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

Efficient Reading of Files into an std::vector

When seeking an optimal method for reading a file into an std::vector, it is crucial to minimize unnecessary copying and maintain efficiency. A common misconception is attempting to reserve space in the vector before reading, but reserve() does not actually insert elements into the vector.

For an optimized solution, the canonical approach involves using iterators:

#include <iterator>

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

In this approach, two iterators are defined. One points to the beginning of the input file stream, and the other to the end. The vector is then constructed by iterating over the range defined by these iterators.

To mitigate potential reallocations during the read process, the reserve() method can be employed:

#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>());

By reserving space within the vector prior to assigning values, the allocation efficiency is improved, reducing the likelihood of memory fragmentation and performance degradation.

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