undefined reference to method generated as lib .a with CMake

Can you explain me why my function AllToAll is undefined in my example? I use CMake to generate a libNeuralNetwork.a which is called by the exemple.

LayerFactory.hpp

#pragma once
#include "LayerModel.hpp"
#include "Layer.hpp"

namespace nn
{
    extern internal::LayerModel AllToAll(int numberOfNeurons, activationFunction activation = sigmoid);
}

LayerFactory.cpp

#include "LayerFactory.hpp"
#include "AllToAll.hpp"

using namespace nn;
using namespace internal;

LayerModel AllToAll(int numberOfNeurons, activationFunction activation)
{
    LayerModel model
    {
        allToAll,
        activation,
        numberOfNeurons
    };
    return model;
}

NeuralNetwork.hpp

#pragma once
#include "layer/LayerModel.hpp"
#include "layer/LayerFactory.hpp"

namespace nn
{
	class NeuralNetwork
	{
    public:
		NeuralNetwork(int numberOfInputs, std::vector<internal::LayerModel> models);
        //...
    };
}

Example.cpp

#include "../src/neural_network/NeuralNetwork.hpp"

using namespace nn;

int example1()
{
	NeuralNetwork neuralNetwork(3, {AllToAll(5), AllToAll(2)});
}

error message:

CMakeFiles/UnitTests.out.dir/ExamplesTest.cpp.o: In function `example1()':
ExamplesTest.cpp:(.text+0x8b3): undefined reference to `nn::AllToAll(int, nn::activationFunction)'

You have declared AllToAll in the top-level namespace and defined it in the nn namespace.

The following will not declare the function in the namespace:

using namespace foo;

extern void Bar();

You need:

namespace foo {
  extern void Bar();
}