Whats the CMake syntax to set and use variables

CMake, the ubiquitous physique scheme generator, empowers builders to negociate the complexities of package tasks crossed divers platforms. Mastering its syntax for mounting and utilizing variables is important for businesslike and adaptable task configurations. This station dives heavy into the nuances of CMake adaptable direction, offering applicable examples and champion practices to elevate your CMake experience.

Mounting Variables

Defining variables inside CMake is simple, using the fit() bid. This bid takes the adaptable sanction and its worth arsenic arguments. Adaptable names are lawsuit-delicate, and it’s champion pattern to usage uppercase for person-outlined variables.

For illustration, to fit the adaptable MY_VARIABLE to the drawstring “Hullo, CMake!”, you would usage:

fit(MY_VARIABLE "Hullo, CMake!")

CMake helps assorted information sorts for adaptable values, together with strings, numbers, lists, and paths. This flexibility permits for sturdy configuration choices.

Utilizing Variables

Accessing the worth saved successful a CMake adaptable is executed utilizing the ${} syntax. Merely enclose the adaptable sanction inside curly braces preceded by a greenback gesture.

Pursuing the former illustration, to mark the worth of MY_VARIABLE to the console, usage the communication() bid:

communication(${MY_VARIABLE})

This volition output “Hullo, CMake!” throughout the CMake configuration procedure. This mechanics allows dynamic configuration primarily based connected adaptable values.

Adaptable Range and Range

Knowing adaptable range is indispensable for predictable behaviour. CMake variables person listing range, which means they are lone accessible inside the listing wherever they are outlined and its subdirectories. This hierarchical construction permits for localized configurations.

The PARENT_SCOPE key phrase permits you to fit a adaptable successful the genitor listing, piece CACHE makes the adaptable persistent crossed aggregate CMake runs. This is peculiarly utile for person-configurable choices.

fit(MY_CACHED_VARIABLE "Cached Worth" CACHE Drawstring "Statement of the adaptable")

This bid units a cached adaptable with a statement, permitting customers to modify its worth done CMake’s GUI oregon bid-formation interface.

Precocious Adaptable Utilization

CMake gives a wealthiness of instructions for manipulating variables, together with conditional logic, database operations, and drawstring processing. These capabilities unlock blase configuration eventualities.

Conditional Logic

The if() bid permits for conditional execution primarily based connected adaptable values. This allows tailoring physique configurations primarily based connected level, compiler, oregon another elements.

if(MSVC) fit(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /MT") endif()

This illustration provides a circumstantial compiler emblem lone once utilizing the Microsoft Ocular C++ compiler. Conditional logic is captious for transverse-level compatibility.

Database Operations

CMake helps database variables, which are sequences of values. Instructions similar database(APPEND ...), database(REMOVE_ITEM ...), and database(Dimension ...) facilitate managing these lists. This is invaluable for managing origin information, libraries, oregon another collections of objects.

Usage fit() to specify variables.
Entree adaptable values with ${}.

Specify your variables.
Usage the variables successful your CMakeLists.txt.
Configure and make your physique scheme.

See a script wherever you demand to conditionally see a room:

if(ENABLE_FEATURE_X) database(APPEND MY_LIBRARIES libfeaturex) endif() target_link_libraries(my_executable ${MY_LIBRARIES})

This dynamically hyperlinks the libfeaturex room lone if the ENABLE_FEATURE_X adaptable is fit. This demonstrates the powerfulness of CMake’s database operations. Leveraging CMake’s constructed-successful instructions empowers you to grip divers configuration necessities effectively.

FAQ

Q: What’s the quality betwixt cached and non-cached variables?

A: Non-cached variables are lone outlined throughout a azygous CMake tally. Cached variables persist their values and tin beryllium modified by the person, making them perfect for configuration choices. Seat this documentation connected the fit bid for much accusation.

Mastering CMake’s adaptable syntax is cardinal for efficaciously managing task configurations. Knowing adaptable range, information varieties, and the disposable instructions permits for creating adaptable and maintainable physique programs. By leveraging these methods, you tin streamline your improvement workflow and guarantee accordant builds crossed antithetic platforms. Research additional CMake options similar capabilities and macros for equal much almighty configuration power. Fit to elevate your CMake abilities? Dive deeper into CMake champion practices and return your tasks to the adjacent flat. For a deeper dive into CMake, research sources similar the authoritative CMake documentation (nexus) and on-line tutorials (nexus). You tin besides discovery invaluable accusation connected Stack Overflow (nexus).

Question & Answer :
I’m asking this arsenic a reminder to myself the adjacent clip I usage CMake. It ne\’er sticks, and Google outcomes aren’t large.

What’s the syntax to fit and usage variables successful CMake?

Once penning CMake scripts location is a batch you demand to cognize astir the syntax and however to usage variables successful CMake.

The Syntax

Strings utilizing fit():

fit(MyString "Any Matter")
fit(MyStringWithVar "Any another Matter: ${MyString}")
fit(MyStringWithQuot "Any punctuation: \"${MyStringWithVar}\"")

Oregon with drawstring():

drawstring(APPEND MyStringWithContent " ${MyString}")

Lists utilizing fit():

fit(MyList "a" "b" "c")
fit(MyList ${MyList} "d")

Oregon amended with database():

database(APPEND MyList "a" "b" "c")
database(APPEND MyList "d")

Lists of Record Names:

fit(MySourcesList "Record.sanction" "Record with Abstraction.sanction")
database(APPEND MySourcesList "Record.sanction" "Record with Abstraction.sanction")
add_excutable(MyExeTarget ${MySourcesList})

The Documentation

The Range oregon “What worth does my adaptable person?”

Archetypal location are the “Average Variables” and issues you demand to cognize astir their range:

Average variables are available to the CMakeLists.txt they are fit successful and every little thing known as from location (add_subdirectory(), see(), macro() and relation()).
The add_subdirectory() and relation() instructions are particular, due to the fact that they unfastened-ahead their ain range.
Which means variables fit(...) location are lone available location and they brand a transcript of each average variables of the range flat they are known as from (known as genitor range).
Truthful if you are successful a sub-listing oregon a relation you tin modify an already present adaptable successful the genitor range with fit(... PARENT_SCOPE)
You tin brand usage of this e.g. successful features by passing the adaptable sanction arsenic a relation parameter. An illustration would beryllium relation(xyz _resultVar) is mounting fit(${_resultVar} 1 PARENT_SCOPE)
Connected the another manus all the things you fit successful see() oregon macro() scripts volition modify variables straight successful the range of wherever they are referred to as from.

2nd location is the “Planetary Variables Cache”. Issues you demand to cognize astir the Cache:

If nary average adaptable with the fixed sanction is outlined successful the actual range, CMake volition expression for a matching Cache introduction.
Cache values are saved successful the CMakeCache.txt record successful your binary output listing.
The values successful the Cache tin beryllium modified successful CMake’s GUI exertion earlier they are generated. So they - successful examination to average variables - person a kind and a docstring. I usually don’t usage the GUI truthful I usage fit(... CACHE Inner "") to fit my planetary and persistant values.

Delight line that the Inner cache adaptable kind does connote Unit
Successful a CMake book you tin lone alteration present Cache entries if you usage the fit(... CACHE ... Unit) syntax. This behaviour is made usage of e.g. by CMake itself, due to the fact that it usually does not unit Cache entries itself and so you tin pre-specify it with different worth.
You tin usage the bid formation to fit entries successful the Cache with the syntax cmake -D var:kind=worth, conscionable cmake -D var=worth oregon with cmake -C CMakeInitialCache.cmake.
You tin unset entries successful the Cache with unset(... CACHE).

The Cache is planetary and you tin fit them literally anyplace successful your CMake scripts. However I would urge you deliberation doubly astir wherever to usage Cache variables (they are planetary and they are persistant). I usually like the set_property(Planetary Place ...) and set_property(Planetary APPEND Place ...) syntax to specify my ain non-persistant planetary variables.

Adaptable Pitfalls and “However to debug adaptable adjustments?”

To debar pitfalls you ought to cognize the pursuing astir variables:

Section variables bash fell cached variables if some person the aforesaid sanction
The find_... instructions - if palmy - bash compose their outcomes arsenic cached variables “truthful that nary call volition hunt once more”
Lists successful CMake are conscionable strings with semicolons delimiters and so the citation-marks are crucial
fit(MyVar a b c) is "a;b;c" and fit(MyVar "a b c") is "a b c"
The advice is that you ever usage citation marks with the 1 objection once you privation to springiness a database arsenic database
Mostly like the database() bid for dealing with lists
The entire range content described supra. Particularly it’s beneficial to usage capabilities() alternatively of macros() due to the fact that you don’t privation your section variables to entertainment ahead successful the genitor range.
A batch of variables utilized by CMake are fit with the task() and enable_language() calls. Truthful it might acquire crucial to fit any variables earlier these instructions are utilized.
Situation variables whitethorn disagree from wherever CMake generated the brand situation and once the the brand information are option to usage.
A alteration successful an situation adaptable does not re-set off the procreation procedure.
Particularly a generated IDE situation whitethorn disagree from your bid formation, truthful it’s beneficial to transportation your situation variables into thing that is cached.

Typically lone debugging variables helps. The pursuing whitethorn aid you:

Merely usage aged printf debugging kind by utilizing the communication() bid. Location besides any fit to usage modules shipped with CMake itself: CMakePrintHelpers.cmake, CMakePrintSystemInformation.cmake
Expression into CMakeCache.txt record successful your binary output listing. This record is equal generated if the existent procreation of your brand situation fails.
Usage variable_watch() to seat wherever your variables are publication/written/eliminated.
Expression into the listing properties CACHE_VARIABLES and VARIABLES
Call cmake --hint ... to seat the CMake’s absolute parsing procedure. That’s kind of the past reserve, due to the fact that it generates a batch of output.

Particular Syntax

Situation Variables
You tin tin publication $ENV{...} and compose fit(ENV{...} ...) situation variables
Generator Expressions
Generator expressions $<...> are lone evaluated once CMake’s generator writes the brand situation (it examination to average variables that are changed “successful-spot” by the parser)
Precise useful e.g. successful compiler/linker bid strains and successful multi-configuration environments
References
With ${${...}} you tin springiness adaptable names successful a adaptable and mention its contented.
Frequently utilized once giving a adaptable sanction arsenic relation/macro parameter.
Changeless Values (seat if() bid)
With if(MyVariable) you tin straight cheque a adaptable for actual/mendacious (nary demand present for the enclosing ${...})
Actual if the changeless is 1, Connected, Sure, Actual, Y, oregon a non-zero figure.
Mendacious if the changeless is zero, Disconnected, Nary, Mendacious, N, Disregard, NOTFOUND, the bare drawstring, oregon ends successful the suffix -NOTFOUND.
This syntax is frequently usage for thing similar if(MSVC), however it tin beryllium complicated for person who does not cognize this syntax shortcut.
Recursive substitutions
You tin concept adaptable names utilizing variables. Last CMake has substituted the variables, it volition cheque once more if the consequence is a adaptable itself. This is precise almighty characteristic utilized successful CMake itself e.g. arsenic kind of a template fit(CMAKE_${lang}_COMPILER ...)
However beryllium alert this tin springiness you a headache successful if() instructions. Present is an illustration wherever CMAKE_CXX_COMPILER_ID is "MSVC" and MSVC is "1":
- if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC") is actual, due to the fact that it evaluates to if("1" STREQUAL "1")
- if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC") is mendacious, due to the fact that it evaluates to if("MSVC" STREQUAL "1")
- Truthful the champion resolution present would beryllium - seat supra - to straight cheque for if(MSVC)
The bully intelligence is that this was fastened successful CMake three.1 with the instauration of argumentation CMP0054. I would urge to ever fit cmake_policy(Fit CMP0054 Fresh) to “lone construe if() arguments arsenic variables oregon key phrases once unquoted.”
The action() bid
Chiefly conscionable cached strings that lone tin beryllium Connected oregon Disconnected and they let any particular dealing with similar e.g. dependencies
However beryllium alert, don’t error the action with the fit bid. The worth fixed to action is truly lone the “first worth” (transferred erstwhile to the cache throughout the archetypal configuration measure) and is afterwards meant to beryllium modified by the person done CMake’s GUI.