Re: Is this a compiler bug?
On Sun, 01 Oct 2006 22:29:00 -0700, "Martin Lafferty" <null@null.com>
wrote:
My compiler is VC++ 2005.
Could some wise soul please explain to me why the following code does
not work as I expect it to?
//Begin snippet
#include <iostream>
#include <tchar.h>
template<typename T>
void Test(const char *Name, const T &Func) {
std::cout<<Name<<std::endl;
Func();
}
template<typename T>
struct func {
const T &_a;
func(const T& A): _a(A) {}
void operator()() const {
std::cout<<"a="<<_a<<std::endl;
}
};
struct int_func: func<int> {
int_func(int a): func<int>(a) {}
};
int _tmain(int argc, _TCHAR* argv[]) {
Test("using func", func<int>(42)); //writes 42 to cout
Test("using int_func", int_func(42)); //writes random int to cout
return 0;
}
//End snippet
Of course I could do this with a typedef but the real code is a bit
more complicated than this simple test case.
The temporary 'int_func' object seems to get created correctly, but as
soon as the constant "using int_func" then _a gets corrupted. Any
insights into what is going on here would be much appreciated.
You're binding a reference to a temporary, and the temporary ceases to
exist after the ctor has completed, leaving _a to be a dangling reference.
I've come to avoid const reference class members and the ctors that
initialize them such as your "func" ctor, because they lead all too
frequently to this problem. Non-const reference members and their
corresponding ctors are fine, because modulo compiler bugs, they won't
permit binding temporaries, but I still comment them to the effect that the
ctor argument must outlive the object being created. If making everything
non-const isn't what you want, I'd recommend changing _a from a const T& to
a const T, which means func will make a copy of its argument. If that won't
work either, then I think you have a design problem, because what you have
is highly error-prone.
--
Doug Harrison
Visual C++ MVP