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In order to improve performance reading from a file, I'm trying to read the entire content of a big (several MB) file into memory and then use a istringstream to access the information.
My question is, which is the best way to read this information and "import it" into the string stream? A problem with this approach (see bellow) is that when creating the string stream the buffers gets copied, and memory usage doubles.
#include <fstream> #include <sstream> using namespace std; int main() { ifstream is; is.open (sFilename.c_str(), ios::binary ); // get length of file: is.seekg (0, std::ios::end); long length = is.tellg(); is.seekg (0, std::ios::beg); // allocate memory: char *buffer = new char [length]; // read data as a block: is.read (buffer,length); // create string stream of memory contents // NOTE: this ends up copying the buffer!!! istringstream iss( string( buffer ) ); // delete temporary buffer delete [] buffer; // close filestream is.close(); /* ================================== * Use iss to access data */ } 
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The std::istringstream is a string class object which is used to stream the string into different variables and similarly files can be stream into strings. Objects of this class use a string buffer that contains a sequence of characters. This sequence of characters can be accessed as a string object.
Use istreambuf_iterator to Read File Into String in C++ istreambuf_iterator is an input iterator that reads successive characters from the std::basic_streambuf object. Thus we can utilize istreambuf_iterator with an ifstream stream and read the whole contents of the file into a std::string .
A stringstream associates a string object with a stream allowing you to read from the string as if it were a stream (like cin). To use stringstream, we need to include sstream header file. The stringstream class is extremely useful in parsing input.
std::ifstream has a method rdbuf(), that returns a pointer to a filebuf. You can then "push" this filebuf into your stringstream:
#include <fstream> #include <sstream> int main() { std::ifstream file( "myFile" ); if ( file ) { std::stringstream buffer; buffer << file.rdbuf(); file.close(); // operations on the buffer... } } EDIT: As Martin York remarks in the comments, this might not be the fastest solution since the stringstream's operator<< will read the filebuf character by character. You might want to check his answer, where he uses the ifstream's read method as you used to do, and then set the stringstream buffer to point to the previously allocated memory.
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