cmake command line -D and using DEFINED

Usage
1

I would like to be able to just define a cmake variable without giving it a value, similar to how the C preprocessor can define variables and check if the variable is defined.

  1. It seems that I have to use -D variable_name=
    instead of just -D variable_name ?

  2. It also seems that If I only check if the variable is defined, cmake-3.17.2
    posts a warning that the variable was not used while cmake-3.18.4 does
    not post the warning ?

2

CMake variables do not have such a state. They either have a value or are undefined. The value can be empty (which might be what you’re looking for). -Dvariable_name= will set it to the empty string.

CMake differences

3.18 might consider the variable “used” in an if (DEFINED) check. I’m not sure what would have changed that.

3

Here is my test script that gives a warning with 3.17.2 and not 3.18.4. Note that it builds in place so use a temporary directory for the test.

#! /bin/bash -e
cat << EOF > CMakeLists.txt
PROJECT(cmake_defined)
IF( DEFINED variable )
    MESSAGE(STATUS "variable is defined")
ELSE( DEFINED variable )
    MESSAGE(STATUS "variable is not defined")
ENDIF( DEFINED variable )
EOF
cmake -D variable= .
4

I’m not sure this is a regression (we don’t guarantee when warnings fire), but thanks for the test case.

Cc: @kyle.edwards @brad.king

5

The command-line syntax for adding a cache entry is -DVAR=$value for some $value, which may or may not be empty. I don’t think we define behavior for plain -DVAR.

my test script that gives a warning with 3.17.2 and not 3.18.4

That script gives me a “Manually-specified variables were not used by the project” warning with 3.17.5, 3.18.5, and 3.19.1. Be sure to use a fresh tree each time because once the variable is in CMakeCache.txt the warning won’t appear again.

6

My mistake. I corrected my test case and got the warning in 3.18.4.

#! /bin/bash -e
if [ -e CMakeCache.txt ]
then
    rm CMakeCache.txt
fi
cat << EOF > CMakeLists.txt
PROJECT(cmake_defined)
IF( DEFINED variable )
    MESSAGE(STATUS "variable is defined")
ELSE( DEFINED variable )
    MESSAGE(STATUS "variable is not defined")
ENDIF( DEFINED variable )
EOF
cmake -D variable= .

Thanks

7

The command line interface explicitly considers -DVAR an error.