problem_6.md

Moves << | Home | >> Tampering

Problem 6: I want a constant vector

2017-09-26

Say we want to print a vector:

ostream &operator <<(ostream &out, const Vector &v) {
    for (size_t i = 0; i < v.size(), ++i) {
        out << v.itemAt(i) << " ";
    }
}

WON'T COMPILE!!! (lushman pls)

• Can't call size() and itemAt() on a const object. What if these methods change fields?
• Since they don't, declare them as const

struct vector {
    ...
    size_t size() const;    // Means these methods will not modify fields
    int &itemAt(size_t i) const;    // Can be called on const objects
    ...
};

size_t vector::size() const {return n};
int &vector:itemAt(size_t i) const {return theVector[i];}

Now the loop will work.

BUT:

void f(const vector &v) {
    v.itemAt(0) = 4;    // Works!! v not very const...
}

• v is a const object - cannot change n, cap, theVector (ptr)
• You can changed items pointed to by theVector

Can we fix this?

struct vector {
    ...
    const int &itemAt(size_t i) const;
};

const int &itemAt(size_t i) const {
    return theVector[i];
}

Now v.itemAt(0) = 4 won't compile if v is const, but it also won't compile if v is not const

To fix: const overloading

struct vector {
    ...
    const int &itemAt(size_t i) const;  // Will be called if the object is const
    int &itemAt(size_t i);  // Will be called if object is non-const
};

inline const int &Vector::itemAt(size_t i) const {return theVector[i];}
inline int &vector::itemAt(size_t) {return theVector[i]};

Putting in inline tells the compile to replace the function call with the function body to save the cost of having to call a function

Merely a suggestion, compiler can choose to ignore it if it sees fit. Good idea for small functions

So now v.itemAt(0) = 4; will only compile if and only if v is non-const

Now let's make it prettier:

struct vector {
    size_t size() const {return n;} // Method body inside class implcity declares the method inline
    const int &operator[](size_t i) const {return theVector[i]};
    int &operator[](size_t i) {return theVector[i];}    
};

ostream &operator<<(ostream &out, const vector &v) {
    for (size_t i = 0; i < v.size(); ++i) {
        out << v[i] << " ";
    }

    return out;
}

---

Moves << | Home | >> Tampering