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>());