Runtime error when using push_back() on smart pointer to vector

Question

I am trying to fill a vector, by dereferencing a smart-pointer. During runtime the program crashes after one iteration of the first "for" loop used for entering the variable input.

using namespace std;

class Measurement
{
protected:
    int sample_size;
    string label;
    shared_ptr <vector<double>> data;
public:
    // parameterised constructor
    Measurement(string pLabel, int pSample_size)
    {
        label = pLabel;
        sample_size = pSample_size;
        cout << "Please input your dataset one entry at a time:" << endl;
        for (int i = 0; i < sample_size; i++)
        {
            double input;
            cin >> input;
            data->push_back(input); // NOT WORKING???
        }
    }
};

int main()
{
    Measurement A("xData", 5);
    return 0;
}

When using the VS debugger it shows that an exception is thrown (Exception thrown: read access violation. std::_Vector_alloc > >::_Myend(...) returned 0xC.) in the vector file, specifically lines 1793 - 1795:

bool _Has_unused_capacity() const _NOEXCEPT
    {   // micro-optimization for capacity() != size()
    return (this->_Myend() != this->_Mylast());

What is the cause of this error?

Doesn't look like you ever allocated the object for the smart pointer. — Cruz Jean
– Cruz Jean, Commented Apr 18, 2019 at 18:06
Dereferencing a nullptr (also true for smart pointers) is Undefined Behaviour. — Jesper Juhl
– Jesper Juhl, Commented Apr 18, 2019 at 18:08

R Sahu · Accepted Answer · 2019-04-18 18:31:39Z

2

A default constructed shared_ptr does not point to anything valid. From https://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr:

Constructs a shared_ptr with no managed object, i.e. empty shared_ptr.

You need to initialize it such that it points to valid object that it manages before you can use the underlying pointer. For example, change the constructor to:

Measurement(string pLabel, int pSample_size) : data(new std::vector<double>()) 
{
   ...
}

or

Measurement(string pLabel, int pSample_size) : data(std::make_shared<std::vector<double>>()) 
{
   ...
}

edited Apr 18, 2019 at 18:31

answered Apr 18, 2019 at 18:11

R Sahu

207k15 gold badges168 silver badges291 bronze badges

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

4 Comments

Cruz Jean Over a year ago

Using new to initialize the smart pointer results in twice the allocations (~2x slower). Using std::make_shared allocates the space for the object and the tag together.

R Sahu Over a year ago

@CruzJean, please clarify what you mean by twice the allocations. You certainly don't mean twice the allocations for the std::vector, do you?

DimChtz Over a year ago

@RSahu By two allocations he means one for the object and one for the reference counter. I don't see any reason to use new and not make_shared here.

user2013904 Over a year ago

Yeah, this was the silly mistake, thanks. Implemented using make_shared instead of new.

DimChtz · Accepted Answer · 2019-04-18 18:11:16Z

1

You need to allocate memory for data before use it:

Measurement(string pLabel, int pSample_size) {
   ...
   data = std::make_shared<vector<double>>();
   ...
}

answered Apr 18, 2019 at 18:11

DimChtz

4,4132 gold badges24 silver badges45 bronze badges

1 Comment

user2013904 Over a year ago

Thanks for the help!

ppetraki · Accepted Answer · 2019-04-18 18:30:07Z

You never initialized the ptr. Below demonstrates the use of default initializers for member variables and member initializer lists.

You could easily add the ptr initialization to the initialization list, but since it doesn't depend on any constructor arguments. It's better to declare it's construction in the manner below to avoid a copy/paste error when creating additional constructors.

#include <iostream>                                                              
#include <vector>                                                                
#include <memory>                                                                

using namespace std;                                                             

class Measurement {                                                              
  protected:                                                                     
    int                        sample_size_;                                     
    string                     label_;                                           
    shared_ptr<vector<double>> data_{make_shared<vector<double>>()};             

  public:                                                                        
    // parameterised constructor                                                 
    Measurement( string pLabel, int pSample_size )                               
      : sample_size_( pSample_size )                                             
      , label_( pLabel )                                                         
    {                                                                            
        cout << "Please input your dataset one entry at a time:" << endl;        
        for ( int i = 0; i < sample_size_; i++ ) {                               
            double input;                                                        
            cin >> input;                                                        
            data_->push_back( input ); // NOT WORKING???                         
        }                                                                        
    }                                                                            

    friend ostream& operator<<( ostream& os, Measurement const& op1 )            
    {                                                                            
        for ( auto& v : *op1.data_ )                                             
            os << v << " ";                                                      
        return os;                                                               
    }                                                                            
};                                                                               

int main()                                                                       
{                                                                                
    Measurement A( "xData", 5 );                                                 
    cout << A << endl;                                                           
    return 0;                                                                    
}

Output:

g++     example.cpp   -o example
Please input your dataset one entry at a time:
1
2
3
4
5
1 2 3 4 5

Collectives™ on Stack Overflow

Runtime error when using push_back() on smart pointer to vector

3 Answers 3

4 Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related