Question

GCC complains about this code even though I compile with -std=c++11 flag, and my gcc version supposedly supports Unrestricted unions (>4.6).

union 
{
    struct 
    {
        float4 I,J,K,T;
    };
    struct 
    {
        float4 m_lines[4];
    };
    struct
    {
        float m16f[16];
    };
    struct 
    {
        float m44f[4][4];
    };
};

Note that float4 has a non-default constructor that takes 0 parameters.

class float4 
{   
    public:
       float4();
 ....
};

Any idea of what I could do ? The error is :

<anonymous union>::<anonymous struct>::I’ with constructor not allowed in anonymous aggregate

Answer 1

The issue here is not the fact that your class float4 has a constructor, making it a non-POD under the old C++03 definition of POD. Rather, the issue is that your union and the structs within your union are anonymous. If you simply name them, it will work:

union u
{
    struct s1
    {
        float4 I,J,K,T;
    };
    struct s2
    {
        float4 m_lines[4];
    };
    struct s3
    {
        float m16f[16];
    };
    struct s4
    {
        float m44f[4][4];
    };
};