sept_25.md

More I/O, stream

Strings in C++

• C used null terminated char arrays
  • Manual memory management
• C++ provides a header
  • Automatic resizing as needed
  • string s = "hello";
    • string s{"Hello"}:
    • "hello" is a C style string since it is just an array of char
  • Compare with ==, ~=, <, >, <=, >=
  • Length: str.length();
  • Characters: str[0]; (Same as C with char[])
  • cin will stop after reading seeing a space
  • getline(cin, s) will get the entire line until a \n is encountered

I/O manipulators (iostream, iomanip)

• Print x in hexadecimal
  • Remember the code is evaluated left-to-right
  • cout << hex << x;
  • Change how cout works
    • All future cout will print in hex
  • cout << dec;
    • Change cout back to decimal
  • cout << showpoint << setprecision(3);
    • Print to three decimal points

Reading/writing files

• The stream abstraction can work on other sources of data
• Header <fstream>
  • ifstream - read from files
  • ofstream - write to files
• Anything we can do with cin(type istream) we can do with myfile(type ifstream)
  • Same for cout & variable of type ofstream
• When is this file close?
  • myfile is stack allocated and thus destroyed when it goes out of scope
  • When it is destroyed, it closes the file

Reading/writing to strings

• Header <sstream>
  • istringstream -- read from strings
  • ostringstream -- write to strings
• buildStrings.cc
  • Using ostringstream and writing to a string

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main () {
  ostringstream ss;
  int lo {1}, hi {100};
  ss << "Enter a # between " << lo << " and " << hi;
  string s {ss.str()};
  cout << s << endl;
}

• getNum.cc
  • Insist user inputs a number

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

int main () {
  int n;
  while (true) {
    cout << "Enter a number:" << endl;
    string s;
    cin >> s;
    // if (cin == EOF) return;
    istringstream ss{s};
    if (ss >> n) break;
    cout << "I said, ";
  }
  cout << "You entered " << n << endl;
}

• readIntsSS.cc

#include <iostream>
#include <sstream>
using namespace std;

int main () {
  string s;
  while (cin >> s) {
    istringstream ss{s};
    int n;
    if (ss >> n) cout << n << endl;
  }
}

Default Arguments

• New in C++

void printFile(string file = "myfile.txt") {
  ifstream f {file};
}

printFile();
printFile("hello.txt"):

• Parameters with default arguments must appear last
  • void test(int num = 5, string str) {}
    • Illegal
  • void test(int num = 5, string str = "foo") {}
    • Legal
    • test(10, "bar");
    • test(10);
    • test();
    • ^ Legal
    • This is illegal: test("bar"); and test(,"bar");
  • Functions can have the same name in C++
    • Function overloading
    • Looks at function signature (looks at parameters types and/or number of parameters)
    • Differing on return type is not enough
    • default args act as if the function with that param only was implemented so it conflicts if you try to overload it

int negate(int i) { return -i; }
bool negate(bool b) { return !b; }