Using string variables instead of string literals causes error

Question

I am following this guide to send email in c++ . You can download the header files : easendmailobj.tlh and easendmail object at here

#include "stdafx.h"
#include <iostream>
#include "easendmailobj.tlh"
using namespace EASendMailObjLib;
using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    //string email = "randomemail1@gmail.com";

    ::CoInitialize( NULL );

    IMailPtr oSmtp = NULL;
    oSmtp.CreateInstance( "EASendMailObj.Mail");
    oSmtp->LicenseCode = _T("TryIt");

    // Set your gmail email address
    oSmtp->FromAddr = _T("randomemail1@gmail.com");

    // Add recipient email address
    oSmtp->AddRecipientEx( _T("randomemail2s@hotmail.com"), 0 );

    // Set email subject
    oSmtp->Subject = _T("simple email from Visual C++ with gmail account");

    // Set email body
    oSmtp->BodyText = _T("this is a test email sent from Visual C++ project with Gmail");

    // Gmail SMTP server address
    oSmtp->ServerAddr = _T("smtp.gmail.com");

    // If you want to use direct SSL 465 port, 
    // Please add this line, otherwise TLS will be used.
    // oSmtp->ServerPort = 465;

    // detect SSL/TLS automatically
    oSmtp->SSL_init();

    // Gmail user authentication should use your 
    // Gmail email address as the user name. 
    // For example: your email is "gmailid@gmail.com", then the user should be "gmailid@gmail.com"
    oSmtp->UserName = _T("somerandomemail@gmail.com");
    oSmtp->Password = _T("somepassword");

    _tprintf(_T("Start to send email via gmail account ...\r\n" ));

    if( oSmtp->SendMail() == 0 )
    {
        _tprintf( _T("email was sent successfully!\r\n"));
    }
    else
    {
        _tprintf( _T("failed to send email with the following error: %s\r\n"),
            (const TCHAR*)oSmtp->GetLastErrDescription());
    }

    if( oSmtp != NULL )
        oSmtp.Release();

    int x;
    cin>>x;

    return 0;
}

Why is there an error when i attempt to use string variables instead of " " ???

#include "stdafx.h"
#include <iostream>
#include "easendmailobj.tlh"
using namespace EASendMailObjLib;
using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    string email = "randomemail1@gmail.com": //string variables

    ::CoInitialize( NULL );

    IMailPtr oSmtp = NULL;
    oSmtp.CreateInstance( "EASendMailObj.Mail");
    oSmtp->LicenseCode = _T("TryIt");

    // Set your gmail email address
    oSmtp->FromAddr = _T(email); // string variable instead of "   " , error is here

    // Add recipient email address
    oSmtp->AddRecipientEx( _T("randomemail2s@hotmail.com"), 0 );

    // Set email subject
    oSmtp->Subject = _T("simple email from Visual C++ with gmail account");

    // Set email body
    oSmtp->BodyText = _T("this is a test email sent from Visual C++ project with Gmail");

    // Gmail SMTP server address
    oSmtp->ServerAddr = _T("smtp.gmail.com");

    // If you want to use direct SSL 465 port, 
    // Please add this line, otherwise TLS will be used.
    // oSmtp->ServerPort = 465;

    // detect SSL/TLS automatically
    oSmtp->SSL_init();

    // Gmail user authentication should use your 
    // Gmail email address as the user name. 
    // For example: your email is "gmailid@gmail.com", then the user should be "gmailid@gmail.com"
    oSmtp->UserName = _T("somerandomemail@gmail.com");
    oSmtp->Password = _T("somepassword");

    _tprintf(_T("Start to send email via gmail account ...\r\n" ));

    if( oSmtp->SendMail() == 0 )
    {
        _tprintf( _T("email was sent successfully!\r\n"));
    }
    else
    {
        _tprintf( _T("failed to send email with the following error: %s\r\n"),
            (const TCHAR*)oSmtp->GetLastErrDescription());
    }

    if( oSmtp != NULL )
        oSmtp.Release();

    int x;
    cin>>x;

    return 0;
}

I get the following error

Error C2065: 'Lemail' : undeclared identifier

Answer 1

A string literal and a std::string are two completely different things. Usually when a function f expects a const char* you can pass to it a string literal or you can use the method c_str() of a string like so f(email.c_str()) .

However the problem here is that _T is not a function. It is a macro and it simply appends a L as prefix to string literals. There is no way you can use it for std::string . You will need to use some functions for the conversion.

Alternatively define a macro that does the same as _T and decides to use wstring or string depending on if UNICODE is defined. Try to read through the code and what does _T do and you should understand what I mean.

Answer 2

It depends on whether UNICODE is defined.

If it is, use std::wstring instead of string. Also, remove the _T wrapping the string variable and add .c_str()

Answer 3

If this is the standard Microsoft junk (more exactly, an experiment that failed—what they wanted to achieve is laudable, but what they actually ended up with is useless extra complexity), you're best off getting rid of it: the function is main (not _tmain ), use char (or wchar_t , depending on what you want, but it must be char in the declaration of main ) instead of _TCHAR , and forget about the _T .

As for the immediate problem, the _T is a macro that will only work on string literals; that's the way it was designed. If you use it on anything else, it may expand to something like Lemail , which of course won't be known.

Answer 4

invalid usage of _T(); it is for string literals.

So that line should be;

oSmtp->FromAddr = email.c_str();

Answer 5

This is the definition of _T :

#if defined(_UNICODE)
#define _T(x) L ##x
#else
#define _T(x) x
#endif

So as you can see, it just appends L infront of anything passed inside it. So if you pass in a string, for example _T("HELLO WORLD") it would translate to L"HELLO WORLD" if _UNICODE is defined.

Similarly if you pass in any other token, _T(email) for example, it would convert it to Lemail which is why you are getting an error.

Work around would be to use std::wstring and std::string as:

#if defined(_UNICODE)
#define String std::wstring
#else
#define String std::string
#endif

and use String email instead.