CODEWITHSUNDEEP
c++undefined-behavior
balki
balki

Reputation: 27694

Is the following undefined behaviour?

The following is an in-lined(defined inside header file) static member function. Is the literal string "MyClass" always guaranteed to be in static memory? If not, will this not be returning a pointer in stack?

const char * className()
{
return "MyClass";
}

Edit:

How about this?

const RWCString& className()
{
return "MyClass";
}

RWCString is an string class which has a implicit constructor which takes a const char*. http://www.roguewave.com/portals/0/products/sourcepro/docs/11/html/toolsref/rwcstring.html

Upvotes: 0

Views: 172

Answers (2)

Matthieu M.
Matthieu M.

Reputation: 300439

The first example:

const char * className()
{
  return "MyClass";
}

is fine. "MyClass" is a literal of type char const[8] which lifetime begins before your code is invoked and ends after your code is done, so no issue.

The second example, however, will not work.

const RWCString& className()
{
  return "MyClass";
}

It requires and object of type RWCString to be constructed within the function in order to be able to return a reference to it. However what is built as a local variable or temporary within a function cannot be returned by reference, so you get undefined behavior (if it compiles).

You can very simply turn it into a "good" function though:

const RWCString& className()
{
  static RWCString const N = "MyClass";
  return N;
}

Here I create a local static object N which will be constructed the first time the function is called. Because it is static its lifetime extends past the call so it is fine to return a reference to it.

EDIT: as Steve pointed out, temporary is more appropriate that local variable here.

Upvotes: 5

Mysticial
Mysticial

Reputation: 471567

Nope. This is completely defined. The string isn't on the stack. It's in global memory. So the pointer it returns is valid. (even better: you declared it const)

Upvotes: 5

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