Use of std::find_if with flexible arrays

Let’s say that I have the following two structures, where struct sB has a flexible array:

struct sA
{
  double valueA;
  double valueB;
};

struct sB
{
  double c;
  size_t arrayMembers;
  sA aM[];
};

I want to iterate over aM and find the first element with valueA > c and valueB > c. I want to do it using std::find_if().

I tried to use std::begin() and std::end():

auto primeElement = std::find_if(std::begin(sBInstance.aM), std::end(sBInstance.aM),
      [&](const sB v){return (v.valueA > c && v.valueB > c)});

However, it doesn’t compile with a compiler error (among other related errors):

‘std::begin’ : no matching overloaded function found

I think that flexible arrays are more C than C++, and C++ iterators maybe are not particularly suited to them, but is there a way I can use std::find_if() on a flexible array? If there is more than one way, which one is the most straightforward?

>Solution :

Standard C++ does not support flexible array members. Those are a C feature that are not part of C++. If you need an object with a variable-length array as a member, use std::vector instead.

If you compiler supports flexible array members as an extension, you’ll need to treat them more like pointers to dynamically-allocated arrays than statically-sized C arrays. Since their size is not known at compile time, std::end cannot statically compute an appropriate offset. Instead, you’ll need to add an offset of arrayMembers to a pointer to the first element of the array:

auto primeElement = std::find_if(
    sBInstance->aM,
    sBInstance->aM + sBInstance->arrayMembers,
    [&](const sA& v){return v.valueA > c && v.valueB > c}
);

This works because pointers are a type of iterator, so pointers to the beginning and one past the end of an array are a valid iterator range to iterate over that array.