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

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 ques开发者_运维知识库tion is not so much about how important that overhead would be. Rather, I'm just curious to know if there's another way.

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>());

If you want true zero-copy reading, that is, to eliminate copying from kernel to user space, just map the file into memory. Write your own mapped file wrapper or use one from boost::interprocess.

If I understand you correctly, you want to read each element but don't want to load it all into the fileContents, correct? I personally don't think this would make unnecessary copies because open files multiple times would decrease performance more. Read once into a fileContentsvector is a reasonable solution in this case.