std::string is contiguous

std::string is contiguous (转)

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:

1	std::string str = "foo";

2	strncpy(&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

02	DWORD bufSize = 0;

03	GetComputerNameA(NULL, &bufSize);

04	if (!bufSize && GetLastError() != ERROR_BUFFER_OVERFLOW) {

05	throw std::runtime_error("GetComputerNameA failed");

06

}

07	// bufSize now contains required size of buffer, including null terminator

08	std::string buf(bufSize, '\0');

09	if (!GetComputerNameA(&buf[0], &bufSize)) {

10	throw std::runtime_error("GetComputerNameA failed");

11

}

12	// bufSize now contains actual size of data

13	buf.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:

• Contiguousness of STL containers and string
• Cringe not: Vectors are guaranteed to be contiguous