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123 Str operations
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If we think about the kind of code we've written that used strings, we can see that we used several operators:
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cin >> s cout << s s[i] s1 + s2 // // // // use use use use the the the the input operator to read a string output operator to write a string index operator to access a character addition operator to concatenate two strings
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All these are binary operators, so that if we define them as functions, each function will have two parameters, one of which may be implicit if the function is a member As we saw in 1124/192, names for overloaded operators are formed by appending the operator symbol to the word operator Hence, operator>> is the name of the function that overloads the input operator, operator[] names the index operation, and so on We may as well start with the index operator, because in 1124/192, we've already seen how to implement this operation, and we know that it must be a member of the class:
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class Str { public: // constructors as before char& operator[](size_type i) { return data[i]; } const char& operator[](size_type i) const { return data[i]; } private: Vec<char> data; };
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The index operators just forward their work to the corresponding Vec operations It is worth noting that, as we did for class Vec, we define two versions of the index operator one that can operate on const objects and the other that cannot By returning a reference to the character, the nonconst version gives write access to the character that it returns The const version returns a reference to a const char, thereby preventing the user from writing to the underlying character We return const char& instead of a plain char for consistency with the standard string class What about the other operators The most interesting problem in defining these functions is deciding whether these operations should be members of the Str class It turns out that answering this question involves different issues for each of these kinds of operators We will address input-output operators first; then, in 1233/218, we'll look at the concatenation operator
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1231 Input-output operators
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In 922/159, we had to decide whether compare should be a member of Student_info We suggested that one way to decide was to ask whether the operation affects the state of the object The input operator certainly changes its object's state After all, we use the input operator to read a new value into a preexisting object Accordingly, we might think that we should make the input operator a member of the Str class However, doing so won't work as we might expect To see why, we have to remember ( 1124/192) how the operands of an expression are bound to the parameters of the overloaded operator function For a binary operation, the left operand is always bound to the first parameter, and the right operand is bound to the second In the case of member operator functions, the first parameter (the left operand) is always the one that is passed implicitly to the member function Thus,
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cin >> s;
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is equivalent to
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cinoperator>>(s);
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which calls the overloaded >> operator defined for the object cin This behavior implies that the >> operator must be a member of class istream Of course, we do not own the definition of istream, so we cannot add this operation to it If instead we make operator>> a member of Str, then our users would have to invoke the operation on behalf of a Str:
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soperator>>(cin);
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or, equivalently,
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s >> cin;
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which would flout the conventions used throughout the library Therefore, we can conclude that the input and, by analogy, the output operator must be a nonmember We can now update our Str class appropriately, by adding declarations for the input and output operators to Strh:
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std::istream& operator>>(std::istream&, Str&); std::ostream& operator<<(std::ostream&, const Str&) ; // added // added
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The output operator is easy to write: It will iterate through the Str, writing a single character at a time:
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ostream& operator<<(ostream& os, const Str& s) { for (Str::size_type i = 0; i != ssize(); ++i) os << s[i]; return os; }
The only catch is that this usage forces us to give Str a size function:
class Str { public: size_type size() const { return datasize(); } // as before };
Despite the simple form of the output operator, we should understand it thoroughly Each time through the loop, we invoke the Str::operator[] to fetch a character to write That operator, in turn, calls Vec::operator[] to obtain the actual value from the underlying vector Similarly, each time through the loop, we determine the size of our Str object by calling ssize(), which calls the size member of the underlying Vec object to determine that object's size