#537 Fixed the Windows DLL build by exporting message::EMPTY_STR and message::EMPTY_BIN

Author: fpagliughi

CHANGELOG.md

@@ -6,6 +6,11 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Upcoming Version 1.5.2 (unreleased)
+ 
+- [#537](https://github.com/eclipse-paho/paho.mqtt.cpp/issues/537) Fixed the Windows DLL build by exporting message::EMPTY_STR and message::EMPTY_BIN 
+
+
 
 ## [Version 1.5.1](https://github.com/eclipse/paho.mqtt.cpp/compare/v1.5.0..v1.5.1) - (2025-02-09)
 

CMakeLists.txt

@@ -49,6 +49,10 @@ if(NOT PAHO_BUILD_SHARED AND NOT PAHO_BUILD_STATIC)
     message(FATAL_ERROR "You must set either PAHO_BUILD_SHARED, PAHO_BUILD_STATIC, or both")
 endif()
 
+# --- Setting naming variables ---
+
+set(PAHO_MQTTPP_GENERATED_DIR ${CMAKE_CURRENT_BINARY_DIR}/generated)
+
 ## --- Find Paho C or build it, if reqested ---
 
 if(PAHO_WITH_SSL)
@@ -137,6 +141,15 @@ if(PAHO_BUILD_TESTS)
     add_subdirectory(test/unit)
 endif()
 
+## --- Install generated header(s) ---
+
+install(
+    DIRECTORY
+        ${PAHO_MQTTPP_GENERATED_DIR}/include/
+    DESTINATION
+        ${CMAKE_INSTALL_INCLUDEDIR}
+)
+
 ## --- Packaging settings ---
 
 if(WIN32)

include/mqtt/message.h

@@ -65,9 +65,9 @@ class message
     /** Initializer for the C struct (from the C library) */
     static constexpr MQTTAsync_message DFLT_C_STRUCT MQTTAsync_message_initializer;
     /** A const string to use for references */
-    static const string EMPTY_STR;
+    PAHO_MQTTPP_EXPORT static const string EMPTY_STR;
     /** A const binary to use for references */
-    static const binary EMPTY_BIN;
+    PAHO_MQTTPP_EXPORT static const binary EMPTY_BIN;
 
     /** The underlying C message struct */
     MQTTAsync_message msg_{DFLT_C_STRUCT};

src/CMakeLists.txt

@@ -28,6 +28,8 @@
 set (THREADS_PREFER_PTHREAD_FLAG ON)
 find_package(Threads REQUIRED)
 
+include(GenerateExportHeader)
+
 ## --- Use object library to optimize compilation ---
 
 set(COMMON_SRC
@@ -77,6 +79,11 @@ if(PAHO_BUILD_SHARED)
         VERSION ${PROJECT_VERSION}
         SOVERSION ${PROJECT_VERSION_MAJOR}
     )
+
+    generate_export_header(paho-mqttpp3-shared
+        BASE_NAME paho_mqttpp
+        EXPORT_FILE_NAME ${PAHO_MQTTPP_GENERATED_DIR}/include/mqtt/export.h
+    )
 endif()
 
 ## --- Build static version of the library, if requested ---
@@ -99,9 +106,15 @@ if(PAHO_BUILD_STATIC)
     if(${PAHO_BUILD_SHARED})
         # This lib should configure for static exports
         target_compile_definitions(paho-mqttpp3-static PRIVATE PAHO_MQTTPP_STATIC)
+        target_compile_definitions(paho-mqttpp3-static PRIVATE PAHO_MQTTPP_STATIC_DEFINE)
     else()
         # If no shared lib, make this one the default target
         add_library(PahoMqttCpp::paho-mqttpp3 ALIAS paho-mqttpp3-static)
+
+        generate_export_header(paho-mqttpp3-static
+            BASE_NAME paho_mqttpp
+            EXPORT_FILE_NAME ${PAHO_MQTTPP_GENERATED_DIR}/include/mqtt/export.h
+        )
     endif()
 
     ## Let the archive use the same base name as the shared lib on *nix systems
@@ -110,6 +123,12 @@ if(PAHO_BUILD_STATIC)
     endif()
 endif()
 
+## --- Make sure at least one target was specified ---
+
+if(NOT PAHO_CPP_TARGETS)
+    message(FATAL_ERROR "No build targets are specified")
+endif()
+
 ## --- Set common properties, etc ---
 
 include(GNUInstallDirs)

src/message.cpp

@@ -17,17 +17,18 @@
  *******************************************************************************/
 
 #include "mqtt/message.h"
+#include "mqtt/export.h"
 
 #include <cstring>
 #include <utility>
 
 namespace mqtt {
 
 // A const string to use for references
-const string message::EMPTY_STR;
+PAHO_MQTTPP_EXPORT const string message::EMPTY_STR;
 
 // A const binary to use for references
-const binary message::EMPTY_BIN;
+PAHO_MQTTPP_EXPORT const binary message::EMPTY_BIN;
 
 /////////////////////////////////////////////////////////////////////////////