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POIs it reasonable to use std::basic_string<t> as a contiguous buffer when targeting C++03?
 

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  1. POIs it reasonable to use std::basic_string<t> as a contiguous buffer when targeting C++03?
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    <p>I know that in C++03, technically the <code>std::basic_string</code> template is not required to have contiguous memory. However, I'm curious how many implementations exist for modern compilers that actually take advantage of this freedom. For example, if one wants to use <code>basic_string</code> to receive the results of some C API (like the example below), it seems silly to allocate a vector just to turn it into a string immediately.</p> <p>Example:</p> <pre><code>DWORD valueLength = 0; DWORD type; LONG errorCheck = RegQueryValueExW( hWin32, value.c_str(), NULL, &amp;type, NULL, &amp;valueLength); if (errorCheck != ERROR_SUCCESS) WindowsApiException::Throw(errorCheck); else if (valueLength == 0) return std::wstring(); std::wstring buffer; do { buffer.resize(valueLength/sizeof(wchar_t)); errorCheck = RegQueryValueExW( hWin32, value.c_str(), NULL, &amp;type, &amp;buffer[0], &amp;valueLength); } while (errorCheck == ERROR_MORE_DATA); if (errorCheck != ERROR_SUCCESS) WindowsApiException::Throw(errorCheck); return buffer; </code></pre> <p>I know code like this might slightly reduce portability because it implies that <code>std::wstring</code> is contiguous -- but I'm wondering just how unportable that makes this code. Put another way, how may compilers actually take advantage of the freedom having noncontiguous memory allows?</p> <hr> <p>EDIT: I updated this question to mention C++03. Readers should note that when targeting C++11, the standard now requires that <code>basic_string</code> be contiguous, so the above question is a non issue when targeting that standard.</p>
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    1. COUnless you're certain that MSVC is successfully giving you the RVO (even though you have two different returns, one a temporary and one a variable name), then you're not "allowed" to worry about an extra copy ;-)
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      1. USSteve Jessop
    2. COI don't believe RVO would optimize a copy between vector and string....
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    3. COWhat I mean is that if the current code has no RVO then it's "create string. Copy it to the return value". You're talking maybe 50% more copying if you change that to "create vector. Copy it to string. Copy it to return value". Or maybe no extra copying at all if you do `return std::wstring(vec.begin(), vec.end());` and get "create vector. Copy it to return value (via RVO)". I'd worry about whether I could detect the speed difference before I worried about how portable the resulting code was. But that's just this example, which is why it's a comment not an answer.
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      1. USSteve Jessop
 

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