error: passing 'const Player' as 'this' argument discards qualifiers [-fpermissive]|

Question

I am doing a school project on C++ and I have an interface defined so I can't change the parameters and types of functions. I am having a problem with this error: error: passing 'const Player' as 'this' argument discards qualifiers [-fpermissive]. I read other questions on stackoverflow about this, but they didn't seem to work. There are 2 classes in this problem. Player extends Character and has abilities. There should be a function that adds abilities to the vector array:

Player& addAbility(const Ability& ability){
    abilities.push_back(ability);

    return *this;
}

The function type and arguments cannot be changed.

After that, there should be a function to get all the abilities:

const vector<Ability>& getAbilities(){
    return abilities;
}

The Ability class looks like this:

class Ability{
public:
    Ability(const string& _name, const float _damage){
        name = _name;
        damage = _damage;
    }
    float getDamage(){
        return damage;
    }

    void setDamage(const float _damage){
        damage = _damage;
    }

    const string& getName(){
        return name;
    }

    void setName(const string& _name){
        name = _name;
    }

private:
    string name;
    float damage;

};

The function type and arguments cannot be changed.

In the class PlayerHelper there should be a function to sum up all the damage of the abilities:

static float getTotalPlayerDamage(const Player& player){
    float sum = 0;
    const vector<Ability>& abilities = player.getAbilities();
    
    for(int i=0;i<abilities.size();i++){
        sum += abilities[i].getDamage();
    }

    return sum;
}

The error is on this line:

const vector<Ability>& abilities = player.getAbilities();

I tried const_cast but it didn't seem to work either. Any help would be greatly appreciated!

The `getAbilities` function needs to be const. `const vector& getAbilities() const { ... }` — Retired Ninja, May 23 '22 at 16:03
`const vector& getAbilities() const{` is the trick. The const after the declaration tells, that the function will not modify it`s object. — Klaus, May 23 '22 at 16:04
"I tried const_cast" - Note to your future self: Usage of `const_cast` is *highly* situational, and "trying" it is almost *always* an indicator of a problem in itself. If you find yourself thinking "I'lll just use const_cast", you're probably already halfway off the rails and hurtling to the river at the bottom of the gorge. — WhozCraig, May 23 '22 at 16:12
@user4581301: Well, it said the function's type and arguments can't be changed. It didn't say anything about CV qualifiers. — Fred Larson, May 23 '22 at 16:12
Does this answer your question? https://stackoverflow.com/questions/5973427/error-passing-xxx-as-this-argument-of-xxx-discards-qualifiers — user253751, May 23 '22 at 16:20

score 1 · Answer 1 · answered May 23 '22 at 16:06

The problem is that player is a const Player& but getAbilities is a non-const member function and thus cannot be called on a const Player.

To solve this, you can make getAbilities a const member function as shown below:

//------------------------------------vvvvv---->const added here
const vector<Ability>& getAbilities() const{
    return abilities;
}

error: passing 'const Player' as 'this' argument discards qualifiers [-fpermissive]|

1 Answers1