How to include header without error?

Question

C++ / C#

There are 4 files

layer.hpp

#pragma once

class Neuron;

class Layer
{
    private:
        Neuron *neurons;
        int size;
    public:
        Layer();
        ~Layer();
};

neuron.hpp

#pragma once

class Layer;

class Neuron
{
    private:
        float *weight;
    public:
        Neuron();
        ~Neuron();
        float execute(const Layer &layer);
};

layer.cpp

#include "layer.hpp"

Layer::Layer(){}
Layer::~Layer(){}

neuron.cpp

#include "neuron.hpp"

Neuron::Neuron() {}
Neuron::~Neuron(){}
float Neuron::execute(const Layer &layer)
{
    return 0.0;
}

and main.cpp

#include "neuron.hpp"
#include "layer.hpp"

int main(int argc, char const *argv[])
{
    Neuron n;
    Layer l;

    return 0;
}

Why does the g++ compiler write that constructors with destructors and a method are not defined?

Answer 1

Why does the g++ compiler write that constructors with destructors and a method are not defined?

g++ main.cpp neuron.cpp layer.cpp -o test