How Can I Efficiently Handle Non-Integer Constants in a C/C Switch Statement?

Switch with Non-Integer Constants in C/C

When working with non-POD (Plain Old Data) constants, it becomes necessary to select between different code paths based on the value of the constant. However, the traditional switch statement only accepts integer values. This presents a challenge for selecting between non-integer constants.

Traditional Approach: Nested if Statements

A simple way to handle this is to use a series of nested if statements, such as:

if( str == "foo" )      ...
else if( str == "bar" ) ...
else                    ...

While this is straightforward, it can become cumbersome and inefficient for large numbers of cases, as it has O(n) complexity where "n" is the number of cases.

Advanced Techniques

To achieve better efficiency, more advanced techniques can be employed. One approach involves using data structures like maps to represent strings as integers and then using a standard switch statement. However, this introduces additional coding complexity.

Compile-Time Magic: Macro and Template Magic

A unique approach is to use macro and template magic to generate an unrolled binary search at compile time. Libraries like fastmatch.h can provide a clean syntax for defining case matches:

NEWMATCH
MATCH("asd")
  some c++ code
MATCH("bqr")
  ... the buffer for the match is in _buf
MATCH("zzz")
  ...  user.YOURSTUFF 
/*ELSE 
  optional
*/
ENDMATCH(xy_match)

This generates a function like xy_match(char *&_buf, T &user) that can be called with ease.

C 11 Updates: Lambdas and Initializer List

With C 11, lambdas and initializer lists provide a cleaner solution:

Switch("ger", {
    {"asdf", []{ printf("0\n"); }},
    {"bde", []{ printf("1\n"); }},
    {"ger", []{ printf("2\n"); }}
}, [](const char *a, const char *b) { return strcmp(a, b) < 0; });

This approach utilizes a binary search on a sorted list of case matches, providing efficient O(log n) complexity.

Compile-Time Trie

As a further advancement, compile-time tries can be leveraged to handle unsorted case-branches. This approach uses advanced C 11 metaprogramming to generate a search trie at compile time. Each trie node contains a switch statement to optimize code generation.

The implementation can be found on GitHub at smilingthax/cttrie.

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