C++ - Redefinition/Previously defined error - c++

Question

Why am I getting this error? I'm just making a class object. What am I missing?
Is it because of the empty body methods and constructor? I'm not sure here.
The tutorial im using is from here:
https://www.youtube.com/watch?v=KkwX7FkLfug
Code:
#include <vector>
#include <iostream>
class Neuron {};
typedef std::vector<Neuron> Layer;
class Net
{
public:
Net(const std::vector<unsigned> &topology) {};
void feedForward(const std::vector<double> &inputVals) {};
void backProp(const std::vector<double> &targetVals) {};
void getResults(std::vector<double> &resultVals) const {};
private:
// [layerNum][neuronNum]
std::vector<Layer> m_layers;
};
void Net::Net(const std::vector<unsigned> &topology)
{
unsigned numLayers = topology.size();
for (unsigned layerNum = 0; layerNum < numLayers; ++layerNum){
m_layers.push_back(Layer());
for (unsigned neuronNum = 0; neuronNum <= topology[layerNum]; ++neuronNum){
m_layers.back().push_back(Neuron());
std::cout << "Made a neuron!" << std::endl;
}
}
}
int main(int argc, char *argv[])
{
std::vector<unsigned> topology;
topology.push_back(3);
topology.push_back(2);
topology.push_back(1);
Net myNet(topology);
std::vector<double> inputVals;
myNet.feedForward(inputVals);
std::vector<double> targetVals;
myNet.backProp(targetVals);
std::vector<double> resultVals;
myNet.getResults(resultVals);
return 0;
}
Running:
g++ e:/something/ProgrammingExt/0a_Testing/cpp/neural_network/neural-net-tutorial.cpp
Getting this error:
ERROR (0.39 seconds): e:/something/ProgrammingExt/0a_Testing/cpp/neural_network/neural-net-
tutorial.cpp:20:52: error: return type specification for constructor invalid
void Net::Net(const std::vector<unsigned> &topology)
^
e:/something/ProgrammingExt/0a_Testing/cpp/neural_network/neural-net-tutorial.cpp:20:6: error: redefinition of ’Net::Net(const std::vector<unsigned int>&)’
void Net::Net(const std::vector<unsigned> &topology)
^~~
e:/something/0a_Testing/cpp/neural_network/neural-net-tutorial.cpp:11:3: note: ’Net::Net(const std::vector<unsigned int>&)’ previously defined here
Net(const std::vector<unsigned> &topology) {};
^~~

Answer 1

The error messages tell you why you are getting them.
ERROR (0.39 seconds): [path]/neural-net-tutorial.cpp:20:52: error: return type specification for constructor invalid
void Net::Net(const std::vector<unsigned> &topology)
^
This is the first error. Constructors return nothing, not even void. So drop the void keyword from the indicated line.
[path]/neural-net-tutorial.cpp:20:6: error: redefinition of ’Net::Net(const std::vector<unsigned int>&)’
void Net::Net(const std::vector<unsigned> &topology)
^~~
[path]/neural-net-tutorial.cpp:11:3: note: ’Net::Net(const std::vector<unsigned int>&)’ previously defined here
Net(const std::vector<unsigned> &topology) {};
^~~
This is the second error. You have two definitions of the constructor for Net that takes a const std::vector<unsigned> & parameter. The duplicate is on line 20, the same line that triggered the earlier error about void. The original is on line 11, where you defined the constructor to have an empty body. There is an extraneous semicolon after this definition, which suggests there might have been an intent to convert the definition to a declaration at some point. That point is now. (The 6 and 3 in the error messages are positions within the indicated line. Your compiler decided to mark the errors at the start of the constructor's name.)
Since you apparently do not want this constructor to have an empty body, change the definition on line 11 from
Net(const std::vector<unsigned> &topology) {};
to a declaration by dropping the function body:
Net(const std::vector<unsigned> &topology);

Answer 2

C++ overloading can't be based on return type. So there are two functions with exact same argument and they are treated as the same function:
Net(const std::vector<unsigned> &topology) {};
and
void Net::Net(const std::vector<unsigned> &topology)
{
unsigned numLayers = topology.size();
for (unsigned layerNum = 0; layerNum < numLayers; ++layerNum){
m_layers.push_back(Layer());
for (unsigned neuronNum = 0; neuronNum <= topology[layerNum]; ++neuronNum){
m_layers.back().push_back(Neuron());
std::cout << "Made a neuron!" << std::endl;
}
}
}
That's why you got redefinition error.
Another problem is that constructor doesn't have a return type, but you give it a "void" for the second definition. That's why you got the first error:
tutorial.cpp:20:52: error: return type specification for constructor invalid
void Net::Net(const std::vector &topology)