Why Does My C Code Throw a 'Multiple Definition” Error?

Multiple Definition Error: A Solution to Conflicting Definitions

In C , when defining functions or classes in headers and implementing them in source files, developers may encounter the error "multiple definition." This error arises when multiple translation units (source files) contain definitions for the same identifier.

Causes of Multiple Definition

The error "multiple definition" occurs when header files containing definitions are included in multiple translation units. This can happen when:

• A header file is included both in a source file and its corresponding header file.
• A definition is provided in a header file instead of a declaration.

Understanding the Error

Consider the code sample provided:

// complex.h
class Complex
{
public:
   friend std::ostream& operator<<(std::ostream&, const Complex&);
};

// complex.cpp
std::ostream& operator<<(std::ostream&, const Complex&);

In this example, the definition of the operator << is in complex.h instead of complex.cpp. This can lead to a multiple definition error because both main.cpp and complex.cpp will attempt to define the << operator for the Complex class.

Solutions to Multiple Definitions

The solutions for the multiple definition error are:

• Use the "inline" Keyword: By declaring the function as inline, you allow multiple translation units to provide the definition without causing a conflict. However, inline functions have some limitations, such as performance impact on large functions.
• Move Definition to Source File: Instead of defining the operator << in the header file, move it to the corresponding source file (complex.cpp). This ensures that the definition only exists in one translation unit.

Using the inline keyword or moving the definition to the source file are the preferred solutions for avoiding the multiple definition error.

Difference in Handling real() and <<

In the code sample, the real() function is declared in complex.h, but not defined. This is why the compiler does not report a multiple definition error for it. Member functions declared in class declarations are implicitly inlined, meaning their definitions are inserted into all translation units that include the header. This is not the case for the operator << function, which is defined in complex.h.

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