c++ - 使用字符串变量而不是字符串文字会导致错误

Question

我正在关注这个 guide在 c++ 中发送电子邮件。您可以在 here 下载头文件:easendmailobj.tlh 和 easendmail object

#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;
}

为什么当我尝试使用字符串变量而不是 ""时出现错误？？？

#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;
}

出现以下错误

Error C2065: 'Lemail' : undeclared identifier

Answer 1

一个字符串文字和一个 std::string是两个完全不同的东西。通常当函数 f期望一个 const char*你可以传递给它一个字符串文字或者你可以使用方法 c_str()像这样的字符串 f(email.c_str()) .

但是这里的问题是_T不是函数。它是一个宏，它只是附加一个 L。作为字符串文字的前缀。您无法将其用于 std::string .您将需要使用一些函数进行转换。

或者定义一个与_T 功能相同的宏并决定使用 wstring或 string取决于是否 UNICODE被定义为。尝试通读代码，看看 _T 的作用是什么？做，你应该明白我的意思。