C++ - std::unique_ptr in vector<> is nullptr

I want to store Particle objects in a vector object, so I can access it later.
These particles (Electrons and Protons) are inherited from the Particle class which contains a toString() virtual method. This toString() method is then overrided in Electron and Proton classes.

Particle

vector

Electrons

Protons

Particle

toString()

toString()

Electron

Proton

When I read the vector container, I want to access to the toString() method specific to Electron or Proton, and not to Particle.

toString()

Electron

Proton

Particle

Apparently, one way is to use std::unique_ptr. Here is the part of the code I try to run:

std::unique_ptr

int main()
/**/
std::vector<std::unique_ptr<Particle>> particles(nbParticles);

particles.push_back(std::unique_ptr<Electron>( new Electron(1.0, 2.0, 3.0)));
particles.push_back(std::unique_ptr<Proton>(new Proton(1.0, 2.0, 3.0)));
particles.push_back(std::unique_ptr<Particle>(new Particle(0.0, 0.0, 1.0, 2.0, 3.0)));

if (particles[0]==nullptr)
std::cout<< "index=0 : nullptr"<<std::endl; //There is a null_ptr at particles[0]


if (particles[2]==nullptr)
std::cout<< "index=2 : nullptr"<<std::endl; //There is not a null_ptr at particles[2]


std::cout<<particles[0]->toString()<<std::endl; //This is what I'm trying to do
/**/



A pointer to a Particle object seems to be fine, but not to an Electron or Proton. I guess there is something wrong with the constructors ?

Particle

Electron

Proton

class Particle

public:
Particle();
Particle(double mass, double charge, double posX, double posY, double posZ);
virtual std::string toString() const;


class Electron : public Particle

public:
Electron(double PosX, double PosY, double PosZ);
virtual std::string toString() const;


class Proton : public Particle

public:
Proton(double PosX, double PosY, double PosZ);
virtual std::string toString() const;



and the definitions:

Particle::Particle(double mass, double charge, double posX, double posY, double posZ) :
m_mass(mass), m_charge(charge),
m_posX(posX), m_posY(posY), m_posZ(posZ)


Electron::Electron(double PosX, double PosY, double PosZ) :
Particle(9.109E-31, -1.602E-19, PosX, PosY, PosZ)

Proton::Proton(double PosX, double PosY, double PosZ) :
Particle(9.109E-31, +1.602E-19, PosX, PosY, PosZ)


Particle

virtual

1 Answer
1

You've made a classic mistake that trips up even the most experienced C++ programmers: you declared the vector with an initial size and then push_backed additional elements to it instead of assigning to the existing elements. Fix this by removing the (nbParticles) from the vector initialization.

push_back

(nbParticles)

Particle

nullptr

nullptr

nbParticles

reserve

particles.reserve(3);

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