[Qpid Wiki] Update of "CppTips/BewareOfStringPromotion" by AlanConway

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The following page has been changed by AlanConway:
http://wiki.apache.org/qpid/CppTips/BewareOfStringPromotion

New page:
`std::string` is a useful tool for simplifying memory management of strings and avoiding unnecessary copies by reference counting. However there is one common gotcha where it ''causes'' unnecessary copies. Consider: 

{{{
void f(const std::string& s) { cout << s << endl };

void g() {
  for (int i = 0; i <  1000; ++i) { f("hello"); };
}
}}}

This actually allocates, copies and deletes 1000 heap buffers with the string "hello"! The problem here is that "hello" is ''not'' an instance of `std::string`. It is a `char[5]` that must be converted to a temporary `std::string` using the appropriate constructor. However `std::string` always wants to manage its own memory, so the constructor allocates a new buffer and copies the string. Once f() returns and we go round the loop again the temporary is deleted along with its buffer.

Here's a better solution:

{{{
void f(const std::string& s) { cout << s << endl };
namespace { const std::string hello("hello"); }
void g() {
  for (int i = 0; i <  1000; ++i) { f(hello); };
}
}}}

This time we have a constant `std::string` that is created once at start up and destroyed once at shut-down. The anonymous namespace makes the constant private to this `.cpp` file so we wont have name clashes. (Its similar to using the `static` keyword on a global declaration in C, but anonymous namespaces are the preferred way to do it in modern C++)