Using += operator with float values - c++

Question

I'm trying to implement a operator function to solve the next error :
error: assignment of member 'Animal::weight' in read-only object weight +=amount*(0.02f);
My Animal.cpp function looks like:
void Animal::feed(float amount) const
{
if (type == "sheep"){
amount=amount*(0.02f);
weight+=amount;
}else if (type == "cow"){
weight +=amount*(0.05f);
}else if (type == "pig"){
weight +=amount*(0.1f);
}
return weight;
}
Animal.h (short version):
class Animal
{
public:
Animal(std::string aType, const char *anSex, float aWeight, QDateTime birthday);
float getWeight() const {return weight;};
void setWeight(float value) {weight = value;};
float feed(float amount) const;
void feedAnimal(float amount);
private:
float weight;
};
float operator+=(const float &weight,const float &amount);
Then I implemented a += operator.
float operator+=(const float &weight,const float &amount);
Which is also included then in the .cpp file:
Animal & operator +=(Animal &animal, float amount){
float w = animal.getWeight();
animal.setWeight(w+amount);
}
I worked with a reference so that the weight is update for every animal. So I can call the function feed, and when I want to know the result I do it with the get function:
float getWeight() const {return weight;};
But for some reason I catch the next error :
'float operator+=(const float&, const float&)' must have an argument of class or enumerated type
float operator+=(const float &weight,const float &amount);
Any solutions for this?
For use the feed function I also have a problem. I have my Farm.cpp class where i loop for all the animals in the farm.
void Farm::feedAllAnimals(float amount)
{
for (auto an : animals) {
if(an != nullptr) {
an->feed(amount);
}
}
std::cout << "all animals fed with " << amount << "kg of fodder";
}
And in my .h file I have those functions :
Public:
void feedAllAnimals(float amount);
Private:
std::vector<std::shared_ptr<const Animal>> animals;
My error:
error: passing 'const Animal' as 'this' argument of 'float Animal::feed(float)' discards qualifiers [-fpermissive] an->feed(amount);
^

Answer 1

You declared function feed as a const member function
void feed(float amount) const;
^^^^^
You may not change the object if it is a constant object.
As for operator
float operator+=(const float &weight,const float &amount);
then you may not overload operators for fundamental types.
I think you mean either the following
Animal & operator +=( Animal &animal, float amount);
For example
Animal & operator +=( Animal &animal, float amount)
{
animal.setWeight( animal.getWeight() + amount );
return animal;
}
or an operator declared as a member function within the class like
Animal & operator +=( float amount );
As for the vector then the template parameter must be without qualifier const if you are going to change objects pointed to by the elements of the evctor
std::vector<std::shared_ptr<Animal>> animals;