Is there a way to get std:string's buffer

问题:

Is there a way to get the "raw" buffer o a std::string?
I'm thinking of something similar to CString::GetBuffer(). For example, with CString I would do:

CString myPath;  
::GetCurrentDirectory(MAX_PATH+1, myPath.GetBuffer(MAX_PATH));  
myPath.ReleaseBuffer();  

So, does std::string have something similar?

回答1:

Use std::vector<char> if you want a real buffer.

#include <vector>
#include <string>

int main(){
  std::vector<char> buff(MAX_PATH+1);
  ::GetCurrentDirectory(MAX_PATH+1, &buff[0]);
  std::string path(buff.begin(), buff.end());
}

Example on Ideone.

回答2:

While a bit unorthodox, it's perfectly valid to use std::string as a linear memory buffer, the only caveat is that it isn't supported by the standard until C++11 that is.

std::string s;
char* s_ptr = &s[0]; // get at the buffer

To quote Herb Sutter,

Every std::string implementation I know of is in fact contiguous and null-terminates its buffer. So, although it isn’t formally guaranteed, in practice you can probably get away with calling &str[0] to get a pointer to a contiguous and null-terminated string. (But to be safe, you should still use str.c_str().)

Probable is key here. So, while it's not a guarantee, you should be able to rely on the principle that std::string is a linear memory buffer and you should assert facts about this in your test suite, just to be sure.

You can always build your own buffer class but when you're looking to buy, this is what the STL has to offer.

回答3:

Not portably, no. The standard does not guarantee that std::strings have an exclusive linear representation in memory (and with the old C++03 standard, even data-structures like ropes are permitted), so the API does not give you access to it. They must be able to change their internal representation to that (in C++03) or give access to their linear representation (if they have one, which is enforced in C++11), but only for reading. You can access this using data() and/or c_str(). Because of that, the interface still supports copy-on-write.

The usual recommendation for working with C-APIs that modify arrays by accessing through pointers is to use an std::vector, which is guaranteed to have a linear memory-representation exactly for this purpose.

To sum this up: if you want to do this portably and if you want your string to end up in an std::string, you have no choice but to copy the result into the string.

回答4:

It has c_str, which on all C++ implementations that I know returns the underlying buffer (but as a const char *, so you can't modify it).

回答5:

According to this MSDN article, I think this is the best approach for what you want to do using std::wstring directly. Second best is std::unique_ptr<wchar_t[]> and third best is using std::vector<wchar_t>. Feel free to read the article and draw you own conclusions.

// Get the length of the text string
// (Note: +1 to consider the terminating NUL)
const int bufferLength = ::GetWindowTextLength(hWnd) + 1;
// Allocate string of proper size
std::wstring text;
text.resize(bufferLength);
// Get the text of the specified control
// Note that the address of the internal string buffer
// can be obtained with the &text[0] syntax
::GetWindowText(hWnd, &text[0], bufferLength);
// Resize down the string to avoid bogus double-NUL-terminated strings
text.resize(bufferLength - 1);

回答6:

 std::string str("Hello world");
 LPCSTR sz = str.c_str();

Keep in mind that sz will be invalidated when str is reallocated or goes out of scope. You could do something like this to decouple from the string:

 std::vector<char> buf(str.begin(), str.end()); // not null terminated
 buf.push_back(0); // null terminated

Or, in oldfashioned C style (note that this will not allow strings with embedded null-characters):

 #include <cstring>

 char* sz = strdup(str.c_str());

 // ... use sz

 free(sz);