UnrealEngineUWP/Engine/Documentation/Source/Programming/Modules/API/ModuleAPI.INT.udn

Availability:Public
Title:Module API Specifiers
Crumbs:%ROOT%, Programming
Description:Exposing classes and functions to other modules.

The easiest way to think about these specifiers is that they're used to tag functions, classes or data as "public" to your module's DLL file. If you mark a function in the Engine module as ENGINE_API, then any module that imports Engine can access that function directly.

These are only used when compiling the engine in "modular" mode (DLL files on desktop platforms). The opposite is what we call "monolithic" mode, which globs all code together in a single executable file. The type of build is controlled by UnrealBuildTool settings and/or platforms and build configuration.

The actual API macro equates to one of the following depending on how the code is being compiled by UBT:

* `__declspec( dllexport )`, when compiling module code in "modular" mode.
* `__declspec( dllimport )`, when including public module headers for a module that you are importing.
* empty, when compiling in "monolithic" mode


API macros only make sense for modules that are statically imported from another module. The Core module is a great example -- almost every other module in UE4 specifies "Core" as a import dependency in their *.Build.cs file.

Many modules never need to be statically imported (e.g. "SceneOutliner" module.) We refer to those modules as dynamically loaded modules. Dynamically loaded modules are awesome because they can be discovered at startup (kind of like a plugin), and often can be reloaded on the fly!

The API macros are mostly used on older code to allow newer modules to access it from their DLL. In newer bits of code, the API macros are used far less, instead setting up nice interface layers to expose functionality across DLL boundaries.