Student_info s; Student_info s2(cin); // // an empty Student_info initialize s2 by reading from cin in Software

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Student_info s; Student_info s2(cin); // // an empty Student_info initialize s2 by reading from cin
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Constructors are distinguished from other member functions in two ways: They have the same name as the name of the class itself, and they have no return type Constructors are similar to other functions in that we can define multiple versions of constructors that differ in terms of the number or type of their arguments With this knowledge, we might update our class to add our two constructors:
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class Student_info { public: Student_info() Student_info(std::istream&); // as before };
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// construct an empty Student_info object // construct one by reading a stream
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951 The default constructor
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The constructor that takes no arguments is known as the default constructor Its job is normally to ensure that its object's data members are properly initialized In the case of Student_info objects, we want to initialize the data to indicate that we haven't yet read a record: We'll want the homework member to be an empty vector, the n member to be an empty string, and the midterm and final members to be initialized to zero:
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Student_info::Student_info(): midterm(O), final(0) { }
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The constructor definition uses some new syntax Between the : and the { is a sequence of constructor initializers, which tell the compiler to initialize the given members with the values that appear between the corresponding parentheses Therefore, this particular default constructor works by explicitly setting the midterm and final grades to 0 Other than that, the constructor does no overt work: The body of the function is empty As we shall see, n and homework are implicitly initialized Understanding constructor initializers is crucial to understanding how to create and initialize objects When we create a new class object, several steps happen in sequence: 1 The implementation allocates memory to hold the object 2 It initializes the object, as directed by the constructor's initializer list 3 It executes the constructor body The implementation initializes every data member of every object, regardless of whether the constructor initializer list mentions those members The constructor body may change these initial values subsequently, but the initialization happens before the constructor body begins execution It is usually better to give a member an initial value explicitly, rather than assigning to it in the body of the constructor By initializing rather than assigning a value, we avoid doing the same work twice We said that constructors exist to ensure that objects are created with their data members in a sensible state In general, this design goal means that every constructor should initialize every data member The need to give members a value is especially critical for members of built-in type If the constructor fails to initialize such members, objects declared at local scope will be initialized with garbage, which is almost never correct We can now understand why we said that the Student_info default constructor did no other "overt" work Although we explicitly initialized only midterm and final, the other data members are initialized implicitly Specifically, n is initialized by the string default constructor, and homework is initialized by the vector default constructor
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952 Constructors with arguments
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Our second Student_info constructor is even easier:
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This constructor delegates the real work to the read function The constructor has no explicit initializer, so the homework and n members will be initialized by the default constructors for vector and string respectively The midterm and final members will have explicit initial values only if the object is being value-initialized This lack of initialization doesn't matter, because read immediately gives these variables new values
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96 Using the Student_info class
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Our new Student_info class is now quite different from the original Student_info structure from 4 Not surprisingly, using the class is different from using the original structure After all, our objective was to prevent users from being able to change our data values, which we accomplished by making them private Instead, we intend for users to write their programs in terms of the interface provided by our class As an example, we can rewrite our original main program from 45/70, which wrote the final grades for the students in a formatted report, to use this version of the class:
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int main() { vector<Student_info> students; Student_info record; string::size_type maxlen = 0; // read and store the data while (recordread(cin)) { maxlen = max(maxlen, recordname()size()) studentspush_back(record); } // alphabetize the student records sort(studentsbegin(), studentsend(), compare); // write the names and grades for (vector<Student_info>::size_type i = 0; i != studentssize(); ++i) { cout << students[i]name() // changed << string(maxlen + 1 - students[i]namesize(), ' '); try { double final_grade = students[i]grade(); // changed streamsize prec = coutprecision(); cout << setprecision(3) << final_grade << setprecision(prec) << endl; } catch (domain_error e) { cout << ewhat() << endl; } } return 0; }
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// //
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