Improving performance of ifstream in C++

gistfile1.cpp

std::ifstream file("test.txt", std::ios::in | std::ios::end);

std::size_t fileSize = file.tellg();
 
std::vector<char> buffer(fileSize);
 
file.seekg(0, std::ios::beg);
 
file.read(buffer.data(), fileSize);
 
std::string str(buffer.begin(), buffer.end());