use operator new. It works with C++ class types that have constructors, including std::string. — Peter
– Peter, Commented Apr 7, 2019 at 6:00
Oh alright! I was used to malloc() and realloc() and just expected it to work here too. Thanks for the help everyone! — 136
– 136, Commented Apr 7, 2019 at 6:03

R Sahu · Accepted Answer · 2019-04-07 06:05:56Z

3

Use of

this->elements = (T *)realloc(this->elements, (this->capacity + 1) * sizeof(T));
this->elements[this->capacity] = pushed;

to manage an array of std::strings is not right. Use of realloc to allocate memory is ok. However, it does not initialize the object properly. The second line is cause for undefined behavior since the object was not initialized properly.

If you are allowed to use std::vector, use it.

template <typename T>
class myStack
{

  private:
    std::vector<T> elements;

  ...
};

Then, use of capacity can be replaced by elements.size().

push can be simplified to:

template <typename T>
void myStack<T>::push(T pushed)
{
    this->elements.push_back(pushed);
}

You'll have to update show() accordingly too.

answered Apr 7, 2019 at 6:05

R Sahu

207k15 gold badges168 silver badges291 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Printing std::strings produces no output

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related