Fix #1215

include/spdlog/common.h

@@ -4,6 +4,7 @@
 #pragma once
 
 #include "spdlog/tweakme.h"
+#include "spdlog/details/null_mutex.h"
 
 #include <atomic>
 #include <chrono>

include/spdlog/details/null_mutex.h

@@ -4,15 +4,16 @@
 #pragma once
 
 #include <atomic>
+#include <utility>
 // null, no cost dummy "mutex" and dummy "atomic" int
 
 namespace spdlog {
 namespace details {
 struct null_mutex
 {
-    void lock() {}
-    void unlock() {}
-    bool try_lock()
+    void lock() const {}
+    void unlock() const {}
+    bool try_lock() const
     {
         return true;
     }
@@ -23,18 +24,24 @@ struct null_atomic_int
     int value;
     null_atomic_int() = default;
 
-    explicit null_atomic_int(int val)
-        : value(val)
+    explicit null_atomic_int(int new_value)
+        : value(new_value)
     {}
 
-    int load(std::memory_order) const
+    int load(std::memory_order = std::memory_order_relaxed) const
     {
         return value;
     }
 
-    void store(int val)
+    void store(int new_value, std::memory_order = std::memory_order_relaxed)
     {
-        value = val;
+        value = new_value;
+    }
+
+    int exchange(int new_value, std::memory_order = std::memory_order_relaxed)
+    {
+        std::swap(new_value, value);
+        return new_value; // return value before the call
     }
 };