C++ How to Read Entire String From File

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Read File Into String in C++

Created: Dec-26, 2020

1. Use istreambuf_iterator to Read File Into String in C++
2. Utilize rdbuf to Read File Into String in C++
3. Use fread to Read File Into String
4. Use read to Read File Into Cord

This commodity will explain several methods of reading the file content into a std::string in C++.

Utilize istreambuf_iterator to Read File Into Cord in C++

istreambuf_iterator is an input iterator that reads successive characters from the std::basic_streambuf object. Thus we can utilise istreambuf_iterator with an ifstream stream and read the whole contents of the file into a std::string.

At first, we open a given file path as an ifstream object. Then we tin can pass istreambuf_iterator<char>(input_file) to the string constructor and go the object we needed in the start place. Note that we are directly passing the string constructor statement to return from the function. The program'due south output should be the contents of the file every bit specified past the filename variable.

              #include <iostream> #include <fstream> #include <sstream>  using std::cout; using std::cerr; using std::endl; using std::string; using std::ifstream; using std::ostringstream;  cord readFileIntoString(const string& path) {     ifstream input_file(path);     if (!input_file.is_open()) {         cerr << "Could not open the file - '"              << path << "'" << endl;         exit(EXIT_FAILURE);     }     return cord((std::istreambuf_iterator<char>(input_file)), std::istreambuf_iterator<char>()); }  int main() {     string filename("input.txt");     string file_contents;      file_contents = readFileIntoString(filename);     cout << file_contents << endl;      leave(EXIT_SUCCESS); }                          

Use rdbuf to Read File Into Cord in C++

The rdbuf function is a built-in method to render a pointer to the stream buffer of the file, which is useful to insert the entire contents of the file using the << operator to the needed object.

In the following instance, we construct an ostringstream object where nosotros insert the rdbuf part's return value. The role itself returns the string object, so the str method is used to get the last render value.

              #include <iostream> #include <fstream> #include <sstream>  using std::cout; using std::cerr; using std::endl; using std::string; using std::ifstream; using std::ostringstream;  string readFileIntoString2(const cord& path) {     auto ss = ostringstream{};     ifstream input_file(path);     if (!input_file.is_open()) {         cerr << "Could not open up the file - '"              << path << "'" << endl;         get out(EXIT_FAILURE);     }     ss << input_file.rdbuf();     render ss.str(); }  int main() {     string filename("input.txt");     string file_contents;      file_contents = readFileIntoString2(filename);     cout << file_contents << endl;      exit(EXIT_SUCCESS); }                          

Use fread to Read File Into String

Another method for reading a file is the C standard library function fread. This method requires relatively legacy functions that are not common in the mod C++ codebases, but information technology offers pregnant speedup performance compared with the previous methods.

fread takes four arguments:

1. A pointer to the buffer where read data is stored.
2. The size of the information item.
3. Number of data items
4. The file pointer from which to read.

Since we are reading the whole file, the file size needs to be retrieved, and it's implemented with the stat Unix arrangement call. Once the file size is retrieved, nosotros pass its value equally the size of the data element to the fread function, and as the number of data items, we specify i.

Note that opened files demand to be closed with the fclose function call, which takes the merely argument of the file pointer.

              #include <iostream> #include <unistd.h> #include <sys/stat.h> #include <fcntl.h>  using std::cout; using std::cerr; using std::endl; using std::string;  string readFileIntoString3(const string& path) {     struct stat sb{};     string res;      FILE* input_file = fopen(path.c_str(), "r");     if (input_file == nullptr) {         perror("fopen");     }      stat(path.c_str(), &sb);     res.resize(sb.st_size);     fread(const_cast<char*>(res.data()), sb.st_size, 1, input_file);     fclose(input_file);      return res; }  int principal() {     cord filename("input.txt");     string file_contents;      file_contents = readFileIntoString3(filename);     cout << file_contents << endl;      leave(EXIT_SUCCESS); }                          

Use read to Read File Into String

The read method is POSIX compliant role call available on various operating systems and can be the most flexible one if the programmer knows to employ it efficiently. fread itself calls read underneath, but this doesn't guarantee the faster operation in all cases, every bit multiple factors play a manus in the efficient apply of such system calls.

The main difference with fread is that read needs a file descriptor statement to indicate to the file from where to read data. File descriptors are special integers associated with the open up file streams that the program might accept during the execution. It can be acquired using the open function call and stored in int blazon. The other two arguments of the read office are the pointer to the buffer where the information will be stored and the number of bytes needed to be read, the latter of which is retrieved with the fstat part call. Note that nosotros are using the string.data as the buffer to store read file contents.

              #include <iostream> #include <unistd.h> #include <sys/stat.h> #include <fcntl.h>  using std::cout; using std::cerr; using std::endl; using std::string;  string readFileIntoString4(const cord& path) {     struct stat sb{};     cord res;      int fd = open(path.c_str(), O_RDONLY);     if (fd < 0) {         perror("open\n");     }      fstat(fd, &sb);     res.resize(sb.st_size);     read(fd, (char*)(res.data()), sb.st_size);     shut(fd);      return res; }  int main() {     string filename("input.txt");     string file_contents;      file_contents = readFileIntoString4(filename);     cout << file_contents << endl;      leave(EXIT_SUCCESS); }                          

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