Conditional declaration refers to a practice where some behavior may or may not be available depending on some parameter.
As with any conditions it should preferably be contained in a way that prevents branching from littering a code base.
Declaration is used to indicate that the behavior that is applied as early as possible such that the underlying logic conditionally exits rather than being conditionally evaluated.
The #ifdef <CONSTANT> preprocessor directive will
ignore/remove the contents through the complementary #endif
if the indicated constant is not defined. The constant itself can then
be provided as an argument or expanded into a template.