Why the switch statement cannot be applied on strings?

Question

Compiling the following code and got the error of type illegal.

int main()
{
    // Compilation error - switch expression of type illegal
    sw         


        

Answer 1

Why not? You can use switch implementation with equivalent syntax and same semantics. The C language does not have objects and strings objects at all, but strings in C is null terminated strings referenced by pointer. The C++ language have possibility to make overload functions for objects comparision or checking objects equalities. As C as C++ is enough flexible to have such switch for strings for C language and for objects of any type that support comparaison or check equality for C++ language. And modern C++11 allow to have this switch implementation enough effective.

Your code will be like this:

std::string name = "Alice";

std::string gender = "boy";
std::string role;

SWITCH(name)
  CASE("Alice")   FALL
  CASE("Carol")   gender = "girl"; FALL
  CASE("Bob")     FALL
  CASE("Dave")    role   = "participant"; BREAK
  CASE("Mallory") FALL
  CASE("Trudy")   role   = "attacker";    BREAK
  CASE("Peggy")   gender = "girl"; FALL
  CASE("Victor")  role   = "verifier";    BREAK
  DEFAULT         role   = "other";
END

// the role will be: "participant"
// the gender will be: "girl"

It is possible to use more complicated types for example std::pairs or any structs or classes that support equality operations (or comarisions for quick mode).

Features

• any type of data which support comparisions or checking equality
• possibility to build cascading nested switch statemens.
• possibility to break or fall through case statements
• possibility to use non constatnt case expressions
• possible to enable quick static/dynamic mode with tree searching (for C++11)

Sintax differences with language switch is

• uppercase keywords
• need parentheses for CASE statement
• semicolon ';' at end of statements is not allowed
• colon ':' at CASE statement is not allowed
• need one of BREAK or FALL keyword at end of CASE statement

For C++97 language used linear search. For C++11 and more modern possible to use quick mode wuth tree search where return statement in CASE becoming not allowed. The C language implementation exists where char* type and zero-terminated string comparisions is used.

Read more about this switch implementation.