tellg() function give wrong size of file?

In C++17 there are std::filesystem file_size methods and functions, so that can streamline the whole task.

• std::filesystem::file_size - cppreference.com
• std::filesystem::directory_entry::file_size - cppreference.com

With those functions/methods there's a chance not to open a file, but read cached data (especially with the std::filesystem::directory_entry::file_size method)

Those functions also require only directory read permissions and not file read permission (as tellg() does)

tellg does not report the size of the file, nor the offset from the beginning in bytes. It reports a token value which can later be used to seek to the same place, and nothing more. (It's not even guaranteed that you can convert the type to an integral type.)

At least according to the language specification: in practice, on Unix systems, the value returned will be the offset in bytes from the beginning of the file, and under Windows, it will be the offset from the beginning of the file for files opened in binary mode. For Windows (and most non-Unix systems), in text mode, there is no direct and immediate mapping between what tellg returns and the number of bytes you must read to get to that position. Under Windows, all you can really count on is that the value will be no less than the number of bytes you have to read (and in most real cases, won't be too much greater, although it can be up to two times more).

If it is important to know exactly how many bytes you can read, the only way of reliably doing so is by reading. You should be able to do this with something like:

#include <limits>

file.ignore( std::numeric_limits<std::streamsize>::max() );
std::streamsize length = file.gcount();
file.clear();   //  Since ignore will have set eof.
file.seekg( 0, std::ios_base::beg );

Finally, two other remarks concerning your code:

First, the line:

*buffer = new char[length];

shouldn't compile: you have declared buffer to be a char*, so *buffer has type char, and is not a pointer. Given what you seem to be doing, you probably want to declare buffer as a char**. But a much better solution would be to declare it as a std::vector<char>& or a std::string&. (That way, you don't have to return the size as well, and you won't leak memory if there is an exception.)

Second, the loop condition at the end is wrong. If you really want to read one character at a time,

while ( file.get( buffer[i] ) ) {
    ++ i;
}

should do the trick. A better solution would probably be to read blocks of data:

while ( file.read( buffer + i, N ) || file.gcount() != 0 ) {
    i += file.gcount();
}

or even:

file.read( buffer, size );
size = file.gcount();

EDIT: I just noticed a third error: if you fail to open the file, you don't tell the caller. At the very least, you should set the size to 0 (but some sort of more precise error handling is probably better).