Effective C++ Item 3 Use const whenever possible

浏览数：101 / 时间：2015年06月08日

1. const return value for overload function

?
class Rational;
 
const Rational operator*(const Rational &lhs, const Rational &rhs);

this kind of code can prevent error like this

?
Rational a, b, c;
(a*b) = c;

I know that you want to use "==" other than "=", but compiler would happily generate code for you and maybe you spend hours to debug. All you need to do is define return value const.

 

2. const member functions

Let‘s see an example

?
class TextBlock {
public:
    ...
    const char& operator[](std::size_t position) const {
        return text[position];
    }
 
    char & operator[](std::size_t position) {
        return text[position];
    }
 
private:
    std::string text;
};

and then to illustrate how const member function works

?
std::cout << tb[0];    // Fine call non-const TextBlock
tb[0] = ‘x‘;    // Fine write non-const TextBlock
std::cout << ctb[0]; // Fine read const TextBlock
ctb[0] = ‘x‘; //error

Be careful that non-const version operatpr[] returns a reference to char not char itself. If member function do return a char, then compile will report error when interpring tb[0] = ‘x‘, because you can‘t change an internal type (char, int, double..). And even compile successfully, tb[0] is just a copy of original object, that won‘t be what you want.

 

3. To avoid duplication code in const and non-const member functions

Typically, corresponding const and non-const have similar functions, and to avoid duplications, we make non-const version of member function call const version with certain cast

?
class TextBlock {
public:
    ...
    const char& operator[](std::size_t position) const {
        ...
        return text[position];
    }
 
    char & operator[](std::size_t position) {
        ...
        return const_cast<char &>
            (static_cast<const TextBlock&> (*this))
                [position];
         
    }
 
private:
    std::string text;
};

As you can see, above has two casts, not one. We want the non-const member function call const member function. But if inside the non-const [] we call [], we recursivly call ourselves. So we need to cast (*this) to const first, and then const_cast is used to remove const of return value

 

4. Use mutable keyword to remove non-static variable of the constaints of constness

?
class TextBlock {
private:
    std::string text;
    mutable std::size_t len;
};

Now mutable can be changed inside a const member function

Effective C++ Item 3 Use const whenever possible,古老的榕树,5-wow.com

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