Efficient file reading in C++11/14

I'm creating a IOManager class in which I have a function to read a file and store it in a buffer. What is the most efficient way of doing that?

I currently have 2 pieces of code:

bool IOManager::readFileToBuffer(std::string filePath, std::vector<unsigned char>& buffer) {
    std::ifstream file(filePath, std::ios::binary);
    if (file.fail()) {
        perror(filePath.c_str());
        return false;
    }

    //seek to the end
    file.seekg(0, std::ios::end);

    //Get the file size
    int fileSize = file.tellg();
    file.seekg(0, std::ios::beg);

    //Reduce the file size by any header bytes that might be present
    fileSize -= file.tellg();

    buffer.resize(fileSize);
    file.read((char *)&(buffer[0]), fileSize);
    file.close();

    return true;
}

and

bool IOManager::readFileToBuffer(std::string filePath, std::vector<char>& buffer) {

    std::ifstream file(filePath, std::ios::binary);

    if (file.fail()) {
        perror(filePath.c_str());
        return false;
    }

    // copies all data into buffer
    std::vector<char> prov(
        (std::istreambuf_iterator<char>(file)),
        (std::istreambuf_iterator<char>()));

    buffer = prov;

    file.close();

    return true;
}

Which one is better? Is this the fastest and most efficient way of doing this according to the C++11/14 standards?

Thanks.