How to write a random access iterator for std::wifstream?

Question

My normal procedure is as follows:

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// I create a wifstream from file
std::wifstream wif(L"...");

// Generate a wstring based on file content
std::wstring m_wstr((istreambuf_iterator<wchar_t>(wif)), istreambuf_iterator<wchar_t>());

// Do stuff on the content using iteraror of wstring
boost::algorithm::boyer_moore<std::wstring::const_iterator>(m_wstr);
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The problem is that the second step takes a lot of time, since the files are huge. Total file content is loaded onto wstring, which isn't normally necessary as the boost::algorithm::boyer_moore skips reading much of it anyway.

So, skipping that step would drastically improve speed, therefore I need a random access iterator over a std::wifstream, e.g., std::wifstream::const_iterator.

How to implement one concisely?