[wpiutil] Make circular buffer classes constexpr (wpilibsuite#7232)

The circular buffer class that uses std::vector internally can be used
in a constant expression as long as it doesn't survive to runtime.

wpiutil/src/main/native/include/wpi/circular_buffer.h

@@ -24,12 +24,12 @@ class circular_buffer {
    *
    * @param size Maximum number of buffer elements.
    */
-  explicit circular_buffer(size_t size) : m_data(size, T{}) {}
+  constexpr explicit circular_buffer(size_t size) : m_data(size, T{}) {}
 
-  circular_buffer(const circular_buffer&) = default;
-  circular_buffer& operator=(const circular_buffer&) = default;
-  circular_buffer(circular_buffer&&) = default;
-  circular_buffer& operator=(circular_buffer&&) = default;
+  constexpr circular_buffer(const circular_buffer&) = default;
+  constexpr circular_buffer& operator=(const circular_buffer&) = default;
+  constexpr circular_buffer(circular_buffer&&) = default;
+  constexpr circular_buffer& operator=(circular_buffer&&) = default;
 
   class iterator {
    public:
@@ -39,20 +39,20 @@ class circular_buffer {
     using pointer = T*;
     using reference = T&;
 
-    iterator(circular_buffer* buffer, size_t index)
+    constexpr iterator(circular_buffer* buffer, size_t index)
         : m_buffer{buffer}, m_index{index} {}
 
-    iterator& operator++() {
+    constexpr iterator& operator++() {
       ++m_index;
       return *this;
     }
-    iterator operator++(int) {
+    constexpr iterator operator++(int) {
       iterator retval = *this;
       ++(*this);
       return retval;
     }
-    bool operator==(const iterator&) const = default;
-    reference operator*() { return (*m_buffer)[m_index]; }
+    constexpr bool operator==(const iterator&) const = default;
+    constexpr reference operator*() { return (*m_buffer)[m_index]; }
 
    private:
     circular_buffer* m_buffer;
@@ -67,69 +67,73 @@ class circular_buffer {
     using pointer = T*;
     using const_reference = const T&;
 
-    const_iterator(const circular_buffer* buffer, size_t index)
+    constexpr const_iterator(const circular_buffer* buffer, size_t index)
         : m_buffer{buffer}, m_index{index} {}
 
-    const_iterator& operator++() {
+    constexpr const_iterator& operator++() {
       ++m_index;
       return *this;
     }
-    const_iterator operator++(int) {
+    constexpr const_iterator operator++(int) {
       const_iterator retval = *this;
       ++(*this);
       return retval;
     }
-    bool operator==(const const_iterator&) const = default;
-    const_reference operator*() const { return (*m_buffer)[m_index]; }
+    constexpr bool operator==(const const_iterator&) const = default;
+    constexpr const_reference operator*() const { return (*m_buffer)[m_index]; }
 
    private:
     const circular_buffer* m_buffer;
     size_t m_index;
   };
 
-  iterator begin() { return iterator(this, 0); }
-  iterator end() { return iterator(this, ::wpi::circular_buffer<T>::size()); }
+  constexpr iterator begin() { return iterator(this, 0); }
+  constexpr iterator end() {
+    return iterator(this, ::wpi::circular_buffer<T>::size());
+  }
 
-  const_iterator begin() const { return const_iterator(this, 0); }
-  const_iterator end() const {
+  constexpr const_iterator begin() const { return const_iterator(this, 0); }
+  constexpr const_iterator end() const {
     return const_iterator(this, ::wpi::circular_buffer<T>::size());
   }
 
-  const_iterator cbegin() const { return const_iterator(this, 0); }
-  const_iterator cend() const {
+  constexpr const_iterator cbegin() const { return const_iterator(this, 0); }
+  constexpr const_iterator cend() const {
     return const_iterator(this, ::wpi::circular_buffer<T>::size());
   }
 
   /**
    * Returns number of elements in buffer
    */
-  size_t size() const { return m_length; }
+  constexpr size_t size() const { return m_length; }
 
   /**
    * Returns value at front of buffer
    */
-  T& front() { return (*this)[0]; }
+  constexpr T& front() { return (*this)[0]; }
 
   /**
    * Returns value at front of buffer
    */
-  const T& front() const { return (*this)[0]; }
+  constexpr const T& front() const { return (*this)[0]; }
 
   /**
    * Returns value at back of buffer
    *
    * If there are no elements in the buffer, calling this function results in
    * undefined behavior.
    */
-  T& back() { return m_data[(m_front + m_length - 1) % m_data.size()]; }
+  constexpr T& back() {
+    return m_data[(m_front + m_length - 1) % m_data.size()];
+  }
 
   /**
    * Returns value at back of buffer
    *
    * If there are no elements in the buffer, calling this function results in
    * undefined behavior.
    */
-  const T& back() const {
+  constexpr const T& back() const {
     return m_data[(m_front + m_length - 1) % m_data.size()];
   }
 
@@ -138,7 +142,7 @@ class circular_buffer {
    *
    * The value at the back is overwritten if the buffer is full.
    */
-  void push_front(T value) {
+  constexpr void push_front(T value) {
     if (m_data.size() == 0) {
       return;
     }
@@ -157,7 +161,7 @@ class circular_buffer {
    *
    * The value at the front is overwritten if the buffer is full.
    */
-  void push_back(T value) {
+  constexpr void push_back(T value) {
     if (m_data.size() == 0) {
       return;
     }
@@ -179,7 +183,7 @@ class circular_buffer {
    * The value at the back is overwritten if the buffer is full.
    */
   template <class... Args>
-  void emplace_front(Args&&... args) {
+  constexpr void emplace_front(Args&&... args) {
     if (m_data.size() == 0) {
       return;
     }
@@ -200,7 +204,7 @@ class circular_buffer {
    * The value at the front is overwritten if the buffer is full.
    */
   template <class... Args>
-  void emplace_back(Args&&... args) {
+  constexpr void emplace_back(Args&&... args) {
     if (m_data.size() == 0) {
       return;
     }
@@ -221,7 +225,7 @@ class circular_buffer {
    * If there are no elements in the buffer, calling this function results in
    * undefined behavior.
    */
-  T pop_front() {
+  constexpr T pop_front() {
     T& temp = m_data[m_front];
     m_front = ModuloInc(m_front);
     m_length--;
@@ -234,15 +238,15 @@ class circular_buffer {
    * If there are no elements in the buffer, calling this function results in
    * undefined behavior.
    */
-  T pop_back() {
+  constexpr T pop_back() {
     m_length--;
     return m_data[(m_front + m_length) % m_data.size()];
   }
 
   /**
    * Resizes internal buffer to given size.
    */
-  void resize(size_t size) {
+  constexpr void resize(size_t size) {
     if (size > m_data.size()) {
       // Find end of buffer
       size_t insertLocation = (m_front + m_length) % m_data.size();
@@ -294,22 +298,22 @@ class circular_buffer {
   /**
    * Empties internal buffer.
    */
-  void reset() {
+  constexpr void reset() {
     m_front = 0;
     m_length = 0;
   }
 
   /**
    * @return Element at index starting from front of buffer.
    */
-  T& operator[](size_t index) {
+  constexpr T& operator[](size_t index) {
     return m_data[(m_front + index) % m_data.size()];
   }
 
   /**
    * @return Element at index starting from front of buffer.
    */
-  const T& operator[](size_t index) const {
+  constexpr const T& operator[](size_t index) const {
     return m_data[(m_front + index) % m_data.size()];
   }
 
@@ -328,15 +332,17 @@ class circular_buffer {
    * @param index Index into the buffer.
    * @return The incremented index.
    */
-  size_t ModuloInc(size_t index) { return (index + 1) % m_data.size(); }
+  constexpr size_t ModuloInc(size_t index) {
+    return (index + 1) % m_data.size();
+  }
 
   /**
    * Decrement an index modulo the size of the buffer.
    *
    * @param index Index into the buffer.
    * @return The decremented index.
    */
-  size_t ModuloDec(size_t index) {
+  constexpr size_t ModuloDec(size_t index) {
     if (index == 0) {
       return m_data.size() - 1;
     } else {