convert string to long long int - c++

Question

I want to read the 3rd line from a text file in as a string, convert it to a long long int, and return that value.
The data on the 3rd line of the text file is 1234567890123456
long long int File::Getline3(int user1, int user3)
{
std::string filename = std::to_string(user1);
std::ifstream fin(filename + ".txt");
fin.getline (line1, 5);
fin.getline (line2, 5);
fin.getline (line3, 20);
fin.close();
// convert line 3 to a string called str
const char *line3;
std::string str(line3);
// convert str to long long int called user3
long long int strtoll(const char *nptr, char **endptr, int base);
char* endptr = NULL;
user3 = strtoll(str.c_str(), &endptr, 10);
return user3;
}
The comments are in to show what I think I'm doing, however I am probably wrong (I'm new to pointers).
I get an "unresolved external" error when I try to build my program.

Answer 1

long long int strtoll(const char *nptr, char **endptr, int base);
This line declares a function. It means that when you call strtoll a few lines down, you're going to be calling that function as it's the most obvious candidate. However, you never defined it.
Instead you meant to call std::strtoll, which is defined (by the standard library), and which will be found through your presumed using namespace std directive if you do not hide it by falsely declaring this non-existent function of your own with the same name. :)
Simply remove the excess declaration:
// convert str to long long int called user3
char* endptr = NULL;
user3 = strtoll(str.c_str(), &endptr, 10);
You have a similar issue with your const char* line3, which you declare inside the function, never assign anything to, then construct a string out of. That's undefined; the pointer is uninitialised. Assuming you have some line3 data member (along with your line1 and line2), again you're hiding it with a local variable of the same name.
Finally, passing user3 in by value is utterly pointless if you're just writing to it and returning it. Remove that parameter.
Putting all this together, your code should probably look like:
// convert line3 to long long int
char* endptr = NULL;
return strtoll(line3, &endptr, 10);
In short, be less declaration-happy!

Answer 2

https://connect.microsoft.com/VisualStudio/feedback/details/758053/missing-strtold-strtoll-strtoull-functions-from-stdlib-h
A work around for Visual C++ that does not have these included.

Answer 3

long long int File::Getline3(int user1)
{
std::string filename = std::to_string(user1);
std::ifstream fin(filename + ".txt");
std::string line1, line2; // place to actually store the lines
getline(fin, line1); // read a line
getline(fin, line2); // read another line
long long int number_on_line_3; // somewhere to store the number
fin >> number_on_line3; // store the number
fin.close();
return number_on_line_3; // return the number
}