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

Question

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?

Answer 1

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().)

"Probably" 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.

Answer 2

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.

Answer 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.

Answer 4

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);

Answer 5

 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);

Answer 6

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).

Answer 7

I think you will be frowned upon by the purists of STD cult for doing this. In any case, its much better to not relay on bloated and generic standard library if you want dynamic string type that can be easily passed to low level API functions that will modify its buffer and size at the same time, without any conversions, than you will have to implement it! Its actually very challenging and interesting task to do. For example in my custom txt type I overload this operators:

ui64 operator~() const; // Size operator
uli32 * operator*();    // Size modification operator
ui64 operator!() const; // True Size Operator
txt& operator--();      // Trimm operator

And also this casts:

operator const char *() const;
operator char *();

And as such, i can pass txt type to low level API functions directly, without even calling any .c_str(). I can then also pass the API function it's true size (i.e. size of buffer) and also pointer to internal size variable (operator*()), so that API function can update amount of characters written, thus giving valid string without the need to call stringlength at all!

I tried to mimic basic types with this txt, so it has no public functions at all, all public interface is only via operators. This way my txt fits perfectly with ints and other fundamental types.