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Home | >> Linear Collections and Modularity

Problem 1: Program Input/Output

2017-09-07

  • Read 2.2, 4.3
  • running a program from the command line

./program-name or path/to/programe-name

Note: . means current directory

Providing input: 2 ways

  1. ./program-name arg1 arg2 ... argn

    • args are written into the program's memory
    • [code][n + 1 (argc)][arg1][arg2][...][argn] (argv)

  2. ./program-name

    • (Then type something)
    • input comes through standard input stream (stdin)
    • keyboard -> program -> stderr (never buffered) OR -> stdout (maybe buffered) -> screen

Redirection ./my-program < infile > outfile 2>errfile

note: 0 and 1 exists respectively, but only 2 is needed since it shares the same operator

Consider (C):

#include <stdio.h>

void echo(FILE *f) {
    for (int c = fgetc(f); c != EOF; c = fgetc(f))
        putchar(c);
    }
}

c is an int and not char because other information cannot be expressed as a char, for example EOF (end of file)

int main(int argc, char *argv[]) {
    if (argc == 1) {
        echo(stdin);
    } else {
        for (int i = 1; i < argc; ++i) {
            FILE *f = fopen(argv[i], "r");
            echo(f);
            fclose(f);   
        }     
    }

    return 0;  // Status code given to shell (echo $?)
}

note:

argv[0] = program-name
...
argv[argc] = NULL

Observe: cmd line args / input from stdin - 2 different programming techniques

To compile: gcc -std=c99 -Wall my program.c -o myprogram gcc translates C source to executable binary -Wall means warn all, gives warning that only appear if asked for -o means what to rename program

This program is a simplification of the linux cat command cat file1 file2 ... filen opens them and prints them one after another

cat - echos stdin cat < file

  • File used as a source for stdin
  • The shell (not cat) opens the file
  • Displays

Can we write the cat program in C++?

  • Already valid C++
  • The "C++" way
    • Command-line args are the same as in C
    • stdin/stdout: #include 
#include <iostream>

int main() {
    int x, y;
    std::cin >> x >> y;
    std::cout << x + y << std::endl;
}

std::cin, std::cout, std::cerr: streams, types std::istream: (cin) std::ostream: (cout, cerr) >> input operator: (cin >> x), populates x as a side-effect, returns cin << output operator: (cout << x + y), prints x + y as a side-effect, returns cout

These operators return cin/cout so they can be chained: From above: std::cin >> x >> y;

File Access

std::ifstream f{"name-of-file"};  // ofstream for output
char c;

while (f >> c) {
    std::cout << c;
}

*f >> c

  • implicity converts to a bool
  • true = read succeded
  • false = read failed

Input: the quick brown fox Output: thequickbrownfox

Stream input slips whitespace (just like scanf)

To include whitespace:

std::ifstream f {"name-of-file"};
f >> std::noskipws;
char c;
...

Note:

  • No explicit calls to fopen/fclose
  • Initializing f {"name-of-file"} opens the file
  • When f's scope ends, the file is closed

Try cat in C++

#include <iostream>

using namespace std;  // Avoids having to say std::

void echo (istream f) {
    char c;
    f >> noskipws;;
    while (f >> c) {
        cout << c;
    }
}

int main(int argc, char *argv[]) {
    if (argc == 1) {
        echo(cin);
    } else {
        for (int i = 1; i < argc; ++i) {
            ifstream f{argv[i]};
            echo(f);
        }
    }
}

Doesn't work, won't even compile!

cin has type istream - echo takes an istream f has type ifstream - is echo(f) as type mismatch?

No! - this is actually fine, ifstream is a subtype of istream

Any ifstream can be treated as an istream

  • Foundational concept in OOP
  • details later

The error is: you can't write a function that takes an istream the way that echo does

Why not?

Compare:

int x;
scanf("%d", &x);

vs

int x;
cin >> x;

C and C++ are pass-by-value languages, so scanf needs the address of x in order to change x via pointers

So why is it not cin >> &x? C++ has another small pointer-like type

References

int y = 10
int &z = y;
  • z is an lvalue reference to y
  • Similar to int *const z = y; but with auto-dereferencing

z = 12;

NOT *z = 12; y is now 12

int *p = &z; // Gives the address of y

In all cases, z acts as if it were y z is an alias ("another name for y")

lvalue references must be initialized to something that has an address

int &z = 4; WRONG int &z = a + b; WRONG

  • Cannot create a pointer to a reference: int &*x;
  • Read right to left:

    • x is a pointer to a reference of an int

  • However you can create a reference to a pointer: int *&x = __;
  • Create a reference to a reference: int &&r = z; (compiles but means something else)
  • Create an array of references: int &r[3] = {...};
  • You can use as function parameters:
void inc(int &n) {
    ++n;
}

int x = 5;
inc(x);

cout << x; // 6

Note: cannot use inc(5) because references require objects with addresses (ie. literals do not)

cin >> x works because x is passed by reference.

istream& operator >> (istream &in, int &n);

Now consider struct ReallyBig {...};

int f(ReallyBig rb) {
    ...
}

Note: struct ReallyBig is not necessary in C++ unlike in C

The case above uses passes by value which copies the huge struct -> this is really slow!

Instead let's pass by reference which doesn't copy it -> this is fast! But this can be dangerous as the function may propogate changes to the caller.

int g (ReallyBig &rb) {
    ...
}

So finally we can use constant references, this is fast and safe!

int h(const ReallyBig &rb) {
    ...
}

Prefer pass-by-const-ref over pass-by-value for anything larger than a pointer, unless the function needs to make a copy anyways.

Also:

int f(int &n) {
    ...
}

int g(const int &n) {
    ...
}

f(5) - (BAD) can't initialize an lvalue reference n to a literal value 5. g(5) - (GOOD) since n can never be changed, the compiler allows this (stored in some temp location so n has something to point to)

Back to cat:

void echo(istream f) {
    ...
}
  • f is passed by value, istream is copied
  • Copying streams is not allowed (C++ has mechanisms to prevent copying)

Works if you pass the stream by reference:

void echo (istream &f) {
    f >> noskipws;
    char c;

    while (f >> c) {
        cout << c;
    }
}

To compile:

  • g++ -std=c++14 -Wall mycat.cc -o mycat
  • OR g++14 mycat.cc -o mycat
  • ./mycat

Separate compilation

Put echo in its own module

echo.h

void echo (istream &f);

echo.cc

#include "echo.h"

void echo(istream &f) {
    ...
}

main.cc

#include <iostream>
#include <fstream>
#include "echo.h"

int main(...) {
    echo(cin);
    ...
    echo(f);
}

Compiling separately:

  • g++14 echo.cc (fails)
    • linking error: no main
  • g++14 main.cc (fails)
    • linking error: no echo

Correct:

g++14 -c echo.cc -> creates echo.o g++14 -c main.cc -> creates main.o (these are object files)

-c indicates only compile, don't link

g++14 echo.o main.o -o mycat (linker)

Advantage:

  • Only have to recompile the parts you change, then relink (no so expensive)
    • Ex. Change echo.cc -> recomplie echo.cc -> relink
  • However if you change echo.h, you must recompile echo.cc and main.cc and relink
    • This is because both files include echo.h
  • What if we don't remember what we changed or what depends on what?
    • Linux tool: make
    • Create a Makefile
mycat: main.o echo.o
    g++ main.o echo.o -o mycat

main.o: main.cc echo.h
    g++ -std=c++14 -Wall -c main.cc

echo.o: echo.cc echo.h
    g++ -std=c++14 -Wall -c echo.cc

.PHONY: clean

clean:
    rm mycat main.o echo.o

Note: cannot use aliases, requires TABS not SPACES

  • targets: mycat, main.o
  • dependencies: everything right of colon
  • recipes: tabbed information

How make works: list a dir in long form ls -l

-rw-r----- 1 j2smith j2smith 25 Sep 9 15:27 echo.cc

  • Based on last modified time
  • Starting at the leaves of the dependency graph
    • if the dependency is newer than the target, rebuild the target ... and so on, up to the root target

ex. echo.cc newer than echo.o -rebuild echo.o echo.o newer than mycat - rebuild mycat

Shortcuts - use variables:

CXX = g++ (name of compiler)
CXXFLAGS = -std=c++14 -Wall
EXEC = myprogram
OBJECTS = main.o echo.o

${EXEC}: ${OBJECTS}
    ${CXX} ${OBJECTS} -o ${EXEC}

main.o: main.cc echo.h
echo.o: echo.cc echo.h

.PHONY: clean

clean: 
    rm ${OBJECTS} ${EXEC}

*Omit recipes - make "guesses" the right one

Writing the dependencies still hard (but compiler can help).

  • g++14 -c -MMD echo.cc: generates echo.o, echo.d
  • cat echo.d -> echo.o echo.cc echo.h
CXX = g++
CXXFLAGS = -std=c++14 -Wall -MMD
EXEC = mycat
OBJECTS = main.o echo.o

DEPENDS = ${OBJECTS:.o=.d}

${EXEC}: ${OBJECTS}
    ${CXX} ${OBJECTS} -o ${EXEC}

-include ${DEPENDS}

.PHONY: clean

clean:
    rm ${EXEC} ${OBJECTS} ${DEPENDS}

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