C++ Expected unqualified-id - c++

Question

I'm am a beginner in C++. I'm using Xcode to compile my code. Right now I am going over variables and doing a brief exercise on the subject. The exercise asks that I ask for the user to input their first and last name as well their age. As a additional requirement I need to use a double floating point for age so I can multiply the age into months. Below is my code:
#include <iostream>
#include <math.h>
int main()
{
std::cout << "Please enter your first name, second name, and age (then press enter).\n";
std::string first;
std::string last;
std::double age;
std::cin >> first >> last >> age;
std::cout << "Hello, " << first << " " << last << ". Your age is " << age << " and you are " << (age * 12) << " months old.";
return 0;
}
I get an error that says that the double is an expected unqualified-id. Can someone point out what I'm doing wrong and the correct way to do this?

Answer 1

double does not live in the std namespace. You need
double age;
You also need to include the string header for std::string. You may get it indirectly from iostream on some implementations, but you cannot rely on that: it is a fluke.

Answer 2

double is a built-in type. It doesn't live in any namespace and doesn't need any qualification! Just remove the std:: in front of double:
double age;
Note, you should test whether your input was actually successful:
if (std::cin >> first >> last >> age) {
// process successful input
}
else {
std::cout << "ERROR: failed to read expected input\n";
}

Answer 3

First of all it is a good idea to include header <string>
#include <string>
If in C# so-called built-in types in realty are aliases for classes as for example double is an alias for System.Double and you can write
System.Double age;
or
double age;
in C++ these types indeed are built in types and use keywords as doubleto specify a type
double age;
Though i do not understand why age should be double because the number of months in year is an integer value.:)

Answer 4

I believe the code below is about ideal for learning how to do this kind of nearly-first C++ program.
The little technical problem with your code was just that double is keyword, not a name defined by the standard library, and hence, not in namespace std.
In addition I’ve added
inclusion of the <string> header, necessary for using std::string in a portable way,
a using namespace std; directive, very handy for small exploration programs (but do not place this in the global namespace in a header file!),
checking of whether input operations succeed (also output can fail but that’s extremely rare).
The way that I check for input operation failure, using boolean "or" (the || operator), is not yet very much used in C++, but is common in some other languages. Essentially the left hand argument of || is converted to bool, since that’s what || requires. The left hand argument is the expression result of some input operation, which in general is a reference to the cin stream, and a bool value is then produced via a defined conversion that is equivalent to writing !cin.fail() (where ! is the logical "not" operation).
E.g., getline( cin, first ) || fail( ... ) reads very nicely as “getline or else fail”, and in addition to reading nicely it’s also visually distinctive, easy to recognize as a failure check.
#include <iostream>
#include <string>
#include <stdlib.h> // exit, EXIT_FAILURE
using namespace std;
// Poor man's way to handle failure, but good enough here:
bool fail( string const& message )
{
cerr << "!" << message << endl;
exit( EXIT_FAILURE );
}
int main()
{
cout << "Please enter your first name: ";
string first;
getline( cin, first )
|| fail( "Sorry, input of your first name failed." );
cout << "Please enter your last name: ";
string last;
getline( cin, first )
|| fail( "Sorry, input of your last name failed." );
cout << "Please enter your age in years: ";
double age;
cin >> age
|| fail( "Sorry, input of your age failed." );
cout << "Hello, " << first << " " << last << "." << endl;
cout
<< "Your age is " << age << " years"
<< " and you are "<< (age*12) << " months old."
<< endl;
}