UrlDownloadToFile to Memory - c++

Question

I'm using URLDownloadToFile() to download images from a web server to a directory on my desktop. If, instead of saving the images to disk, I wanted to read them into memory (like a byte array or base64 string or something), is there a function similar to URLDownloadToFile() that can achieve this?

Answer 1

There is URLOpenStream(), URLOpenBlockingStream() and URLOpenPullStream() which allow you to download into memory.
From these three, URLOpenBlockingStream() seems to be the most straightforward to use as it returns an IStream pointer from which you can synchronously read in a loop. Although it's not a single do-all function like URLDownloadToFile() it's not much difficult to use.
Here is a complete example console application for URLOpenBlockingStream(). It downloads from an URL and writes the response to stdout. Instead of this, you could store the response in a std::vector or do whatever you like with it.
#include <Windows.h>
#include <Urlmon.h> // URLOpenBlockingStreamW()
#include <atlbase.h> // CComPtr
#include <iostream>
#pragma comment( lib, "Urlmon.lib" )
struct ComInit
{
HRESULT hr;
ComInit() : hr( ::CoInitialize( nullptr ) ) {}
~ComInit() { if( SUCCEEDED( hr ) ) ::CoUninitialize(); }
};
int main(int argc, char* argv[])
{
ComInit init;
// use CComPtr so you don't have to manually call Release()
CComPtr<IStream> pStream;
// Open the HTTP request.
HRESULT hr = URLOpenBlockingStreamW( nullptr, L"http://httpbin.org/headers", &pStream, 0, nullptr );
if( FAILED( hr ) )
{
std::cout << "ERROR: Could not connect. HRESULT: 0x" << std::hex << hr << std::dec << "\n";
return 1;
}
// Download the response and write it to stdout.
char buffer[ 4096 ];
do
{
DWORD bytesRead = 0;
hr = pStream->Read( buffer, sizeof(buffer), &bytesRead );
if( bytesRead > 0 )
{
std::cout.write( buffer, bytesRead );
}
}
while( SUCCEEDED( hr ) && hr != S_FALSE );
if( FAILED( hr ) )
{
std::cout << "ERROR: Download failed. HRESULT: 0x" << std::hex << hr << std::dec << "\n";
return 2;
}
std::cout << "\n";
return 0;
}

Answer 2

There is no single do-all function in the Win32 API to download a URL resource into memory, like URLDownloadToFile() does for a file. You will have to write your own function that requests the URL and then reads the data into memory.
For example, using InternetOpenUrl() or HttpOpenRequest()/HttpSendRequest(), and then InternetReadFile(). See Downloading Resources from the WWW on MSDN for more details.
Or using any other HTTP/S library/API of your choosing.