Clarify pop() preconditions

rigtorp · Sep 25, 2023 · 4066113 · 4066113
1 parent 945c7c5
commit 4066113
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Showing 3 changed files with 8 additions and 4 deletions.
diff --git a/.gitignore b/.gitignore
@@ -2,4 +2,5 @@ build
 cmake-build-*/
 *~
 .idea/
-.vscode/
+.vscode/
+.cache/
diff --git a/README.md b/README.md
@@ -67,8 +67,10 @@ See `src/SPSCQueueExample.cpp` for the full example.
 
 - `void pop();`
 
-  Dequeue first item of queue. Invalid to call if queue is
-  empty. Requires `std::is_nothrow_destructible<T>::value == true`.
+  Dequeue first item of queue. You must ensure that the queue is non-empty
+  before calling pop. This means that `front()` must have returned a
+  non-`nullptr` before each call to `pop()`. Requires
+  `std::is_nothrow_destructible<T>::value == true`.
 
 - `size_t size();`
 

diff --git a/include/rigtorp/SPSCQueue.h b/include/rigtorp/SPSCQueue.h
@@ -180,7 +180,8 @@ template <typename T, typename Allocator = std::allocator<T>> class SPSCQueue {
     static_assert(std::is_nothrow_destructible<T>::value,
                   "T must be nothrow destructible");
     auto const readIdx = readIdx_.load(std::memory_order_relaxed);
-    assert(writeIdx_.load(std::memory_order_acquire) != readIdx);
+    assert(writeIdx_.load(std::memory_order_acquire) != readIdx &&
+           "Can only call pop() after front() has returned a non-nullptr");
     slots_[readIdx + kPadding].~T();
     auto nextReadIdx = readIdx + 1;
     if (nextReadIdx == capacity_) {