Difference between revisions of "CMake:VariablesListsStrings"

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==Scope of variables in CMake==
{{CMake/Template/Moved}}


In CMake variables don't have to be declared, they are created upon their first usage:
This page has moved [https://gitlab.kitware.com/cmake/community/wikis/doc/cmake/VariablesListsStrings here].
 
src/foo/CMakeLists.txt:
 
<pre>
set(SomeVariable "Hello world")
</pre>
 
This creates (if it didn't exist yet) the variable <tt>SomeVariable</tt>.
All variables in CMake are '''global'''.
Global means that the variables exist in the file where they have been created, in all subdirectories connected using ADD_SUBDIRECTORY() or SUBDIRS(), and in all included files in any of these directories. They don't propagate up to the parent directories.
Also if the value of a variable is changed in a subdirectory, the change doesn't propagate up to the variable in the parent directory.
 
==Strings vs. lists==
 
In CMake all variables are of the type string. Nevertheless CMake can deal also with lists.
If a string contains semicolons, these semicolons can be interpreted as separators of string values.
 
<pre>
set(MyString "Hello world")
</pre>
 
This sets MyString to "Hello world", which is always only one string.
 
<pre>
set(MyList Hello world)
</pre>
 
This sets MyList to a list with the items "Hello" and "world". It will be stored as a string "Hello;world".
 
==Emulating maps==
 
Since CMake only has the string type (and somewhat lists), you can't use maps directly, but you can emulate them.
 
Let's say you'd like to do the following:
 
<pre>
void InsertIntoMap(string key, string value)
{
  MyMap[key]=value;
}
 
string k = "foo";
string v = "bar";
InsertIntoMap(k, v);
</pre>
 
you can do the following in CMake:
 
<pre>
MACRO(INSERT_INTO_MAP _KEY _VALUE)
  SET("MyMap_${_KEY}" "${_VALUE}")
ENDMACRO(INSERT_INTO_MAP)
 
SET(MyKey "foo")
SET(MyValue "bar")
INSERT_INTO_MAP("${MyKey}" "${MyValue}")
</pre>
 
You can test for the existence of a key using IF(DEFINED ...)
 
==Boolean values in CMake==
 
The following values are interpreted as ''false'' by CMake. Upper/lower casing doesn't matter:
* OFF
* FALSE
* N
* NO
* 0
* "" (empty string)
* a null variable, such as produced by "set(variable)".
* NOTFOUND
* any string ending in -NOTFOUND
 
''True'':
* ON
* TRUE
* Y
* YE
* YES
* 1
* everything not listed under ''false''
 
==Using CMake regexps==
 
==Escaping==
Sometimes you need to escape a $
* In a regex, outside of [] you need \\$
* In a regex, inside of [] you do not need anything. [$] is fine.
* In a non-regex string, in front of a curly bracket you need \${
<pre>
set(blah "whatever but \${do_not_evaluate}")
</pre>
* In a regex string, in front of a curly bracket you need \\\${
<pre>
string(REGEX REPLACE
  ".*whatever but \\\${do not evaluate}"
</pre>
 
==The CMake cache==
 
CMake uses an on-disk cache file to store variables and their values for later runs. This file is located in CMAKE_BINARY_DIR and is named CMakeCache.txt.
The variables in this file are global to the whole build tree.
 
Most configure-checks save their result in this file, so they don't have to be executed again on later CMake runs.
 
{{CMake/Template/Footer}}

Latest revision as of 15:41, 30 April 2018


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