wstring is undefined

#include <string>
#include <vector>
#include <windows.h>
#include <time.h>
#include <map>

struct file_data 
{ 
    wstring sLastAccessTime; 
    __int64 nFileSize      ; 
}; 
 
int GetFileList(const wchar_t *searchkey, std::map<std::wstring, file_data> &map) 
{ 
    WIN32_FIND_DATA fd; 
    HANDLE h = FindFirstFile(searchkey,&fd); 
    if(h == INVALID_HANDLE_VALUE) 
    { 
        return 0; // no files found 
    } 
    while(1) 
    { 
        wchar_t buf[128]; 
        FILETIME ft = fd.ftLastWriteTime; 
        SYSTEMTIME sysTime; 
        FileTimeToSystemTime(&ft, &sysTime); 
        wsprintf(buf, L"%d-%02d-%02d",sysTime.wYear, sysTime.wMonth, sysTime.wDay); 
 
        file_data filedata; 
        filedata.sLastAccessTime= buf; 
        filedata.nFileSize      = (((__int64)fd.nFileSizeHigh) << 32) + fd.nFileSizeLow; 
 
        map[fd.cFileName]= filedata; 
 
        if (FindNextFile(h, &fd) == FALSE) 
            break; 
    } 
    return map.size(); 
} 
 
int main() 
{ 
    std::map<std::wstring, file_data> map; 
    GetFileList(L"C:\\Users\\DS\\Downloads\\*.zip", map); 
    GetFileList(L"C:\\Users\\DS\\Downloads\\*.txt", map); 
 
    for(std::map<std::wstring, file_data>::const_iterator it = map.begin(); 
        it != map.end(); ++it) 
    { 
        MessageBoxW(NULL,it->first.c_str(),L"File Name",MB_OK); 
        MessageBoxW(NULL,it->second.sLastAccessTime.c_str(),L"File Date",MB_OK); 
    } 
 
    return 0; 
}