How to Write the Range-based For-Loop With Argv?

Question

From the c++0x Wikipedia site:

int my_array[5] = {1, 2, 3, 4, 5};
for (int &x : my_array) {
    x *= 2;
}

So why does this code not work?

int main(int argc, char* argv[])
{
    for (char *arg : argv)
    {
        // Do something.
    }
}

Error:

main.cpp:36: error: no matching function for call to ‘begin(char**&)’

I am using Qt with g++ 4.6.1 on Ubuntu 11.10.

Additional Information

Is There a Range Class in C++0x

Range-Based For-Loop Statement Definition Redundance

Answer 1

Usually, the first thing I do with argc and argv is this:

std::vector<std::string> arguments(argv, argv + argc);

Now I have a vector of strings to work with and I can easily use not only the range-based for loops, but also C++ standard library facilities.

for(std::string& s : arguments) {
    // do stuff...
}

The wikipedia code works because the type of my_array is a variable of array type. The original code does not work, because argv is not an array. The syntax char* argv[] may make it look like it is an array, but that's just a sad artifact of C syntax. char* argv[] is exactly the same as char** argv. argv is not an array; it's actually just a pointer.

The range-based for loop works on:

• arrays;
• any type that has member functions begin() and end() that return iterators;
• any type for which exist non-member functions begin and end that can be called like begin(x) and end(x), with x being the thing that you're iterating over.

As you can see, pointers are not part of this list.

Answer 2

You don't, because the system can't tell how long argv is at compile time. Someone can likely find the proper section of the standard to quote you about this.

There is a way around it though, and that's to create a custom class to wrap argv. It's not even that hard.

class argv_range {
 public:
   argv_range(int argc, const char * const argv[])
        : argc_(argc), argv_(argv)
   {
   }

   const char * const *begin() const { return argv_; }
   const char * const *end() const { return argv_ + argc_; }

 private:
   const int argc_;
   const char * const *argv_;
};

Here's how you use it:

for (const char *arg: argv_range(argc, argv)) {
   // Do something.
}

Yeah, I use a lot of consts. Basically, argv is an array of character pointers, none of which should be modified, each pointing to a string, none of the characters of which should be modified either.