Narrowing conversions in C++0x. Is it just me, or does this sound like a breaking change?

Question

C++0x is going to make the following code and similar code ill-formed, because it requires a so-called narrowing conversion of a double to a int<

Answer 1

I ran into this breaking change when I used GCC. The compiler printed an error for code like this:

void foo(const unsigned long long &i)
{
    unsigned int a[2] = {i & 0xFFFFFFFF, i >> 32};
}

In function void foo(const long long unsigned int&):

error: narrowing conversion of (((long long unsigned int)i) & 4294967295ull) from long long unsigned int to unsigned int inside { }

error: narrowing conversion of (((long long unsigned int)i) >> 32) from long long unsigned int to unsigned int inside { }

Fortunately, the error messages were straightforward and the fix was simple:

void foo(const unsigned long long &i)
{
    unsigned int a[2] = {static_cast<unsigned int>(i & 0xFFFFFFFF),
            static_cast<unsigned int>(i >> 32)};
}

The code was in an external library, with only two occurrences in one file. I don't think the breaking change will affect much code. Novices might get confused, though.

Answer 2

A practical instance that I have encountered:

float x = 4.2; // an input argument
float a[2] = {x-0.5, x+0.5};

The numeric literal is implicitly double which causes promotion.