static_cast and explicit conversion operators

Question

I have found that the following code gives a compile error on gcc 4.7.3 but not on clang 3.3:

#include <cstdint>                                          

struct X {
    explicit operator uint32_t() { return 0; }
};

int main() {
    static_cast< int >( X() );
    return 0;
}

The question is, which is right? Gcc 4.7.3 says:

testcast.cpp:8:29: error: invalid static_cast from type 'X' to type 'int'

What I think happens is that clang uses the uint32_t operator to get an unsigned and then implicitly converts that to int. I suspect the spec does not leave this undefined, and as such I'd expect one of the compilers to be wrong.

In this case, I think GCC is right in rejecting this code. Chained-conversion is not allowed: means, if A can convert into B which can convert into C, it does NOT mean A can directly convert into C. — Sarfaraz Nawaz
– Sarfaraz Nawaz, Commented Nov 19, 2013 at 16:47
@chris, yes that will work on both, but that's not quite what I'm asking -- this happens inside a relatively deep stack of templates, and clang will call a different function from gcc because it has a different idea of what is a valid expression. I like the gcc behaviour more in this case I guess, as I don't want the unsigned->signed conversion to happen "behind my back" (the conversions are explicit for a reason). But I'd rather know for sure which is right. :-) — mornfall
– mornfall, Commented Nov 19, 2013 at 16:54
@mornfall, I'm implying that GCC is correct, but I'm not completely sure. If Clang 3.4 rejects it, too, that's almost certainly the correct behaviour. — Qaz
– Qaz, Commented Nov 19, 2013 at 17:00
Tip-of-trunk clang rejects this with a very similar error message. — Howard Hinnant
– Howard Hinnant, Commented Nov 19, 2013 at 17:51

masoud · Accepted Answer · 2013-11-19 16:53:38Z

3

You must explicitly cast it to uint32_t, otherwise a compile error. You should try this:

static_cast< uint32_t >( X() );

So, If it doesn't make error in clang-3.3, it seems as a bug.

Observation: GCC and Clang-3.4 both reject the code and make compile errors.

edited Nov 19, 2013 at 16:53

answered Nov 19, 2013 at 16:41

masoud

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2 Comments

mornfall Over a year ago

The links you provided show a failure in both, but completely unrelated to my question (gcc is running in C++98 mode failing to even recognize the "explicit" keyword; clang says main.cpp:8:23: error: unknown type name 'uint32_t')

mornfall Over a year ago

See here -- I don't know what version of clang that web runs, but it compiles the code without complaint if you actually tell it that it is C++11. (edit: it says clang version 3.4 (trunk 184460); hence even clang 3.4 claims the code is OK)

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