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            清風竹林

            ぷ雪飄絳梅映殘紅
               ぷ花舞霜飛映蒼松
                 ----- Do more,suffer less

            std::string is contiguous (轉)

            原文地址: http://hpyblg.wordpress.com/2010/06/03/stdstring-is-contiguous/

            You can safely assume that the memory buffer used by std::string is contiguous. Specifically, the address of the string’s first character can be used as the address for the whole string, just like a C-style char array:

            1std::string str = "foo";
            2strncpy(&str[0], "bar", 3); // str now contains "bar".

            Why is this safe? The current C++ standard apparently doesn’t guarantee that the string is stored contiguously, but it is in all known implementations. Additionally, the next C++ standard (C++0x) will make this guarantee. So the above usage is valid on all present and future C++ implementations.

            Why is this important? It’s common for functions, especially in the Windows API, to “return” strings by copying them into a buffer passed to the function. Since the memory buffer used in std::string is contiguous you can safely pass it to the function, after resizing the string to the correct size.

            A typical usage for Windows API functions:

            01// get required buffer size
            02DWORD bufSize = 0;
            03GetComputerNameA(NULL, &bufSize);
            04if (!bufSize && GetLastError() != ERROR_BUFFER_OVERFLOW) {
            05  throw std::runtime_error("GetComputerNameA failed");
            06}
            07// bufSize now contains required size of buffer, including null terminator
            08std::string buf(bufSize, '\0');
            09if (!GetComputerNameA(&buf[0], &bufSize)) {
            10  throw std::runtime_error("GetComputerNameA failed");
            11}
            12// bufSize now contains actual size of data
            13buf.resize(bufSize);
            14// now use buf as a regular std::string

            This is cumbersome but actually easier than plain C code, since you don’t have to manage the memory yourself.

            Note that the expression &str[0] is valid only if str isn’t empty. Also, everything I’ve said also applies to std::wstring, the wide-character version of std::string.

            References:

            posted on 2011-05-26 20:52 李現民 閱讀(606) 評論(3)  編輯 收藏 引用 所屬分類: 絕對盜版

            評論

            # re: std::string is contiguous (轉) 2011-05-27 10:35 oldman

            為什么不用 str.data() 呢?  回復  更多評論   

            # re: std::string is contiguous (轉) 2011-05-27 10:45 李現民

            @oldman
            &str[0]的數據是可以修改的, 而str.data()的數據不能修改  回復  更多評論   

            # re: std::string is contiguous (轉) 2011-05-27 10:50 oldman

            哦,明白你的意思了。我的意思是說既然現在大家對于data的實現也是直接拿內部數據返回的,拿出來去掉const屬性,一樣可以用。  回復  更多評論   

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