L4.Streams

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Streams

A stream is an abstraction for input/output.

You can think of it as a source(input) or destination(output) of characters of indefinite length.

The Idea Behind Streams

How does the stream work?

• Input from user is in text form(string).
• Output to user is in text form(string).
• Intermediate computation needs to be done in object form.

Types of Streams

Output Streams

Can only receive data.

• The std::cout stream is an example of an output stream.
• All output streams are of type std::ostream.

We can use a std::ostream for more than just printing to a console.

We can send the data to a file using a std::ofstream,which is a special type of std::ostream.

Input Streams

Can only give data.

• The std::cin stream is an example of an output stream.
• All output streams are of type std::istream.

We can use a std::istream for more than just printing to a console.

We can read the data from a file using a std::ifstream,which is a special type of std::istream.

Think that when we use >> to read data from the shell,we will only get a single word,not the whole line.

So if we want to read a whole line,we can use std::getline() function.

Stream Internals

We know that writing to a console/file is a slow operation. If the program had to write each character immediately,it would be very slow.

So what we do is to write to a buffer.

• Accumulate characters in a buffer.
• When buffer is full,write out all contents of the buffer to the output device at once.

State Bits

Stream have four bits to indicate their state. - G(Good bit):ready for read/write. - F(Fail bit):previous operation failed,all future operations frozen. - E(EOF bit):previous operation reached the end of buffer content. - B(Bad bit):external error,likely irrecoverable.

When we read the data,we have the following 4 steps:

1. Read data.
2. Check if data is vaild,if not break.
3. Use data.
4. Go back to step 1.

while(true) {
    stream >> temp;
    if(stream.fail()) break;
    do_something(temp);
}

If we convert streams to bool,we get the following:

while(true) {
    stream >> temp;
    if(!stream) break;
    do_something(temp);
}

We konw that stream >> temp returns the stream itself,so we can use it as a condition.

while(true) {
    if(!(stream >> temp)) break;
    do_something(temp);
}

One more step,we can simplify the logic:

while(stream >> temp) {
    do_something(temp);
}

Stream Manipulators

There are some special functions that can be used to modify the behavior of the stream.

What we are familiar with is std::endl,which is used to end the line.

Stream Manipulators

CommonNumericPadding

• endl : insert a newline and flushed the stream.
• ws : skips all whitespace until it finds another char.
• boolalpha : print true or false for bools.

• boolalpha : print true or false for bools.
• hex : print numbers in hex.
• setprecision : adjust the precision numbers print with.

• setw : set the width of the next output.
• setfill : set the fill character for padding.

For example:

Example

example1example2example3example4

codeOutput

std::cout << "[" << std::setw(10) << "Hi" << "]" << std::endl;

[        Hi]

codeOutput

std::cout << "[" << std::left << std::setw(10) << "Hi" << "]" << std::endl;

[Hi        ]

codeOutput

std::cout << "[" << std::left << std::setfill('-') << std::setw(10) << "Hi" << "]" << std::endl;

[Hi--------]

codeOutput

std::cout << std::hex << 10;
std::cout << std::oct << 10;
std::cout << std::dec << 10;

a
12
10

If you are interested in the details of the manipulators,you can refer to the manipulators.

Std::stringstream

We can use std::stringstream to convert between strings and other types.

#include <sstream>

int main() 
{
    std::string line = "137 2.718 Hello";
    std::stringstream stream(line);

    int myInt;
    double myDouble;
    std::string myString;
    stream >> myInt >> myDouble >> myString;

    std::cout << myInt << std::endl;
    std::cout << myDouble << std::endl;
    std::cout << myString << std::endl;
}

When to use stringstream

• If you need error checking for user input,best practice is to:
  • use getline to retrieve a line from cin.
  • create a stringstream with the line.
  • parse the line using a stringstream,usually with >>.
• Use state bits to control streams and perform error-checking.
  • fail bit can check type mismatches.
  • eof bit can check if you consumed all input.

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