Warning: should not initialize a non-const reference with a temporary

I get the warning from the title on Sun Studio 12.1 with the following snippet:

#include <vector>

std::vector<int> g()
{
  std::vector<int> result;
  result.push_back(5);
  return result;
}

int main()
{
  int b = g()[0];  // <- Warning in this 开发者_开发知识库line

  return b;
}

Warning text is:

Warning: should not initialize a non-const reference with a temporary.

While I know that initialising a non-const reference with a temporary is a bad thing, I cannot see how that happens here. I know that [0] returns a reference to the first element of the vector which itself is temporary, but I fail to see what the problem is.

Can somebody explain

• Why does to compiler complain?
• Is it a legitimate warning?
  • If yes, what do I have to change?
  • If no, how can I silence it elegantly?

---

No, it's not legitimate. The return value of g() is a temporary, but it is not const - you just can't get a non-const reference to it. The non-const member operator[] is perfectly valid to call here, and the double to integer conversion is just as safe.

---

This Sun compiler looks very weird, it doesn't seem legitimate at all to me. Ideone has no problem compiling it.

Regarding the silencing part:

std::vector<double> const tmp = g();
int b = tmp[0];

That is, introducing a named variable instead of leaving the temporary floating.

EDIT:

As suggested in comments, const-qualifying the return value might help.

std::vector<double> const g();

int main() {
  int b = g()[0];
  return b;
}

---

Yes, it does initialize a non-const reference with a temporary. But only conceptually during overload resolution and not actually. The compiler should not warn about it.

In overload resolution, the operator[] has this function parameter signature

operator[](std::vector<int>&, std::vector<int>::size_type);

The first parameter will receive the temporary returned by g(), but as said, that's fine and C++ makes specifically an exception for that reference, which is the so-called "implicit object parameter", so that overload resolution accepts the temporary argument.