Why can't the switch statement be applied to strings?

The reason why has to do with the type system. C/C++ doesn't really support strings as a type. It does support the idea of a constant char array but it doesn't really fully understand the notion of a string.

In order to generate the code for a switch statement the compiler must understand what it means for two values to be equal. For items like ints and enums, this is a trivial bit comparison. But how should the compiler compare 2 string values? Case sensitive, insensitive, culture aware, etc ... Without a full awareness of a string this cannot be accurately answered.

Additionally, C/C++ switch statements are typically generated as branch tables. It's not nearly as easy to generate a branch table for a string style switch.

As mentioned previously, compilers like to build lookup tables that optimize switch statements to near O(1) timing whenever possible. Combine this with the fact that the C++ Language doesn't have a string type - std::string is part of the Standard Library which is not part of the Language per se.

I will offer an alternative that you might want to consider, I've used it in the past to good effect. Instead of switching over the string itself, switch over the result of a hash function that uses the string as input. Your code will be almost as clear as switching over the string if you are using a predetermined set of strings:

enum string_code {
    eFred,
    eBarney,
    eWilma,
    eBetty,
    ...
};

string_code hashit (std::string const& inString) {
    if (inString == "Fred") return eFred;
    if (inString == "Barney") return eBarney;
    ...
}

void foo() {
    switch (hashit(stringValue)) {
    case eFred:
        ...
    case eBarney:
        ...
    }
}

There are a bunch of obvious optimizations that pretty much follow what the C compiler would do with a switch statement... funny how that happens.

C++

constexpr hash function:

constexpr unsigned int hash(const char *s, int off = 0) {                        
    return !s[off] ? 5381 : (hash(s, off+1)*33) ^ s[off];                           
}                                                                                

switch( hash(str) ){
case hash("one") : // do something
case hash("two") : // do something
}

Update:

The example above is C++11. There constexpr function must be with single statement. This was relaxed in next C++ versions.

In C++14 and C++17 you can use following hash function:

constexpr uint32_t hash(const char* data, size_t const size) noexcept{
    uint32_t hash = 5381;

    for(const char *c = data; c < data + size; ++c)
        hash = ((hash << 5) + hash) + (unsigned char) *c;

    return hash;
}

Also C++17 have std::string_view, so you can use it instead of const char *.

In C++20, you can try using consteval.

The problem is that for reasons of optimization the switch statement in C++ does not work on anything but primitive types, and you can only compare them with compile time constants.

Presumably the reason for the restriction is that the compiler is able to apply some form of optimization compiling the code down to one cmp instruction and a goto where the address is computed based on the value of the argument at runtime. Since branching and and loops don't play nicely with modern CPUs, this can be an important optimization.

To go around this, I am afraid you will have to resort to if statements.

In C++ and C switches only work on integer types. Use an if else ladder instead. C++ could obviously have implemented some sort of swich statement for strings - I guess nobody thought it worthwhile, and I agree with them.

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.

I think the reason is that in C strings are not primitive types, as tomjen said, think in a string as a char array, so you can not do things like:

switch (char[]) { // ...
switch (int[]) { // ...

In c++ strings are not first class citizens. The string operations are done through standard library. I think, that is the reason. Also, C++ uses branch table optimization to optimize the switch case statements. Have a look at the link.

http://en.wikipedia.org/wiki/Switch_statement

Late to the party, here's a solution I came up with some time ago, which completely abides to the requested syntax.

#include <uberswitch/uberswitch.hpp>

int main()
{
    uswitch (std::string("raj"))
    {
        ucase ("sda"): /* ... */ break;  //notice the parenthesis around the value.
    }
}

Here's the code: https://github.com/falemagn/uberswitch

In C++ you can only use a switch statement on int and char

You could put the strings in an array and use a constexpr to convert them to indices at compile time.

constexpr const char* arr[] = { "bar", "foo" };
constexpr int index(const char* str) { /*...*/ }

do_something(std::string str)
{
    switch(quick_index(str))
    {
        case index("bar"):
            // ...
            break;

        case index("foo"):
            // ...
            break;

        case -1:
        default:
            // ...
            break;
    }

For quick_index, which doesn't have to be constexpr, you could e.g. use an unordered_map to do it O(1) at runtime. (Or sort the array and use binary search, see here for an example.)

Here's a full example for C++11, with a simple custom constexpr string comparer. Duplicate cases and cases not in the array (index gives -1) will be detected at compile time. Missing cases are obviously not detected. Later C++ versions have more flexible constexpr expressions, allowing for simpler code.

#include <iostream>
#include <algorithm>
#include <unordered_map>

constexpr const char* arr[] = { "bar", "foo", "foobar" };

constexpr int cmp(const char* str1, const char* str2)
{
    return *str1 == *str2 && (!*str1 || cmp(str1+1, str2+1));
}

constexpr int index(const char* str, int pos=0)
{
    return pos == sizeof(arr)/sizeof(arr[0]) ? -1 : cmp(str, arr[pos]) ? pos : index(str,pos+1);
}

int main()
{
    // initialize hash table once
    std::unordered_map<std::string,int> lookup;
    int i = 0;
    for(auto s : arr) lookup[s] = i++;
    auto quick_index = [&](std::string& s)
        { auto it = lookup.find(s); return it == lookup.end() ? -1 : it->second; };
    
    // usage in code
    std::string str = "bar";
    
    switch(quick_index(str))
    {
        case index("bar"):
            std::cout << "bartender" << std::endl;
            break;

        case index("foo"):
            std::cout << "fighter" << std::endl;
            break;

        case index("foobar"):
            std::cout << "fighter bartender" << std::endl;
            break;
            
        case -1:
        default:
            std::cout << "moo" << std::endl;
            break;
    }
}

hare's comment to Nick's solution is really cool. here the complete code example (in C++11):

constexpr uint32_t hash(const std::string& s) noexcept
{
    uint32_t hash = 5381;
    for (const auto& c : s)
        hash = ((hash << 5) + hash) + (unsigned char)c;
    return hash;
}

constexpr inline uint32_t operator"" _(char const* p, size_t) { return hash(p); }

std::string s = "raj";
switch (hash(s)) {
case "sda"_:
    // do_something();
    break;
default:
    break;
}

More functional workaround to the switch problem:

class APIHandlerImpl
{

// define map of "cases"
std::map<string, std::function<void(server*, websocketpp::connection_hdl, string)>> in_events;

public:
    APIHandlerImpl()
    {
        // bind handler method in constructor
        in_events["/hello"] = std::bind(&APIHandlerImpl::handleHello, this, _1, _2, _3);
        in_events["/bye"] = std::bind(&APIHandlerImpl::handleBye, this, _1, _2, _3);
    }

    void onEvent(string event = "/hello", string data = "{}")
    {
        // execute event based on incomming event
        in_events[event](s, hdl, data);
    }

    void APIHandlerImpl::handleHello(server* s, websocketpp::connection_hdl hdl, string data)
    {
        // ...
    }

    void APIHandlerImpl::handleBye(server* s, websocketpp::connection_hdl hdl, string data)
    {
        // ...
    }
}

You can use switch on strings. What you need is table of strings, check every string

char** strings[4] = {"Banana", "Watermelon", "Apple", "Orange"};

unsigned get_case_string(char* str, char** _strings, unsigned n)
{
    while(n)
    {
        n--
        if(strcmp(str, _strings[n]) == 0) return n;
    }
    return 0;
}

unsigned index = get_case_string("Banana", strings, 4);

switch(index)
{
    case 1: break;/*Found string `Banana`*/
    default: /*No string*/
}

You can't use string in switch case.Only int & char are allowed. Instead you can try enum for representing the string and use it in the switch case block like

enum MyString(raj,taj,aaj);

Use it int the swich case statement.