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c++optimizationendiannessrapidjson
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kyb

Reputation: 8211

Is there any conversions when storing/reading integers of different size to/from union?

Reading Rapidjson code I found some interesting optimization with "type punning".

    // By using proper binary layout, retrieval of different integer types do not need conversions.
    union Number {
#if RAPIDJSON_ENDIAN == RAPIDJSON_LITTLEENDIAN
        struct I {
            int i;
            char padding[4];
        }i;
        struct U {
            unsigned u;
            char padding2[4];
        }u;
#else
        struct I {
            char padding[4];
            int i;
        }i;
        struct U {
            char padding2[4];
            unsigned u;
        }u;
#endif
        int64_t i64;
        uint64_t u64;
        double d;
    };  // 8 bytes

It looks like only BE CPUs are affected by this optimization. How does this increases performance? I'd like to test but do not have BE machine.

Wikipedia says:

While not allowed by C++, such type punning code is allowed as "implementation-defined" by the C11 standard[15] and commonly used[16] in code interacting with hardware.[17]

So is it legal in C++? I believe in absolute most cases it works fine. But should I use it in new code?

Upvotes: 0

Views: 45

Answers (1)

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

So is it legal in C++?

No, it isn't legal in c++ (Wikipedia also already stated "While not allowed by C++ ...").

In c++ a union is just reserving memory for the contained union members, such that it is enough to fit the largest member. That memory is shared by all members.

Accessing a different member from the union as was used to initialize it, is undefined behavior. You need to decide beforehand with which union members to work, if these are shared by any functions (this is often done using a type discriminator).

Upvotes: 1

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