VecArray.h

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#ifndef FWCore_Utilities_VecArray_h
#define FWCore_Utilities_VecArray_h

#include <utility>
#include <stdexcept>
#include <string>
#include <cstddef>

namespace edm {
  template <typename T, unsigned int N>
  class VecArray {
    T data_[N];
    unsigned int size_;

  public:
    using value_type = T;
    using size_type = unsigned int;
    using difference_type = std::ptrdiff_t;
    using reference = value_type&;
    using const_reference = const value_type&;
    using pointer = value_type*;
    using const_pointer = const value_type*;
    using iterator = value_type*;
    using const_iterator = const value_type*;

    VecArray(): data_{}, size_{0} {}

    // Not range-checked, undefined behaviour if access beyond size()
    reference operator[](size_type pos) { return data_[pos]; }
    // Not range-checked, undefined behaviour if access beyond size()
    const_reference operator[](size_type pos) const { return data_[pos]; }
    // Undefined behaviour if size()==0
    reference front() { return data_[0]; }
    // Undefined behaviour if size()==0
    const_reference front() const { return data_[0]; }

    // Undefined behaviour if size()==0
    reference back() { return data_[size_-1]; }
    // Undefined behaviour if size()==0
    const_reference back() const { return data_[size_-1]; }
    pointer data() { return data_; }
    const_pointer data() const { return data_; }

    iterator begin() noexcept { return data_; }
    const_iterator begin() const noexcept { return data_; }
    const_iterator cbegin() const noexcept { return data_; }

    iterator end() noexcept { return begin()+size_; }
    const_iterator end() const noexcept { return begin()+size_; }
    const_iterator cend() const noexcept { return cbegin()+size_; }

    constexpr bool empty() noexcept { return size_ == 0; }
    constexpr size_type size() noexcept { return size_; }
    static constexpr size_type capacity() noexcept { return N; }

    void clear() {
      size_ = 0;
    }

    // Throws if size()==N
    void push_back(const T& value) {
      if(size_ >= N)
        throw std::length_error("push_back on already-full VecArray (N="+std::to_string(N)+")");
      push_back_unchecked(value);
    }

    // Undefined behaviour if size()==N
    void push_back_unchecked(const T& value) {
      data_[size_] = value;
      ++size_;
    }

    // Undefined behaviour if size()==0
    void pop_back() {
      --size_;
    }

    void resize(unsigned int size) {
      if(size > N)
        throw std::length_error("Requesting size "+std::to_string(size)+" while maximum allowed is "+std::to_string(N));

      while(size < size_)
        pop_back();
      size_ = size;
    }

    void swap(VecArray& other)
      noexcept(noexcept(std::swap(data_, other.data_)) &&
               noexcept(std::swap(size_, other.size_)))
    {
      std::swap(data_, other.data_);
      std::swap(size_, other.size_);
    }
  };
}

#endif