Can C 11 String Literals Be Used for Compile-Time String Hashing?

Compile-Time String Hashing Using C 11 String Literals

Question:

Is it possible to calculate a string's hash at compile time using C 11's new string literals?

Answer:

Yes, it is possible, but requires caution, as it may not work with all compilers.

Operator:

The operator for compile-time string hashing is not explicitly defined. Instead, a constant expression is generated using a compile-time hash function that takes a string literal as input.

Use Cases:

One potential use case is to create a switch statement that can perform different actions based on the hash of a string:

void foo(const std::string& value)
{
   switch(std::hash<value>())
   {
      case "one"_hash: one(); break;
      case "two"_hash: two(); break;
      default: other(); break;
   }
}

Example Implementation:

Here is an example implementation using a constexpr function:

template<size_t idx>
constexpr uint32_t crc32(const char * str)
{
    // Compile-time CRC32 implementation
    return (crc32<idx-1>(str) >> 8) ^ crc_table[(crc32<idx-1>(str) ^ str[idx]) & 0x000000FF];
}

template<>
constexpr uint32_t crc32<size_t(-1)>(const char * str)
{
    // Stop recursion
    return 0xFFFFFFFF;
}

#define COMPILE_TIME_CRC32_STR(x) (crc32<sizeof(x) - 2>(x) ^ 0xFFFFFFFF)

This function can be used as follows:

enum TestEnum
{
    CrcVal01 = COMPILE_TIME_CRC32_STR("stack-overflow"),
};

In this example, CrcVal01 will be evaluated to the compile-time hash of the string "stack-overflow."

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