Question

I’m doing something really simple: slurping an entire text file from disk into a std::string. My current code basically does this:

std::ifstream f(filename); return std::string(std::istreambuf_iterator<char>(f), std::istreambuf_iterator<char>()); 

It’s very unlikely that this will ever have any kind of performance impact on the program, but I still got curious whether this is a slow way of doing it.

Is there a risk that the construction of the string will involve a lot of reallocations? Would it be better (that is, faster) to use seekg()/tellg() to calculate the size of the file and reserve() that much space in the string before doing the reading?

Answer 1

I benchmarked your implementation(1), mine(2), and two others(3 and 4) that I found on stackoverflow.

Results (Average of 100 runs; timed using gettimeofday, file was 40 paragraphs of lorem ipsum):

• readFile1: 764
• readFile2: 104
• readFile3: 129
• readFile4: 402

The implementations:

string readFile1(const string &fileName) {     ifstream f(fileName.c_str());     return string(std::istreambuf_iterator<char>(f),             std::istreambuf_iterator<char>()); }  string readFile2(const string &fileName) {     ifstream ifs(fileName.c_str(), ios::in | ios::binary | ios::ate);      ifstream::pos_type fileSize = ifs.tellg();     ifs.seekg(0, ios::beg);      vector<char> bytes(fileSize);     ifs.read(&bytes[0], fileSize);      return string(&bytes[0], fileSize); }  string readFile3(const string &fileName) {     string data;     ifstream in(fileName.c_str());     getline(in, data, string::traits_type::to_char_type(                       string::traits_type::eof()));     return data; }  string readFile4(const std::string& filename) {     ifstream file(filename.c_str(), ios::in | ios::binary | ios::ate);      string data;     data.reserve(file.tellg());     file.seekg(0, ios::beg);     data.append(istreambuf_iterator<char>(file.rdbuf()),                 istreambuf_iterator<char>());     return data; }