CS11 Intro C++: Lecture 3

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File I/O

Use fstream like iostream.

  • ifstream: for reading
  • ofstream: for writing
  • fstream: for both
vector<double> read_data(string filename) {
  ifstream ifs{filename};
  vector<double> data;

  // Make sure the file was opened successfully
  if (!ifs.is_open()) // or if(!ifs)
    throw illegal_argument("Couldn't open file");

  // Read data until we hit EOF
  while (ifs.good()) { // or while(ifs)
    double v;
    ifs >> v;
  return data;

Streams can be used in conditional expressions. (Check the comment above.)

ifstream destructor will automatically close the file when the ifs object goes out of scope. If it need to be closed, there is a member function close().

Variable Scope

A variable’s scope is the part of the program where the variable is accessible.

  • Narrow the scope as small as possible

    • Declare variables when/where you actually need them
    • Reduce bug, name conflicts, etc.
  • When an object variable goes out of scope, its destructor is called automatically (only for non-primitive types)

Function Arguments

Arguments are pased by value as a default. The function receives a copy of the arguments, not the original.

Passing arguments by references will be preferred when the data is too big to copy it.

// use `&`
double compare_something_heavy(vector<double> &values) {

The function can change the original value when it passed by reference.

It is able to specify that argument’s value cannot change by using const modifier.

double compute_average(const vector<double> &values) {
  // Function signature need to be matched declaration and definition


  • Passing an object

    • Pass by reference and state const if there is no modification.
    • Pass by value if the mutation should not affect the caller.
  • Passing a primitive

    • Pass by value because it is generally small and fast enough.
    • Pass by reference if it need to be modified.

const and user-defined classes

Define const on member functions when the member functions are not mutating any data.

class Point {
  // ...
  double get_x() const; // Accessors
  double get_y() const;
  // ...

double Point::get_x() const {
  // ...
// ...

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