C++ union element size based on other's element size

[piot...@gmail.com]

Recently I have come across some discrepancies between gcc and clang. Consider the following piece of code:

struct S
{
    union
    {
        int arr1[10];
        char arr2[sizeof(arr1)];
    };
};

It compiles successfully with gcc 4.9.2 in c++03 and c++11 mode. However when I change S to be a template like so:

template <size_t N>
struct S
{
    union
    {   
        int arr1[N];
        char arr2[sizeof(arr1)];
    };  
};

I get the following error output:

error: int S<10ul>::<anonymous union>::arr1 [10]’ is inaccessible

int arr1[N];

error: within this context

char arr2[sizeof(arr1)];

Clang compiles both versions only in c++11 mode. Is this a legal piece of code? There is an open question on this particular problem on stackoverflow. Some people quoted the standard however none of them seem to give a definitive answer.

Thanks,

Piotr

[Richard Smith]

On Mon, Mar 2, 2015 at 4:26 PM, <piot...@gmail.com> wrote:

Recently I have come across some discrepancies between gcc and clang. Consider the following piece of code:

struct S
{
    union
    {
        int arr1[10];
        char arr2[sizeof(arr1)];
    };
};

It compiles successfully with gcc 4.9.2 in c++03 and c++11 mode. However when I change S to be a template like so:

template <size_t N>
struct S
{
    union
    {   
        int arr1[N];
        char arr2[sizeof(arr1)];
    };  
};

I get the following error output:

error: int S<10ul>::<anonymous union>::arr1 [10]’ is inaccessible

int arr1[N];

error: within this context

char arr2[sizeof(arr1)];

Clang compiles both versions only in c++11 mode. Is this a legal piece of code?

This is ill-formed in C++98/C++03 due to the use of the non-static data member 'arr1' outside of a member function. It is valid in C++11, in either templated or non-templated form.

There is an open question on this particular problem on stackoverflow. Some people quoted the standard however none of them seem to give a definitive answer.

Thanks,

Piotr

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[piot...@gmail.com]

Can you please provide a quote from the standard explaining what you said in your email?

Thank,

Piotr