How Do Inline Namespaces Solve Library Versioning Problems in C ?

What are Inline Namespaces For?

Inline namespaces address a need for library versioning without relying on platform-specific binary executable format features. They allow library authors to create nested namespaces whose members appear as if they were directly under the enclosing namespace.

Library Versioning with Inline Namespaces

Consider a vector implementation in the STL header:

namespace std {
    // Pre-C++98
    #if __cplusplus < 1997L
        inline
    #endif
    namespace pre_cxx_1997 {
        // ...
    }
    // C++98/03
    #if __cplusplus >= 1997L
        #if __cplusplus == 1997L
            inline
        #endif
        namespace cxx_1997 {
            // ...
        }
    #endif
}

By using inline namespaces, the library author can introduce new versions of the vector implementation across different standard versions:

// C++11
inline
namespace cxx_2011 {
    // ...
}

Implications of Non-Inline Nested Namespaces

Without inline namespaces, versioning using nested namespaces requires using namespace declarations. This approach, however, can expose the implementation detail that the vector is not declared directly under namespace std.

Potential Pitfalls

The lack of a defined scheme for inline namespace names in the standard limits their use to third-party libraries only. Additionally, inline namespaces can potentially lead to issues with the specialization of std templates, as the true namespace in which they are defined is not exposed when using nested namespaces without explicit inlining.

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