36

Now I have a function that has to return a string. I saw a particular implementation where he returns a const char * from the function.

Something like this:

const char * GetSomeString() 
{ 
  ........   
  return somestlstring.c_str(); 
}

SomeOtherFoo ()
{
  const char * tmp = GetSomeString();
  string s = tmp;
}

Now I felt there is something potentially wrong with this. Is my gut feel right? or Is this a perfectly safe code?

Kindly give me ur suggestions. I have a feeling return const char * this way might result in havoc..

Thanks, Arjun

AMM
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6 Answers6

33

Depending on what somestlstring is and what is being done there.

If it is a local variable you are returning a pointer into memory that is being released when GetSomeString completes, so it is a dangling pointer and an error.

It all boils down to the lifetime of somestlstring and the operations you perform on it. The pointer returned by .c_str() is guaranteed to be valid only up to the next mutating operation in the string. So if something changes somestlstring from the call to .c_str() and before s is constructed you will be in undefined behavior land.

David Rodríguez - dribeas
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8

If you are asking about the lifetime of the const char * returned by the std::string c_str() function, it is valid until you modify the string you obtained it from, or until the string is destroyed. Returning it from a function is OK (though I would say not great practice), provided you bear those two facts in mind.

3

This is OK under the conditions @Neil elaborated on. However a better way would be to return a reference to the string

string const& GetSomeString() 
{ 
  ........   
  return somestlstring; 
}

string s = GetSomeString();

Still keeping in mind that ´somestlstring` must not be a local automatic variable but stored somewhere else in a namespace or a class. Otherwise you can return the string by value

string GetSomeString() 
{ 
  ........   
  return somestlstring; // can be a local automatic variable
}

string s = GetSomeString();
Johannes Schaub - litb
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2

It's not great - how long does the memory for your string stick around? Who is responsible for deleting it? Does it need to be deleted at all?

You're better off returning a string object that is responsible for allocating and freeing the string memory - this could be a std::string, or a QString (if you're using Qt), or CString (if you're using MFC / ATL).

on a slightly different note, will your string ever be unicode? Most string classes can deal transparently with unicode data, but const char will not...

Thomi
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2

It depends upon where the somestlstring variable is located.

If it is a variable locale to the GetSomeString() function, then this is plainly wrong. Indeed, the somestlstring variable is destroyed at the end of the function, and thus the const char * points to something that does not exist anymore.

If it is a global variable, then this code is right.

Didier Trosset
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2

To add some scenarios in which this would be ok:

  • somestlstring is a global variable initialized in the same translation unit (.cpp) as GetSomeString()
  • somestlstring is a non-static class member, and GetSomeString is a member of that class. In that case, the lifetime of the pointer returned must be documented (basically - as others said - until the strign changes or the object is destroyed)
  • you are returning a char const * to a literal or compile-time initialized string
peterchen
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