C / C++ / MFC

George_George wrote:

1. I have debugged that for small buffer (size <= 15), string will use buffer on stack, char array, and for larger buffer (>15), the space on heap is used (and destructor of string will free the space on heap), correct?

George_George wrote:

2. What is the function of the internal member variable _Bx._Buf and _Bx._Ptr? I have posted my debug results and why sometimes _Bx._Ptr is bad ptr and sometimes _Bx._Ptr has valid content.

George_George wrote:

1. I have debugged that for small buffer (size <= 15), string will use buffer on stack, char array, and for larger buffer (>15), the space on heap is used (and destructor of string will free the space on heap), correct?

George_George wrote:

2. What is the function of the internal member variable _Bx._Buf and _Bx._Ptr? I have posted my debug results and why sometimes _Bx._Ptr is bad ptr and sometimes _Bx.

union _Bxty
{  // storage for small buffer or pointer to larger one
  _Elem _Buf[_BUF_SIZE];
  _Elem *_Ptr;
} _Bx;

George_George wrote:

1.

So, your answer to my question 1 is _Buf is used to hold small sized content, and _Ptr is used to hold larger sizes content. Small sized content is on stack and larger sized content is on heap, correct?

George_George wrote:

2.

What is your answer to my original question item 2? I think because when the content is small, only _Buf is used and _Ptr is empty, so _Ptr is bad ptr? Correct understanding?

union Foo
{
  char a[4];
  char *p;
};
void main()
{
  Foo f;
  f.a[0]='m';
  f.a[1]='s';
  f.a[2]='d';
  f.a[3]='n';
}

George_George wrote:

1.

In item 2, you mean when the content is big, the first 4 bytes of _Buf is actually the address of the heap (i.e. the value of _Ptr) where the content resides?

George_George wrote:

2.

In my original code, I return local variable b to main function variable s1, is it dangerous code?

George_George wrote:

when return b is called it creates a new temp instance and making s1 in main references it

#include <string>

using namespace std;

string& foo()
{
	string b;
	b.append ("msdn");
	// at this point _Bx._Buf content is msdn, _Bx._Ptr is bad ptr
	b.append (".microsoft");
	// at this point _Bx._Buf content is msdn.microsoft, _Bx._Ptr is bad ptr
	b.append (".com");
	// at this point  _Bx._Buf content is msdn.microsoft.com, _Bx._Ptr content is msdn.microsoft.com

	return b;
}

int main()
{
	string& s1 = foo();
	
	return 0;
}

George_George wrote:

1.

So, in my original code, there are 3 instances,

- instance b and copied b to a new temp instance as the return value;
- instance s1 which is copy contructed from the temp instance.

Correct understanding?

George_George wrote:

2.

If I change the code in the following way, return the reference to local variable b, is it safe code?

int main()
{
  string& s1 = foo();
  cout << s1 << endl;	
  return 0;
}

George_George wrote:

1.

For your comments, I think using reference is better since it will save instance creation and useless copy?

George_George wrote:

2.

I have tried to use cout, and the output is correct "msdn.microsoft.com", what is wrong?

#include <string>
#include <iostream>

using namespace std;

void foo(string& b)
{
	b.append ("msdn");
	// _Bx._Buf content is msdn, _Bx._Ptr is bad ptr
	b.append (".microsoft");
	// _Bx._Buf content is msdn.microsoft, _Bx._Ptr is bad ptr
	b.append (".com");
	// _Bx._Buf content is msdn.microsoft.com, _Bx._Ptr content is msdn.microsoft.com

	return;
}

int main()
{
	string b;
	
	foo (b);
	
	cout << b << endl;

	return 0;
}

George_George wrote:

Is it the best solution -- I mean saving the time to create new instance and safe code?