Problem with URLDownloadToFile

Hello,
my problem with URLDownloadToFile is that I can't use variables to set my url and my save-path. Both variables are defined as std::string.

HRESULT hr = URLDownloadToFile ( NULL, _T(file.c_str), _T(savepath.c_str()), 0, NULL );


I've got the following errors:

[BCC32 Error] E2451 Undefined symbol 'Lfile'
[BCC32 Error] E2451 Undefined symbol 'Lsavepath'


When I change the variables to constant text (like "http:/www.test.com/test.xy and C:\test.xy) it will compile correctly. What can I do?