cpp17 · advanced

C++17: inline Variables

C++17 inline variables extend the ODR-merging guarantee of inline functions to objects: a variable marked inline may be defined in a header and included in many translation units, and the linker collapses the copies into a single object. This fixes a long-standing pain — before C++17, a non-const static data member had to be declared in the class and defined in exactly one .cpp, so forgetting the out-of-line definition caused a linker error while defining it in the header violated the One Definition Rule, a constant headache for header-only libraries. Now a static data member can be written as static inline int n = 0; directly in the class body, and namespace-scope inline variables provide header-friendly globals. Additionally, constexpr static data members are implicitly inline as of C++17, so they no longer require a separate definition either. Use inline variables for header-only library globals and singletons, configuration constants kept beside their type, and self-defining static members.

The code

// config.h  — included by many .cpp files
struct Config {
    static inline int max_conns = 64; // C++17: define IN the header
};

inline int g_counter = 0; // one shared global, header-only

// pre-C++17: declare in header, DEFINE in exactly one .cpp,
// or risk multiple-definition (ODR) link errors

What this lesson walks through

1. 01The old static-member pain
2. 02inline variables fix it
3. 03Gotcha — without inline = ODR violation
4. 04When to use

Before C++17, a non-const static data member had to be declared in the class (header) and defined in exactly one translation unit (.cpp). Forget the out-of-line definition and you get a linker error; put it in the header and every TU defines it, violating the One Definition Rule. Header-only libraries fought this constantly.