Alternatives to IMPORTED_IMPLIB for Linking External Libraries in CMake

What it is

• Imported targets represent external libraries that you're linking against in your project but haven't built yourself within CMake.
• IMPORTED_IMPLIB is a target property used with CMake's imported targets.

Purpose

• The linker uses this library during the linking stage to resolve symbols from the external library.
• It specifies the location of the import library (.lib on Windows, .a on Unix-like systems) for the imported target.

How it works

1. • You use the add_library command with the IMPORTED keyword to create an imported target for the external library.

   add_library(my_external_lib SHARED IMPORTED)
   

2. Set IMPORTED_IMPLIB
   

   • Use the set_target_properties command to set the IMPORTED_IMPLIB property for the imported target.
   • You can provide the full path to the import library:

   set_target_properties(my_external_lib PROPERTIES IMPORTED_IMPLIB "/path/to/my_external_lib.lib")
   

Configuration-Specific Properties

• You can also use MAP_IMPORTED_CONFIG_<CONFIG> to map your project's configurations to those of the imported library.

• Use properties like IMPORTED_IMPLIB_DEBUG and IMPORTED_IMPLIB_RELEASE for configuration-specific paths.

  set_target_properties(my_external_lib PROPERTIES
      IMPORTED_IMPLIB_DEBUG "/path/to/debug/my_external_lib.lib"
      IMPORTED_IMPLIB_RELEASE "/path/to/release/my_external_lib.lib")
  

• CMake allows you to set different import library paths for different build configurations (e.g., Debug, Release).

Example

add_library(opencv_core SHARED IMPORTED)
set_target_properties(opencv_core PROPERTIES
    IMPORTED_LOCATION "${OpenCV_LIBRARY}"  # Assuming OpenCV_LIBRARY is set by find_package(OpenCV)
    IMPORTED_IMPLIB "${OpenCV_IMPLIB}"  # Assuming OpenCV_IMPLIB is set by find_package(OpenCV))
)

Key Points

• CMake's package managers (like find_package) often set these properties automatically.
• Consider configuration-specific paths for different build configurations.
• IMPORTED_IMPLIB is crucial for linking against external libraries using CMake.

---

Basic Example

This example links against an external library named libfoo.a located in the system's default library search path:

add_library(my_executable EXECUTABLE main.cpp)

# Create an imported target for libfoo.a
add_library(libfoo SHARED IMPORTED)

# Assume libfoo.a is in the default search path (no need to set IMPORTED_LOCATION)
set_target_properties(libfoo PROPERTIES IMPORTED_IMPLIB "libfoo.lib")  # Windows

# Or set IMPORTED_IMPLIB for Unix-like systems if needed:
# set_target_properties(libfoo PROPERTIES IMPORTED_IMPLIB "libfoo.a")

# Link my_executable with libfoo
target_link_libraries(my_executable PRIVATE libfoo)

Configuration-Specific Paths

This example shows how to set different import library paths for Debug and Release configurations:

add_library(my_library SHARED IMPORTED)

set_target_properties(my_library PROPERTIES
    IMPORTED_LOCATION "${LIBRARY_DIR}/${MY_LIBRARY_NAME}"  # Assuming LIBRARY_DIR and MY_LIBRARY_NAME are set
    IMPORTED_IMPLIB_DEBUG "${LIBRARY_DIR}/Debug/${MY_LIBRARY_NAME}.lib"  # Windows Debug path
    IMPORTED_IMPLIB_RELEASE "${LIBRARY_DIR}/Release/${MY_LIBRARY_NAME}.lib"  # Windows Release path
)

Using find_package

find_package(OpenCV REQUIRED)  # Assuming OpenCV is installed and find_package finds it

add_executable(my_program main.cpp)
target_link_libraries(my_program PRIVATE OpenCV::core)  # Link against OpenCV's core library

• This allows you to link with OpenCV without explicitly specifying import library paths.
• The find_package(OpenCV) command likely sets OpenCV_LIBRARY and OpenCV_IMPLIB variables, which are then used to set IMPORTED_LOCATION and IMPORTED_IMPLIB properties for the OpenCV::core target.

• Consult the documentation of your package manager (e.g., OpenCV's find_package module) for details on how it sets target properties.
• Adjust these examples based on your specific library names and locations.

---

Rely on find_package Modules

• This is the preferred approach as it simplifies linking with external libraries and reduces manual configuration.
• These modules typically set relevant properties like IMPORTED_LOCATION and IMPORTED_IMPLIB based on your system's library setup.
• CMake's package managers like find_package(OpenCV) usually handle locating and configuring imported libraries automatically.

Use IMPORTED_LOCATION for Frameworks (Apple/XCFrameworks)

• This approach avoids the need for IMPORTED_IMPLIB for these specific frameworks.
• When you set IMPORTED_LOCATION to the directory containing the framework, CMake will automatically find the necessary files (headers and resources) within that directory.
• If you're dealing with Apple frameworks or XCFrameworks, IMPORTED_LOCATION can often be sufficient.

• However, this is not recommended as it relies on a system-specific setup and can lead to portability issues.
• In rare cases, if you're absolutely certain the import library (libfoo.lib on Windows, libfoo.a on Unix-like systems) resides in a standard library search path on your system, you might omit both IMPORTED_IMPLIB and IMPORTED_LOCATION.

Scenario	Preferred Approach	Additional Notes
General Case	find_package(library)	Leverage package manager for automatic configuration
Apple Frameworks	IMPORTED_LOCATION	Set to framework directory; no need for IMPORTED_IMPLIB
System-Wide Search	Not Recommended	Can lead to portability issues