DevTut

# C++ Streams

# String streams

std::ostringstream (opens new window) is a class whose objects look like an output stream (that is, you can write to them via operator<<), but actually store the writing results, and provide them in the form of a stream.

Consider the following short code:

#include <sstream>
#include <string>                                                                                                                          

using namespace std;

int main()
{
    ostringstream ss;
    ss << "the answer to everything is " << 42;
    const string result = ss.str(); 
}

The line

ostringstream ss;

creates such an object. This object is first manipulated like a regular stream:

ss << "the answer to everything is " << 42;

Following that, though, the resulting stream can be obtained like this:

const string result = ss.str();

(the string result will be equal to "the answer to everything is 42").

This is mainly useful when we have a class for which stream serialization has been defined, and for which we want a string form. For example, suppose we have some class

class foo 
{   
    // All sort of stuff here.
};  

ostream &operator<<(ostream &os, const foo &f);

To get the string representation of a foo object,

foo f;

we could use

ostringstream ss; 
ss << f;
const string result = ss.str();

Then result contains the string representation of the foo object.

# Printing collections with iostream

# Basic printing

std::ostream_iterator allows to print contents of an STL container to any output stream without explicit loops. The second argument of std::ostream_iterator constructor sets the delimiter. For example, the following code:

std::vector<int> v = {1,2,3,4};
std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " ! "));

will print

1 ! 2 ! 3 ! 4 !

# Implicit type cast

std::ostream_iterator allows to cast container's content type implicitly. For example, let's tune std::cout to print floating-point values with 3 digits after decimal point:

std::cout << std::setprecision(3);
std::fixed(std::cout);

and instantiate std::ostream_iterator with float, while the contained values remain int:

std::vector<int> v = {1,2,3,4};
std::copy(v.begin(), v.end(), std::ostream_iterator<float>(std::cout, " ! "));

so the code above yields

1.000 ! 2.000 ! 3.000 ! 4.000 !

despite std::vector holds ints.

# Generation and transformation

std::generate, std::generate_n and std::transform functions provide a very powerful tool for on-the-fly data manipulation. For example, having a vector:

std::vector<int> v = {1,2,3,4,8,16};

we can easily print boolean value of "x is even" statement for each element:

std::boolalpha(std::cout); // print booleans alphabetically
std::transform(v.begin(), v.end(), std::ostream_iterator<bool>(std::cout, " "),
[](int val) {
    return (val % 2) == 0;
});

or print the squared element:

std::transform(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "),
[](int val) {
    return val * val;
});

Printing N space-delimited random numbers:

const int N = 10;
std::generate_n(std::ostream_iterator<int>(std::cout, " "), N, std::rand);

# Arrays

As in the section about reading text files, almost all these considerations may be applied to native arrays. For example, let's print squared values from a native array:

int v[] = {1,2,3,4,8,16};
std::transform(v, std::end(v), std::ostream_iterator<int>(std::cout, " "),
[](int val) {
    return val * val;
});

# Reading a file till the end

# Reading a text file line-by-line

A proper way to read a text file line-by-line till the end is usually not clear from ifstream documentation. Let's consider some common mistakes done by beginner C++ programmers, and a proper way to read the file.

# Lines without whitespace characters

For the sake of simplicity, let's assume that each line in the file contains no whitespace symbols.

ifstream has operator bool(), which returns true when a stream has no errors and is ready to read. Moreover, ifstream::operator >> returns a reference to the stream itself, so we can read and check for EOF (as well as for errors) in one line with very elegant syntax:

std::ifstream ifs("1.txt");
std::string s;
while(ifs >> s) {
    std::cout << s << std::endl;
}

# Lines with whitespace characters

ifstream::operator >> reads the stream until any whitespace character occurs, so the above code will print the words from a line on separate lines. To read everything till the end of line, use std::getline instead of ifstream::operator >>. getline returns reference to the thread it worked with, so the same syntax is available:

while(std::getline(ifs, s)) {
    std::cout << s << std::endl;
}

Obviously, std::getline should also be used for reading a single-line file till the end.

# Reading a file into a buffer at once

Finally, let's read the file from the beginning till the end without stopping at any character, including whitespaces and newlines. If we know the exact file size or upper bound of the length is acceptable, we can resize the string and then read:

s.resize(100);
std::copy(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>(),
    s.begin());

Otherwise, we need to insert each character to the end of the string, so std::back_inserter is what we need:

std::copy(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>(),
    std::back_inserter(s));

Alternatively, it is possible to initialize a collection with stream data, using a constructor with iterator range arguments:

std::vector v(std::istreambuf_iterator<char>(ifs),
    std::istreambuf_iterator<char>());

Note that these examples are also applicable if ifs is opened as binary file:

std::ifstream ifs("1.txt", std::ios::binary);

# Copying streams

A file may be copied to another file with streams and iterators:

std::ofstream ofs("out.file");
std::copy(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>(),
    std::ostream_iterator<char>(ofs));
ofs.close();

or redirected to any other type of stream with a compatible interface. For example Boost.Asio network stream:

boost::asio::ip::tcp::iostream stream;
stream.connect("example.com", "http");
std::copy(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>(),
    std::ostream_iterator<char>(stream));
stream.close();

# Arrays

As iterators might be thought of as a generalization of pointers, STL containers in the examples above may be replaced with native arrays. Here is how to parse numbers into array:

int arr[100];
std::copy(std::istream_iterator<char>(ifs), std::istream_iterator<char>(), arr);

Beware of buffer overflow, as arrays cannot be resized on-the-fly after they were allocated. For example, if the code above will be fed with a file that contains more than 100 integer numbers, it will attempt to write outside the array and run into undefined behavior.

# Parsing files

# Parsing files into STL containers

istream_iterators are very useful for reading sequences of numbers or other parsable data into STL containers without explicit loops in the code.

Using explicit container size:

std::vector<int> v(100);
std::copy(std::istream_iterator<int>(ifs), std::istream_iterator<int>(),
    v.begin());

or with inserting iterator:

std::vector<int> v;
std::copy(std::istream_iterator<int>(ifs), std::istream_iterator<int>(),
    std::back_inserter(v));

Note that the numbers in the input file may be divided by any number of any whitespace characters and newlines.

# Parsing heterogeneous text tables

As istream::operator>> reads text until a whitespace symbol, it may be used in while condition to parse complex data tables. For example, if we have a file with two real numbers followed by a string (without spaces) on each line:

1.12 3.14 foo
2.1 2.2 barr

it may be parsed like this:

std::string s;
double a, b;
while(ifs >> a >> b >> s) {
    std::cout << a << " " << b << " " << s << std::endl;
}

# Transformation

Any range-manipulating function may be used with std::istream_iterator ranges. One of them is std::transform, which allows to process data on-the-fly. For example, let's read integer values, multiply them by 3.14 and store the result into floating-point container:

std::vector<double> v(100);
std::transform(std::istream_iterator<int>(ifs), std::istream_iterator<int>(),
v.begin(),
[](int val) {
    return val * 3.14;
});

# Remarks

Default constructor of std::istream_iterator constructs an iterator which represents the end of the stream. Thus, std::copy(std::istream_iterator<int>(ifs), std::istream_iterator<int>(), .... means to copy from the current position in ifs to the end.

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