CMake Lesson 2 - PUBLIC, PRIVATE, and INTERFACE

Modern CMake is target-oriented. Build properties are attached to targets and propagated according to three keywords.

---

PRIVATE

Only this target needs it.

target_include_directories(calcpack
    PRIVATE
        src
)

Only calcpack can use headers from src/.

Consumers do not inherit this path.

Typical uses:

• Internal headers
• Internal compile definitions
• Implementation-only libraries

---

PUBLIC

This target needs it, and everyone using this target also needs it.

target_include_directories(calcpack
    PUBLIC
        include
)

Both

calcpack

and

applications linking calcpack

receive the include directory.

Typical uses:

• Public API headers
• Public compile definitions
• Public library dependencies

---

INTERFACE

This target does not need it, but consumers do.

Example:

add_library(math INTERFACE)
 
target_include_directories(math
    INTERFACE
        include
)

Useful for:

• Header-only libraries
• Interface-only compile options
• Interface compile definitions

---

Propagation

PRIVATE

App
 
calcpack
  ▲
  │
uses include path

Only the library receives the property.

---

PUBLIC

App
  ▲
  │
calcpack
  ▲
  │
include path

Both receive the property.

---

INTERFACE

App
  ▲
  │
Header-only library

Only consumers receive the property.

---

Applies to More Than Include Directories

The same keywords are used with:

target_link_libraries()
target_compile_definitions()
target_compile_options()
target_sources()

Example:

target_compile_definitions(calcpack
    PUBLIC
        CALCPACK_VERSION=1
)

Both the library and its users can access:

CALCPACK_VERSION

---

Rule of Thumb

Keyword	Used by this target	Used by consumers
PRIVATE	✅	❌
PUBLIC	✅	✅
INTERFACE	❌	✅

---

Memory Trick

• PRIVATE → Only I need it.
• PUBLIC → I need it, and anyone using me needs it too.
• INTERFACE → I don’t need it, but my users do.

---

Typical Usage

Situation	Keyword
Internal implementation headers (src/)	PRIVATE
Public API headers (include/)	PUBLIC
Header-only library	INTERFACE
Internal helper library	PRIVATE
Dependency exposed in public headers	PUBLIC

---

Key Takeaways

• Modern CMake revolves around targets, not global settings.
• Build properties propagate according to PRIVATE, PUBLIC, or INTERFACE.
• Think in terms of who needs the property, rather than memorizing syntax.