Boundary checking in operator[] about gsl-lite

[martinmoene]

span is part of the GSL Bounds safety profile.

Quoting:

Bounds.1: Don't use pointer arithmetic. Use span instead.
Reason
Pointers should only refer to single objects, and pointer arithmetic is fragile and easy to get wrong. span is a bounds-checked, safe type for accessing arrays of data.

Microsoft's GSL indeed performs the check in:

#include <gsl.h>
#include <iostream>

int use( gsl::span<int> s, int i ) 
{
    return s[i];
}

int main()
{
    int arr[] = { 1, 2, 3, };

    try
    {
        (void) use( arr, 0 );
        (void) use( arr, 1 );
        (void) use( arr, 2 );
        (void) use( arr, 3 );
    }
    catch( std::exception const & e )
    {
        std::cout << "Error: " << e.what();
    }
}

Compile (g++ (GCC) 5.2.0) and run (M-GSL):

prompt> g++ -Wall -std=c++14 -DGSL_THROW_ON_CONTRACT_VIOLATION -I../../M-GSL/include -o main.exe main.cpp && main.exe
Error: GSL: Precondition failure at ../../M-GSL/include/span.h: 388

Compile (g++ (GCC) 5.2.0) and run (gsl-lite):

prompt> g++ -Wall -std=c++14 -DGSL_THROW_ON_CONTRACT_VIOLATION -I../../gsl-lite/include -o main.exe main.cpp && main.exe
Error: GSL: Precondition failure at ../../gsl-lite/include/gsl/gsl-lite.h: 835