Is it reasonable to use std::basic_string as a contiguous buffer when targeting C++03?

问题

I know that in C++03, technically the std::basic_string 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 basic_string 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.

Example:

DWORD valueLength = 0;
DWORD type;
LONG errorCheck = RegQueryValueExW(
        hWin32,
        value.c_str(),
        NULL,
        &type,
        NULL,
        &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,
            &type,
            &buffer[0],
            &valueLength);
} while (errorCheck == ERROR_MORE_DATA);

if (errorCheck != ERROR_SUCCESS)
    WindowsApiException::Throw(errorCheck);

return buffer;

I know code like this might slightly reduce portability because it implies that std::wstring 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?

EDIT: I updated this question to mention C++03. Readers should note that when targeting C++11, the standard now requires that basic_string be contiguous, so the above question is a non issue when targeting that standard.

回答1:

I'd consider it quite safe to assume that std::string allocates its storage contiguously.

At the present time, all known implementations of std::string allocate space contiguously.

Moreover, the current draft of C++ 0x (N3000) [Edit: Warning, direct link to large PDF] requires that the space be allocated contiguously (§21.4.1/5):

The char-like objects in a basic_string object shall be stored contiguously. That is, for any basic_string object s, the identity &*(s.begin() + n) == &*s.begin() + n shall hold for all values of n such that 0 <= n < s.size().

As such, the chances of a current or future implementation of std::string using non-contiguous storage are essentially nil.

回答2:

A while back there was a question about being able to write to the storage for a std::string as if it were an array of characters, and it hinged on whether the contents of a std::string were contiguous:

Is it legal to write to std::string?

My answer indicated that according to a couple well regarded sources (Herb Sutter and Matt Austern) the current C++ standard does require std::string to store its data contiguous under certain conditions (once you call str[0] assuming str is a std::string) and that that fact pretty much forces the hand of any implementation.

Basically, if you combine the promises made by string::data() and string::operator[]() you conclude that &str[0] needs to return a contiguous buffer. Therefore Austern suggests that the committee just make that explicit, and apparently that's what'll happen in the 0x standard (or are they calling it the 1x standard now?).

So strictly speaking an implementation doesn't have to implement std::string using contiguous storage, but it has to do so pretty much on demand. And your example code does just that by passing in &buffer[0].

Links:

Herb Sutter's comment
Matt Austern's C++ Standard Library Defect Report
previous SO answer

回答3:

Edit: You want to call &buffer[0], not buffer.data(), because [] returns a non-const reference and does notify the object that its contents can change unexpectedly.

It would be cleaner to do buffer.data(), but you should worry less about contiguous memory than memory shared between structures. string implementations can and do expect to be told when an object is being modified. string::data specifically requires that the program not modify the internal buffer returned.

VERY high chances that some implementation will create one buffer for all strings uninitialized besides having length set to 10 or whatever.

Use a vector or even an array with new[]/delete[]. If you really can't copy the buffer, legally initialize the string to something unique before changing it.

回答4:

The result is undefined and I would not do it. The cost of reading into a vector and then converting to a string is trivial in modern c++ heaps. VS the risk that your code will die in windows 9

also, doesnt that need a const_cast on &buffer[0]?

回答5:

Of course, allocating a vector here is silly. Using std::wstring here is not wise also. It's better to use a char array to call the winapi. construct a wstring when returning value.

来源：https://stackoverflow.com/questions/2256160/is-it-reasonable-to-use-stdbasic-stringt-as-a-contiguous-buffer-when-targeti

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