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