.NET WebClient.DownloadData(url) alternative in C++/WinAPI? - c++

Question

How can I get contents of a file online with C++?

Answer 1

There are a number of ways you can do this.
WinInet
First off Windows has a built-in API allowing you to make HTTP requests, which is reasonably simple to use. I use this simple wrapper class to download files using it:
/**
* Simple wrapper around the WinInet library.
*/
class Inet
{
public:
explicit Inet() : m_hInet(NULL), m_hConnection(NULL)
{
m_hInet = ::InternetOpen(
"My User Agent",
INTERNET_OPEN_TYPE_PRECONFIG,
NULL,
NULL,
/*INTERNET_FLAG_ASYNC*/0);
}
~Inet()
{
Close();
if (m_hInet)
{
::InternetCloseHandle(m_hInet);
m_hInet = NULL;
}
}
/**
* Attempt to open a URL for reading.
* #return false if we don't have a valid internet connection, the url is null, or we fail to open the url, true otherwise.
*/
bool Open(LPCTSTR url)
{
if (m_hInet == NULL)
{
return false;
}
if (url == NULL)
{
return false;
}
m_hConnection = ::InternetOpenUrl(
m_hInet,
url,
NULL /*headers*/,
0 /*headers length*/,
INTERNET_FLAG_NO_AUTH | INTERNET_FLAG_NO_CACHE_WRITE | INTERNET_FLAG_NO_COOKIES | INTERNET_FLAG_NO_UI,
reinterpret_cast<DWORD_PTR>(this));
return m_hConnection != NULL;
}
/**
* Read from a connection opened with Open.
* #return true if we read data.
*/
bool ReadFile(LPVOID lpBuffer, DWORD dwNumberOfBytesToRead, LPDWORD dwRead)
{
ASSERT(m_hConnection != NULL);
return ::InternetReadFile(m_hConnection, lpBuffer, dwNumberOfBytesToRead, dwRead) != 0;
}
/**
* Close any open connection.
*/
void Close()
{
if (m_hConnection != NULL)
{
::InternetCloseHandle(m_hConnection);
m_hConnection = NULL;
}
}
private:
HINTERNET m_hInet;
HINTERNET m_hConnection;
};
Usage of this is very simple:
Inet inet;
if (inet.Open(url))
{
BYTE buffer[UPDATE_BUFFER_SIZE];
DWORD dwRead;
while (inet.ReadFile(&buffer[0], UPDATE_BUFFER_SIZE, &dwRead))
{
// TODO: Do Something With buffer here
if (dwRead == 0)
{
break;
}
}
}
LibCurl
If you'd rather avoid Windows-specific APIs then you could do a lot worse than use the libcurl library to get files using a variety of protocols, including HTTP. There is a good sample showing how to retrieve a URL directly into memory (avoiding downloading to disk): getinmemory sample.

Answer 2

Use the following functions.
WinHttpConnect
WinHttpOpenRequest
WinHttpSendRequest
WinHttpReceiveResponse
WinHttpQueryDataAvailable
WinHttpReadData