411 REFERENCE · Wikibooks
C++ Language/Variables/Initialization
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A constant variable (marked by the const keyword) must be initialized.
If a static-lifetime variable is not manually initialized, then it will be automatically initialized to 0.
If a local object variable is not manually initialized, then its default-constructor will construct it.
If a local primitive variable is not manually initialized, then its value will be undefined until something gets assigned to it.
Additional information about initialization (includes interactive examples)