Search code examples
c++compiler-warningsobject-lifetimeconst-reference

Warning C4172: Returning a reference to const std::string bound to a local variable. How safe is it?

I was just building one of our projects at work and I see a new function was added:

const std::string& ClassName::MethodName() const
{
   return "";
}

The compiler gives a warning:

Warning C4172: returning address of local variable or temporary

I think the compiler is right. How safe is this function?

Note that the function doesn't return const char* which would be OK inasmuch as string literals have static storage duration. It returns a reference to const std::string

Solution

  • Yes it is not safe.
    Returning address of a local variable or temporary and dereferencing it results in Undefined Behavior.

    As you commented:
    Yes, the lifetime of the temporary bound to a constant reference increases till the lifetime of constant. But that needs the caller to accept the return value in a const reference, So by itself the function won't be safe.

    From the C++ Standard:
    C++03 12.2 Temporary objects:

    The second context is when a reference is bound to a temporary. The temporary to which the reference is bound or the temporary that is the complete object to a subobject of which the temporary is bound persists for the lifetime of the reference except as specified below...

    A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the constructor exits. A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of the full expression containing the call.A temporary bound to the returned value in a function return statement (6.6.3) persists until the function exits