Nontrivial C++ programs are generally divided into multiple translation units that are later linked together to form an executable. To support such a model, C++ restricts named object definitions to ensure that linking will behave deterministically by requiring a single definition for an object across all translation units. This model is called the one-definition rule (ODR), which is defined by the C++ Standard, [basic.def.odr], in paragraph 4 [ISO/IEC 14882-2014].:
Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program; no diagnostic required. The definition can appear explicitly in the program, it can be found in the standard or a user-defined library, or (when appropriate) it is implicitly defined. An inline function shall be defined in every translation unit in which it is odr-used.
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