ReadFile std::unique_ptr vs. std::vector vs. std::string - c++

Question

Which one of the following would be the most efficient and why? I'm leaning towards the unique_ptr because I think that there is no copy being done when returning the data read. It's just a transfer of ownership of a pointer.
For example, I think that the string version would create a temporary string, read data into that, and upon returning, it'd copy its data into the assigned result?
However, I'm not sure that I'm right. Any ideas what I mean and which is the best?
UniquePtr:
std::unique_ptr<const char[]> ReadFile(const char* FileName)
{
std::fstream file(FileName, std::ios::in);
if (file.is_open())
{
file.seekg(0, std::ios::end);
std::size_t size = file.tellg();
std::unique_ptr<char[]> result(new char[size]);
file.seekg(0, std::ios::beg);
file.read(result.get(), size);
file.close();
return std::move(result);
}
return nullptr;
}
Vector:
std::string ReadFile(const char* FileName)
{
std::fstream file(FileName, std::ios::in);
if (file.is_open())
{
file.seekg(0, std::ios::end);
std::vector<std::int8_t> buffer(file.tellg());
file.seekg(0, std::ios::beg);
file.read(result.data(), result.size());
file.close();
return std::string(result.data());
}
return std::string();
}
String:
std::string ReadFile(const char* FileName)
{
std::fstream file(FileName, std::ios::in);
if (file.is_open())
{
std::string result = std::string();
file.seekg(0, std::ios::end);
result.resize(file.tellg());
file.seekg(0, std::ios::beg);
file.read(&result[0], result.size());
file.close();
return result;
}
return std::string();
}

Answer 1

std::string ReadFile(const char* FileName)
{
std::fstream file(FileName, std::ios::in);
std::string result = std::string();
if (file.is_open())
{
file.seekg(0, std::ios::end);
result.resize(file.tellg());
file.seekg(0, std::ios::beg);
file.read(&result[0], result.size());
file.close();
}
return result;
}
I have no proof , but if function has only one return , copy elision might be implemented by compiler , if there are 2 returns inside function ,
copy elision might not work as expected ~~~

Answer 2

The
return std::string(result.data());
will only work if the data is null-terminated.
Apart from that complication it needlessly copies from a vector to a string.
The std::string code is most natural for reading text (text mode open of file).
You won't see much of a performance difference since the file i/o dwarfs the rest, but anyway with C++03 you will most likely get Return Value Optimization (depends on compiler), and with C++11 you will get move optimization of the result if you just use std::move.