What is the One-Definition Rule in C and What Happens if it's Violated?

Decoding the One-Definition Rule in C

The One-Definition Rule is a crucial concept in C to ensure the integrity of code and prevent ambiguity. It stipulates that within a translation unit, there must be a singular definition for each variable, function, class type, enumeration type, and template.

Official Definition

The definitive source for the One-Definition Rule lies within the C Standard, section 3.2:

• "No translation unit shall contain more than one definition of any variable, function, class type, enumeration type or template."

Implications

This rule prohibits multiple definitions within a single translation unit. However, it permits multiple occurrences in different translation units as long as the definitions are identical.

In addition, for non-inline functions and objects used within a program, there must be exactly one definition. This definition can reside in the current program, standard library, or user-defined library. If appropriate, the definition can also be implicitly generated.

Inline Functions

Inline functions are an exception to the One-Definition Rule. They must be defined in every translation unit where they are used to ensure that their implementation is readily available to the compiler.

Consequences of Violation

Ignoring the One-Definition Rule can lead to compilation errors and unpredictable program behavior. For instance, multiple definitions of the same variable can result in conflicts during program execution.

By adhering to the One-Definition Rule, programmers can avoid such errors and maintain the consistency and reliability of their C code.

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