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CMake's if command [1] supports several signatures, starting with

if(<constant>)
if(<variable|string>)
if(NOT <expression>)

How to negate the first two?

If the CMake documentation is correct (which in my experience is far from certain), then my question boils down to:

How to convert a constant, a variable, or a string X into an expression, with the additional requirement that X is to be evaluated as a boolean?

[1] https://cmake.org/cmake/help/latest/command/if.html

Joachim W
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  • `if(NOT )`, `if(NOT )`. Actually, `` in the third example is just a placeholder for any parameter, which can be passed to `if`. – Tsyvarev Sep 27 '18 at 12:15
  • if that is the full story, then why not paste it as an answer, which I then quickly accept? – Joachim W Sep 27 '18 at 13:37
  • " is just a placeholder for any parameter" - a prime example for the many unsaid things that make it so difficult to learn CMake from the docs. – Joachim W Sep 27 '18 at 13:40

1 Answers1

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Actually, <expression> is just a placeholder for any parameter, which can be passed to if. Even the list of possible if constructions is titled as "Possible expressions are".

if(NOT <constant>) # Revert 'if(<constant>)'
if(NOT <variable|string>) # Revert 'if(NOT <variable|string>)'
Tsyvarev
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