12. File

[ITHelpself]

std::ofstream os("foo.txt");
if(os.is_open()){
 os << "Hello World!";
}

[ITHelpself]

std::ofstream os("foo.txt");
if(os.is_open()){
 char data[] = "Foo";
 // Writes 3 characters from data -> "Foo".
 os.write(data, 3);
}

[ITHelpself]

os << "Hello Badbit!"; // This operation might fail for any reason.
if (os.bad())
 // Failed to write!

[ITHelpself]

std::ifstream ifs("foo.txt"); // ifstream: Opens file "foo.txt" for reading only.
std::ofstream ofs("foo.txt"); // ofstream: Opens file "foo.txt" for writing only.
std::fstream iofs("foo.txt"); // fstream: Opens file "foo.txt" for reading and writing.
std::ifstream ifs;
ifs.open("bar.txt"); // ifstream: Opens file "bar.txt" for reading only.
std::ofstream ofs;
ofs.open("bar.txt"); // ofstream: Opens file "bar.txt" for writing only.
std::fstream iofs;
iofs.open("bar.txt"); // fstream: Opens file "bar.txt" for reading and writing

[ITHelpself]

// Try to read the file 'foo.txt'.
std::ifstream ifs("fooo.txt"); // Note the typo; the file can't be opened.
// Check if the file has been opened successfully.
if (!ifs.is_open()) {
// The file hasn't been opened; take appropriate actions here.
throw CustomException(ifs, "File could not be opened");
}

[ITHelpself]

// Open the file 'c:\\folder\\foo.txt' on Windows.
std::ifstream ifs("c:\\\\folder\\\\foo.txt"); // using escaped backslashes
Version ≥ C++11
or use raw literal:
// Open the file 'c:\\folder\\foo.txt' on Windows.
std::ifstream ifs(R"(c:\\folder\\foo.txt)"); // using raw literal
or use forward slashes instead:
// Open the file 'c:\\folder\\foo.txt' on Windows.
std::ifstream ifs("c:/folder/foo.txt");
Version ≥ C++11

[ITHelpself]

// Define variables.
std::ifstream is("foo.txt");
std::string firstname, lastname;
int age, bmonth, bday, byear;
// Extract firstname, lastname, age, bday month, bday day, and bday year in that order.
// Note: '>>' returns false if it reached EOF (end of file) or if the input data doesn't
// correspond to the type of the input variable (for example, the string "foo" can't be
// extracted into an 'int' variable).
while (is >> firstname >> lastname >> age >> bmonth >> bday >> byear)
// Process the data that has been read.

[ITHelpself]

// Opens 'foo.txt'.
std::ifstream is("foo.txt");
std::string whole_file;
// Sets position to the end of the file.
is.seekg(0, std::ios::end);
// Reserves memory for the file.
whole_file.reserve(is.tellg());
// Sets position to the start of the file.
is.seekg(0, std::ios::beg);
// Sets contents of 'whole_file' to all characters in the file.
whole_file.assign(std::istreambuf_iterator<char>(is),
std::istreambuf_iterator<char>());

[ITHelpself]

std::ifstream is("foo.txt");
// The function getline returns false if there are no more lines.
for (std::string str; std::getline(is, str);) {
// Process the line that has been read.
}

[ITHelpself]

std::ifstream is("foo.txt");
char str[4];
// Read 4 characters from the file.
is.read(str, 4);

[ITHelpself]

is.read(str, 4); // This operation might fail for any reason.
if (is.fail())
// Failed to read!

[ITHelpself]

std::ofstream os("foo.txt", std::ios::out | std::ios::trunc);
std::ifstream is;
is.open("foo.txt", std::ios::in | std::ios::binary);

[ITHelpself]

std::ifstream f("file.txt");
if (f)
{
std::stringstream buffer;
buffer << f.rdbuf();
f.close();
// The content of "file.txt" is available in the string `buffer.str()`
}

[ITHelpself]

std::ifstream f("file.txt");
if (f)
{
std::string str((std::istreambuf_iterator<char>(f)),
std::istreambuf_iterator<char>());
// Operations on `str`...
}

[ITHelpself]

std::ifstream f("file.txt");
if (f)
{
f.seekg(0, std::ios::end);GoalKicker.com – C++ Notes for Professionals 59
const auto size = f.tellg();
std::string str(size, ' ');
f.seekg(0);
f.read(&str[0], size);
f.close();
// Operations on `str`...
}

[ITHelpself]

#include <iostream>
#include <fstream>
#include <locale>
int main()
{
std::cout << "User-preferred locale setting is "
<< std::locale("").name().c_str() << std::endl;
// Write a floating-point value using the user's preferred locale.
std::ofstream ofs1;
ofs1.imbue(std::locale(""));
ofs1.open("file1.txt");
ofs1 << 78123.456 << std::endl;
// Use a specific locale (names are system-dependent)
std::ofstream ofs2;
ofs2.imbue(std::locale("en_US.UTF-8"));
ofs2.open("file2.txt");
ofs2 << 78123.456 << std::endl;
// Switch to the classic "C" locale
std::ofstream ofs3;
ofs3.imbue(std::locale::classic());
ofs3.open("file3.txt");
ofs3 << 78123.456 << std::endl;
}

[ITHelpself]

while (!f.eof())
{
// Everything is OK
f >> buffer;
// What if *only* now the eof / fail bit is set?
/* Use `buffer` */
}
You could correctly write:
while (!f.eof())
{
f >> buffer >> std::ws;
if (f.fail())
break;
/* Use `buffer` */
}
but
while (f >> buffer)
{
/* Use `buffer` */
}
is simpler and less error prone

[ITHelpself]

std::ofstream os("foo.txt");
os << "Hello World!" << std::flush;
char data[3] = "Foo";
os.write(data, 3);
os.flush();
There is a stream manipulator std::endl that combines writing a newline with flushing the stream:
// Both following lines do the same thing
os << "Hello World!\n" << std::flush;
os << "Hello world!" << std::endl

[ITHelpself]

std::ifstream file("file3.txt");
std::vector<std::string> v;
std::string s;
while(file >> s) // keep reading until we run out
{
v.push_back(s);
}

[ITHelpself]

std::ifstream file("file3.txt");
std::vector<std::string> v(std::istream_iterator<std::string>{file},
std::istream_iterator<std::string>{});

[ITHelpself]

// Unfortunately there is no built in type that reads line using >>
// So here we build a simple helper class to do it. That will convert
// back to a string when used in string context.
struct Line
{
// Store data here
std::string data;
// Convert object to string
operator std::string const&() const {return data;}
// Read a line from a stream.
friend std::istream& operator>>(std::istream& stream, Line& line)
{
return std::getline(stream, line.data);
}
};
std::ifstream file("file3.txt");
// Read the lines of a file into a container.
std::vector<std::string> v(std::istream_iterator<Line>{file},
std::istream_iterator<Line>{});

[ITHelpself]

std::ifstream src("source_filename", std::ios::binary);
std::ofstream dst("dest_filename", std::ios::binary);
dst << src.rdbuf();

[ITHelpself]

std::fileystem::copy_file("source_filename", "dest_filename");

[ITHelpself]

std::string const prepared_data = prepare_data();
{
// Open a file for writing.
std::ofstream output("foo.txt");
// Write data.
output << prepared_data;
} // The ofstream will go out of scope here.
// Its destructor will take care of closing the file properly.

[ITHelpself]

// Open the file "foo.txt" for the first time.
std::ofstream output("foo.txt");
// Get some data to write from somewhere.
std::string const prepared_data = prepare_data();
// Write data to the file "foo.txt".
output << prepared_data;
// Close the file "foo.txt".
output.close();
// Preparing data might take a long time. Therefore, we don't open the output file stream
// before we actually can write some data to it.
std::string const more_prepared_data = prepare_complex_data();
// Open the file "foo.txt" for the second time once we are ready for writing.
output.open("foo.txt");
// Write the data to the file "foo.txt".
output << more_prepared_data;
// Close the file "foo.txt" once again.
output.close();

[ITHelpself]

struct info_type
{
std::string name;
int age;
float height;
// we define an overload of operator>> as a friend function which
// gives in privileged access to private data members
friend std::istream& operator>>(std::istream& is, info_type& info)
{
// skip whitespace
is >> std::ws;
std::getline(is, info.name);
is >> info.age;
is >> info.height;
return is;
}
};
void func4()
{
auto file = std::ifstream("file4.txt");
std::vector<info_type> v;
for(info_type info; file >> info;) // keep reading until we run out
{
// we only get here if the read succeeded
v.push_back(info);
}
for(auto const& info: v)
{
std::cout << " name: " << info.name << '\n';
std::cout << " age: " << info.age << " years" << '\n';
std::cout << "height: " << info.height << "lbs" << '\n';
std::cout << '\n';
}
}