Efficient way of reading a file into an std::vector<char>?

Question

I'd like to avoid unnecessary copies. I'm aiming for something along the lines of:

std::ifstream testFile( "testfile", "rb" ); std::vector<char> fileContents; int fileSize = getFileSize( testFile ); fileContents.reserve( fileSize ); testFile.read( &fileContents[0], fileSize ); 

(which doesn't work because reserve doesn't actually insert anything into the vector, so I can't access [0]).

Of course, std::vector<char> fileContents(fileSize) works, but there is an overhead of initializing all elements (fileSize can be rather big). Same for resize().

This question is not so much about how important that overhead would be. Rather, I'm just curious to know if there's another way.

Answer 1

The canonical form is this:

#include<iterator> // ...  std::ifstream testFile("testfile", std::ios::binary); std::vector<char> fileContents((std::istreambuf_iterator<char>(testFile)),                                std::istreambuf_iterator<char>()); 

If you are worried about reallocations then reserve space in the vector:

#include<iterator> // ...  std::ifstream testFile("testfile", std::ios::binary); std::vector<char> fileContents; fileContents.reserve(fileSize); fileContents.assign(std::istreambuf_iterator<char>(testFile),                     std::istreambuf_iterator<char>());