How to Efficiently Read a File into a std::vector?

Efficient Reading of a File into std::vector

In certain scenarios, reading a file into a std::vector efficiently is crucial to avoid unnecessary copies and overheads. One approach involves using the reserve() method to allocate space for the vector, followed by using read() to populate the vector.

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

However, this approach fails when resizing the vector with reserve(), as it does not actually insert elements into the vector. As a result, attempting to access fileContents[0] will cause an error.

Alternative Approach Using Iterators

A more comprehensive solution involves the usage of iterators. Using an input file stream, the following snippet allows for efficient file reading:

#include<iterator>

//...

std::ifstream testFile("testfile", std::ios::binary);
std::vector<char> fileContents((std::istreambuf_iterator<char>(testFile)),
                              std::istreambuf_iterator<char>());

This approach utilizes istreambuf_iterator to iterate over the input file stream and insert elements directly into the vector.

Handling Reallocations

If reallocations are a concern, reserve() can be used to pre-allocate 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>());

In this variation, reserve() is used to allocate space based on the known file size, and assign() is employed to populate the vector using iterators.

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