Evaluate a string with a switch in C++ [duplicate]

Question

I want to evaluate a string with a switch but when I read the string entered by the user throws me the following error.

#include<iostream> using namespace std;      int main() {         string a;         cin>>a;         switch (string(a)) {         case "Option 1":             cout<<"It pressed number 1"<<endl;             break;         case "Option 2":             cout<<"It pressed number 2"<<endl;             break;         case "Option 3":             cout<<"It pressed number 3"<<endl;             break;         default:             cout<<"She put no choice"<<endl;             break;         }         return 0;     } 

error: invalid cast from type 'std::string {aka std::basic_string}' to type 'int

Answer 1

As said before, switch can be used only with integer values. So, you just need to convert your "case" values to integer. You can achieve it by using constexpr from c++11, thus some calls of constexpr functions can be calculated in compile time.

something like that...

switch (str2int(s)) {   case str2int("Value1"):     break;   case str2int("Value2"):     break; } 

where str2int is like (implementation from here):

constexpr unsigned int str2int(const char* str, int h = 0) {     return !str[h] ? 5381 : (str2int(str, h+1) * 33) ^ str[h]; } 

Another example, the next function can be calculated in compile time:

constexpr int factorial(int n) {     return n <= 1 ? 1 : (n * factorial(n-1)); }    int f5{factorial(5)}; // Compiler will run factorial(5)  // and f5 will be initialized by this value.  // so programm instead of wasting time for running function,  // just will put the precalculated constant to f5  

Answer 2

You can map the strings to enum values, then switch on the enum:

enum Options {     Option_Invalid,     Option1,     Option2,     //others... };  Options resolveOption(string input);  //  ...later...  switch( resolveOption(input) ) {     case Option1: {         //...         break;     }     case Option2: {         //...         break;     }     // handles Option_Invalid and any other missing/unmapped cases     default: {         //...         break;     } } 

Resolving the enum can be implemented as a series of if checks:

 Options resolveOption(std::string input) {     if( input == "option1" ) return Option1;     if( input == "option2" ) return Option2;     //...     return Option_Invalid;  } 

Or a map lookup:

 Options resolveOption(std::string input) {     static const std::map<std::string, Option> optionStrings {         { "option1", Option1 },         { "option2", Option2 },         //...     };      auto itr = optionStrings.find(input);     if( itr != optionStrings.end() ) {         return itr->second;     }     return Option_Invalid;  }