c++ - Using += operator with float values -

i'm trying implement operator function solve 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 implemented += operator.

float operator+=(const float &weight,const float &amount); 

which included in .cpp file:

 animal & operator +=(animal &animal, float amount){     float w = animal.getweight();     animal.setweight(w+amount);     } 

i worked reference weight update every animal. can call function feed, , when want know result function:

float getweight() const {return weight;}; 

but reason catch next error :

 'float operator+=(const float&, const float&)' must have argument of class or enumerated type  float operator+=(const float &weight,const float &amount); 

any solutions this?

for use feed function have problem. have farm.cpp class loop animals in farm.

void farm::feedallanimals(float amount)   {   (auto : animals) {           if(an != nullptr) {                  an->feed(amount);           }    }   std::cout << "all animals fed " << amount << "kg of fodder";   } 

and in .h file have functions :

public:     void feedallanimals(float amount);  private:  std::vector<std::shared_ptr<const animal>> animals; 

my error:

error: passing 'const animal' 'this' argument of 'float animal::feed(float)' discards qualifiers [-fpermissive] an->feed(amount);                              ^ 

you declared function feed const member function

void feed(float amount) const;                         ^^^^^ 

you may not change object if constant object.

as operator

float operator+=(const float &weight,const float &amount); 

then may not overload operators fundamental types. think mean either following

animal & operator +=( animal &animal, float amount); 

for example

animal & operator +=( animal &animal, float amount) {     animal.setweight( animal.getweight() + amount );      return animal; } 

or operator declared member function within class like

animal & operator +=( float amount ); 

as vector template parameter must without qualifier const if going change objects pointed elements of evctor

std::vector<std::shared_ptr<animal>> animals;