what does a const char** look like?

Question

I have a class which takes the main command line arguments (eg, -B, -a, etc) and does something with them, but also i would like to be able to instantiate that function without passing the command line arguments, so for example:

constructor:

myClass(int argc, const char **argv){
    <initialise class with flags from argv>
}

myClass() : myClass(2, "-B") {}
}

Here i am trying to instantiate myClass with the flag "-B", but it keeps giving me the error:

no known conversion for argument 3 from 'const char [3]' to 'const char**'

so i was wondering what i need to do to pass a value in as const char**?

Answer 1

First level is pointer to first pointer to char * . Second level is pointer to the first const char of c-string.

> cdecl explain "const char ** args"
declare args as pointer to pointer to const char

If you have -std=c++11 available, you can use this example (but it can be rewritten to use old standard):

#include <iostream>
#include <vector>
#include <string>    

class test {
  public:
    test(const std::vector<std::string> & args) {
      for (auto & arg : args) {
        std::cout << arg << "\n";
      }
    }

    test() : test{{"-B"}} {}
};

int main(int argc, const char ** argv) {
  test sth{{argv+1, argc+argv}}; // skip program name here
  test sth_else;

}

Answer 2

const char** is pointer to const char* . In your case, you intend to pass multiple arguments as part of argv , so you can pass something like below:

const char* argv[] = {"<program name>", "B"};
myClass m(2, argv);

_{Note: const char** x & const char* x[] are same. The 2nd syntax is helpful when one wants to "represent" an array.}

Here I am giving a way to mimic the main(int, char**) function argument for your internal test. If you want to pass from default constructor to argument constructor then all the above stuff will have to go global.