C++: is return value a L-value?

Question

Consider this code:

struct foo
{
  int a;
};

foo q() { foo f; f.a =4; return f;}

int main()
{
  foo i;
  i.a = 5;
  q() = i;
}

No compile

Answer 1

One interesting application of this:

void f(const std::string& x);
std::string g() { return ""; }

...

f(g() += "");

Here, g() += modifies the temporary, which may be faster that creating an additional temporary with + because the heap allocated for g()'s return value may already have enough spare capacity to accommodate .

See it run at ideone.com with GCC / C++11.

Now, which computing novice said something about optimisations and evil...? ;-].