begin() and end() is defined for arrays in C++11 and above

I was a little confused the first time I saw range-based for-loop in C++11 on “Tour of C++” that it works right out of the box for both recognized array and STL containers and yet the text says it requires begin() and end() to be defined for the object for range-based for-loop to run.

I later learned that despite typical STL usage example writes v.begin(), v.end(), the most bulletproof way is to write begin(v), end(v) instead (Herb Sutter recommends it). Then I started to suspect that C++11 must have defined free-form (non-member) begin(), end() functions that takes in arbitrary recognized arrays. I pulled up my code editor and ran this:

#include <iostream>
int main()
{
    int v[4]={1,2,3,4};
    std::cout << *(std::crend(v)-1) << std::endl;

    return 0;
}

It compiled and ran uneventfully, printing ‘1’ as expected (I’m using crend(), to see if they implemented the more obscure ones). It makes more sense now why range-based for-loop works for arbitrary recognized arrays without making an exception to the begin(), end() requirement.

To confirm that it is the case (since “Tour of C++” didn’t say anything about why arbitrary array works for range-based for loop), I looked up the STL source code from libc++ in LLVM, namely <iterator>, and saw this:

template <class T, size_t N> constexpr T* begin(T (&array)[N]);

Bingo! There’s a mechanism to do so. But before I close, Stephan T. Lavavej (Mr. STL) mentioned that the template quoted above is no longer required (by the standard) to implement range-based for-loop in C++11.

Now the conclusion becomes that begin(), end() that takes in recognized arrays exist (which completes the logic behind range-based for-loop), but the range-based for loop can (and typically will) handle recognized arrays without these templated functions defined.

 

37 total views, 0 views today