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Streams

  • Version
    • Lin
    • 2023-01-08
    • learning Streams
    • review

Info

  • What are streams?
  • Output streams
  • Input streams
  • String streams!

What are streams?

Definition

Stream:an abstraction for input/output. Streams convert between data and the string representation of data.

std::cout is an output stream. It has type std::ostream

Two ways to classify streams

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Output Streams?

1.Output Streams

  • Have type std::ostream

  • You can only send data to the stream

    • Interact with the stream using the << operator

    • Converts any type into string and sends it to the stream

  • std::cout is the output stream that goes to the console

std::cout << 5 << std::endl; 
 // converts int value 5 to string “5”
 // sends “5” to the console output stream

2.Output File Streams

  • Have type std::ofstream

  • You can only send data to file using the << operator

    • Converts data of any type into a string and sends it to the file stream

  • Must initialize your own ofstream object linked to your file 

    std::ofstream out(“out.txt”);
// out is now an ofstream that outputs to out.txt
out << 5 << std::endl; // out.txt contains 5

std::cout is a global constant object that you get from #include

To use any other output stream, you must first initialize it!

Input streams

Definition

  • Have type std::istream
    • You can only receive strings using the >> operator
      • Receives a string from the stream and converts it to data
    • std::cin is the input stream that gets input from the console

>> << operator

  • “>>” is the stream extraction operator or simply extraction operator
    • Used to extract data from a stream and place it into a variable
  • “<<” is the stream insertion operator or insertion operator
    • Used to insert data into a stream usually to output the data to a file, console, or string

std::cin

std::cin is an input stream. It has type std::istream

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Example

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Example

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std::getline()

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How it works:

  • Clears contents in str

  • Extracts chars from is and stores them in str until:

    • End of file reached, sets EOF bit (checked using is.eof())

    • Next char in is is delim, extracts but does not store delim

    • str out of space, sets FAIL bit (checked using is.fail())

    • If no chars extracted for any reason, FAIL bit set

Tip

  • In contrast,">>" only reads until it hits whitespace(so can't read a sentence in one go)

  • But ">>" can convert data to built-in types (like ints) while getline can only produce strings

  • And ">>" only stops reading at predefined whitespace while getline can stop reading at any delimiter you define

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下一次cin才会把whitespace comsume掉

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Input File Streams

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std::cin is a global constant object that you get from #include iostream

Stringstreams

Definition

A stream that can read from or write to a string object

Allows you to perform input/output operations on a string as if it were a stream 

std::string input = "123";
std::stringstream stream(input);
int number;
stream >> number;
std::cout << number << std::endl; // Outputs "123"

Example

If you only want to read OR write data:

Read only: std::istringstream

  • Give any data type to the istringstream, it’ll store it as a string!

Write only: std::ostringstream

  • Make an ostringstream out of a string, read from it word/type by word/type!

Follows same patterns as the other i/ostreams!

#include <iostream>
#include <fstream>
#include <sstream>

int main() {
    std::ifstream file("data.txt");
    std::string line;
    int lineNum = 1;
    while (std::getline(file, line)) {
        std::stringstream ss(line);
        int a, b, c;
        ss >> a >> b >> c;
        std::cout << "The sum of the numbers on line " <<  lineNum << " is: " << a + b + c << std::endl;
        lineNum++;
    }

    return 0;
}
/*
data.txt
1 2 3
4 5 6
7 8 9
*/

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Recap

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