C++ Expected unqualified-id

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;
}